:xrefstyle: full
:sectnums!:
== Jakarta Expression Language, Version 6.1

Copyright (c) 2013, 2025 Oracle and/or its affiliates and others.
All rights reserved.

Eclipse is a registered trademark of the Eclipse Foundation. Jakarta
is a trademark of the Eclipse Foundation. Oracle and Java are
registered trademarks of Oracle and/or its affiliates. Other names
may be trademarks of their respective owners. 

The Jakarta Expression Language Team - {revdate}

Comments to: el-dev@eclipse.org

== Preface

This is the Expression Language specification
version 6.1, developed by the Jakarta Expression Language Team under the Eclipse
Foundation Specification Process.

=== Historical Note

The Expression Language (EL) was originally inspired by both
ECMAScript and the XPath expression languages. During its inception, the
experts involved were very reluctant to design yet another expression
language and tried to use each of these languages, but they fell short
in different areas.

The JSP Standard Tag Library (JSTL) version 1.0
(based on JSP 1.2) was therefore first to introduce an Expression
Language to make it easy for page authors to access and manipulate
application data without having to master the complexity associated with
programming languages such as Java and JavaScript.

Given its success, the EL was subsequently moved
into the JSP specification (JSP 2.0/JSTL 1.1), making it generally
available within JSP pages (not just for attributes of JSTL tag
libraries).

JavaServer Faces 1.0 defined a standard
framework for building User Interface components, and was built on top
of JSP 1.2 technology. Because JSP 1.2 technology did not have an
integrated expression language and because the JSP 2.0 EL did not meet
all of the needs of Faces, an EL variant was developed for Faces 1.0.
The Faces expert group (EG) attempted to make the language as compatible
with JSP 2.0 as possible but some differences were necessary.

It was obviously desirable to have a single,
unified expression language that meets the needs of the various web-tier
technologies. The Faces and JSP EGs therefore worked together on the
specification of a unified expression language, defined in JSR 245, and
which took effect for the JSP 2.1 and Faces 1.2 releases.

The JSP/JSTL/Faces expert groups also
acknowledged that the EL is useful beyond their own
specifications. The 3.0 specification was the first JSR that defined the
Expression Language as an independent specification, with no
dependencies on other technologies.

This specification is now developed under the Eclipse
Foundation Specification Process. Together with the Test Compatibility
Kit (TCK) which tests that a given implementation meets the requirements
of the specification, and Compatible Implementations (CIs) that
implement this specification and which pass the TCK, this specification
defines the Jakarta standard for Expression Language.

<<<
=== Typographical Conventions

[width="100%",cols="50%,50%",options="header",]
|===
|Font Style |Uses
| _Italic_ | _Emphasis, definition of term._
| `Monospace` | `Syntax, code examples, attribute names, Java language
types, API, enumerated attribute values.`
|===

=== Comments

We are interested in improving this
specification and welcome your comments and suggestions. We have a
GitHub project with an issue tracker and a mailing list for comments
and discussions about this specification.

Project: https://github.com/jakartaee/expression-language

Mail alias for comments: el-dev@eclipse.org

:sectnums:
== Language Syntax and Semantics

=== Overview

The EL was originally designed as a simple
language to meet the needs of the presentation layer in web
applications. It features:

* A simple syntax restricted to the evaluation of expressions

* Variables and nested properties

* Relational, logical, arithmetic, conditional, and empty operators

* Functions implemented as static methods on Java classes

* Lenient semantics where appropriate default values and type
conversions are provided to minimize exposing errors to end users

as well as

* A pluggable API for resolving variable references into Java objects
and for resolving the properties applied to these Java objects

* An API for deferred evaluation of expressions that refer to either
values or methods on an object

* Support for lvalue expressions (expressions a value can be assigned
to)

These last three features are key additions
to the JSP 2.0 EL resulting from the EL alignment work done in the JSP
2.1 and Faces 1.2 specifications.

EL 3.0 added features to enable EL to be used
as a stand-alone tool. It introduced APIs for direct evaluation of EL
expressions and manipulation of EL environments. It also added some
powerful features to the language, such as the support of operations for
collection objects.

EL 4.0 implemented the transition from the `javax` namespace to the
`jakarta` namespace.

EL 5.0 made Java 11 the minimum version and implemented a number of
enhancement requests from the user community. Deprecated code was removed
and use of FeatureDescriptor was deprecated with a view to removing the
dependency on the `java.desktop` module.

EL 6.0 made Java 17 the minimum version, removed deprecated code and made the
dependency on the `java.desktop` module optional. New resolvers have been added
for `java.lang.Record` and `java.lang.Optional` instances and support for the
`length` property has been added to arrays.

==== EL in a nutshell

The syntax is quite simple. Model objects are
accessed by name. A generalized `[]` operator can be used to access
maps, lists, arrays of objects and properties of a JavaBeans object, and
to invoke methods in a JavaBeans object; the operator can be nested
arbitrarily. The `.` operator can be used as a convenient shorthand for
property access when the property name follows the conventions of Java
identifiers, but the `[]` operator allows for more generalized access.
Similarly, the `.` operator can also be used to invoke methods, when the
method name is known, but the `[]` operator can be used to invoke methods
dynamically.

Relational comparisons are allowed using the
standard Java relational operators. Comparisons may be made against
other values, or against boolean (for equality comparisons only),
string, integer, or floating point literals. Arithmetic operators can be
used to compute integer and floating point values. Logical operators are
available.

The EL features a flexible architecture where
the resolution of model objects (and their associated properties and
methods), functions, and variables are all performed through a pluggable
API, making the EL easily adaptable to various environments.

=== EL Expressions

An EL expression is specified either as an
_eval-expression_, or as a _literal-expression_. The EL also supports
_composite expressions_, where multiple EL expressions
(eval-expressions and literal-expressions) are grouped together.

An EL expression is parsed as either a _value
expression_ or a _method expression_. A value expression refers to a value,
whereas a method expression refers to a method on an object. Once
parsed, the expression can optionally be evaluated one or more times.

Each type of expression (eval-expression,
literal-expression, and composite expression) is described in its own
section below.

==== Eval-expressions

An eval-expression is formed by using the
constructs `$\{expr}` or `#\{expr}`. Both constructs are parsed and
evaluated in exactly the same way by the EL, even though they might
carry different meanings in the technology that is using the EL.

For instance, by convention the Jakarta EE web
tier specifications use the `$\{expr}` construct for immediate
evaluation and the `\#{expr}` construct for deferred evaluation. This
difference in delimiters points out the semantic differences between the
two expression types in the Jakarta EE web tier. Expressions delimited by
`#{}` are said to use “deferred evaluation” because the expression is
not evaluated until its value is needed by the system. Expressions
delimited by `${}` are said to use “immediate evaluation” because the
expression is compiled when the JSP page is compiled and it is executed
when the JSP page is executed. More on this in
<<Syntax restrictions>>.

Other technologies may choose to use the same
convention. It is up to each technology to enforce its own restrictions
on where each construct can be used.

In some EL APIs, especially those introduced
in EL 3.0 to support stand-alone use, the EL expressions are specified
without `${}` or `#{}` delimiters.

*Nested eval-expressions, such as `${item[$\{i}]}`, are illegal.*

===== Eval-expressions as value expressions

When parsed as a value expression, an
eval-expression can be evaluated as either an _rvalue_ or an _lvalue_. An
_rvalue_ is an expression that would typically appear on the right side of
the assignment operator. An _lvalue_ would typically appear on the left
side.

For instance, all EL expressions in JSP 2.0
are evaluated by the JSP engine immediately when the page response is
rendered. They all yield rvalues.

In the following JSTL action:

* `<c:out value="${customer.name}"/>`

the expression `${customer.name}` is
evaluated by the JSP engine and the returned value is fed to the tag
handler and converted to the type associated with the attribute
(`String` in this case).

Faces, on the other hand, supports a full UI
component model that requires expressions to represent more than just
rvalues. It needs expressions to represent references to data structures
whose value could be assigned, as well as to represent methods that
could be invoked.

For example, in the following Faces code sample:

[source,html]
----
<h:form>
  <h:inputText
    id="email"
    value="#{checkOutFormBean.email}"
    size="25" maxlength="125"
    validator="#{checkOutFormBean.validateEmail}"/>
</h:form>
----

when the form is submitted, the “apply
request values” phase of Faces evaluates the EL expression
`#{checkOutFormBean.email}` as a reference to a data structure whose
value is set with the input parameter it is associated with in the form.
The result of the expression therefore represents a reference to a data
structure, or an lvalue, the left hand side of an assignment
operation.

When that same expression is evaluated during
the rendering phase, it yields the specific value associated with the
object (rvalue), just as would be the case with JSP.

The valid syntax for an lvalue is a subset of
the valid syntax for an rvalue. In particular, an lvalue can only
consist of either a single variable (e.g. `$\{name}`) or a property
resolution on some object, via the `.` or `[]` operator (e.g.
`${employee.name}`). Of course, an EL function or method that returns
either an object or a name can be part of an lvalue.

When parsing a value expression, an expected
type is provided. In the case of an rvalue, the expected type is what
the result of the expression evaluation is coerced to. In the case of
lvalues, the expected type is ignored and the provided value is coerced
to the actual type of the property the expression points to, before that
property is set. The EL type conversion rules are defined in
<<Type Conversion>>. A few sample
eval-expressions are shown in the following table.


.Sample eval-expressions
[width="100%",cols="34%,33%,33%",options="header",]
|===
|Expression
|Expected Type
|Result
|`${customer.name}`
|`String`
|`Guy Lafleur`

_Expression evaluates to a String. No
conversion necessary._

|`${book}`
|`String`
|`Wonders of the World`

_Expression evaluates to a `Book` object (e.g.
`com.example.Book`). Conversion rules result in the evaluation of
`book.toString()`, which could, for example, yield the book title._

|===


===== Eval-expressions as method expressions

In some cases, it is desirable for an EL
expression to refer to a method instead of a model object.

For instance, in JSF, a component tag also
has a set of attributes for referencing methods that can perform certain
functions for the component associated with the tag. To support these
types of expressions, the EL defines method expressions (EL class
`MethodExpression`).

In the above example, the validator attribute
uses an expression that is associated with type `MethodExpression`.
Just as with ``ValueExpression``s, the evaluation of the expression
(calling the method) is deferred and can be processed by the underlying
technology at the appropriate moment within its life cycle.

A method expression shares the same syntax as
an lvalue. That is, it can only consist of either a single variable
(e.g. `$\{name}`) or a property resolution on some object, via the `.`
or `[]` operator (e.g. `${employee.name}`). Information about the
expected return type and parameter types is provided at the time the
method is parsed.

A method expression is evaluated by invoking
its referenced method or by retrieving information about the referenced
method. Upon evaluation, if the expected signature is provided at parse
time, the EL API verifies that the method conforms to the expected
signature, and there is therefore no coercion performed. If the expected
signature is not provided at parse time, then at evaluation, the method
is identified with the information of the parameters in the expression,
and the parameters are coerced to the respective formal types.
The method must be identified using the same process as that described in
section 15.12.2 of the Java Language Specification, version 11 with the
addition that coercion resolution is considered to be less specific than
overload resolution.


==== Literal-expressions

A literal-expression does not use the
`$\{expr}` or `#{expr}` constructs, and simply evaluates to the text of
the expression, of type `String`. Upon evaluation, an expected type of
something other than `String` can be provided. Sample
literal-expressions are shown in the following table.


.Sample literal-expressions
[width="100%",cols="34%,33%,33%",options="header",]
|===
|Expression
|Expected Type
|Result
|`Aloha!`
|`String`
|`Aloha!`
|`true`
|`Boolean`
|`Boolean.TRUE`
|===

To generate literal values that include the
character sequence \"``${``" or \"``#{``", the developer can choose to
use a composite expression as shown here:

* `${'${'}exprA}`

* `\#{'#{'}exprB}`
 
The resulting values would
then be the strings `${exprA}` and `#\{exprB}`.

Alternatively, the escape characters `\$` and
`\#` can be used to escape what would otherwise be treated as an
eval-expression. Given the literal-expressions:

* `\${exprA}`

* `\#{exprB}`

The resulting values would again be the
strings `$\{exprA}` and `#\{exprB}`.

A literal-expression can be used anywhere a
value expression can be used. A literal-expression can also be used as a
method expression that returns a non-void return value. The standard EL
coercion rules (see <<Type Conversion>>)
then apply if the return type of the method expression is not
`java.lang.String`.

Note that when EL is integrated into other technologies, such as JSP,
that integration may not include literal-expressions. Where integrations
do not include literal-expressions, those integrating technologies will
define their own specification, including escaping rules, for handling
text outside of EL and the escaping rules described above will not apply.

==== Composite expressions

The EL also supports _composite expressions_,
where multiple EL expressions are grouped together. With composite
expressions, eval-expressions are evaluated from left to right, coerced
to ``String``s (according to the EL type conversion rules), and
concatenated with any intervening literal-expressions.

For example, the composite expression
`"$\{firstName} $\{lastName}"` is composed of three EL expressions:
eval-expression `"$\{firstName}"`, literal-expression `" "`, and
eval-expression `"$\{lastName}"`.

Once evaluated, the resulting `String` is
then coerced to the expected type, according to the EL type conversion
rules. A sample composite expression is shown in the following table.

.Sample composite expression
[width="100%",cols="34%,33%,33%",options="header",]
|===
|Expression
|Expected Type
|Result
|`Welcome ${customer.name} to our site`
|`String`
|`Welcome Guy Lafleur to our site`

_``${customer.name}`` evaluates to a `String` which
is then concatenated with the literal-expressions. No conversion
necessary._

|===


*It is illegal to mix `${}` and `\#{}`
constructs in a composite expression*. This restriction is imposed to
avoid ambiguities should a user think that using `${expr}` or
`#{expr}` dictates how an expression is evaluated. For instance, as was
mentioned previously, the convention in the Jakarta EE web tier specifications
is for `${}` to mean immediate evaluation and for `#{}` to mean
deferred evaluation. This means that in EL expressions in the Jakarta EE web
tier, a developer cannot force immediate evaluation of some parts of a
composite expression and deferred evaluation of other parts. This
restriction may be lifted in future versions to allow for more advanced
EL usage patterns.

For APIs prior to EL 3.0, a composite
expression can be used anywhere an EL expression can be used except for
when parsing a method expression. Only a single eval-expression can be
used to parse a method expression.

Some APIs in EL 3.0 onwards use only single
eval-expressions, and not the composite expressions. However, there is
no loss in functionality, since a composite expression can be specified
with a single eval-expressions, by using the string concatenation
operators, introduced in EL 3.0. For instance, the composite expression:

* `Welcome ${customer.name} to our site`

can be written as:

* `${'Welcome ' += customer.name += ' to our site'}`.

==== Syntax restrictions

While `${}` and `#{}` eval-expressions are
parsed and evaluated in exactly the same way by the EL, the underlying
technology is free to impose restrictions on which syntax can be used
according to where the expression appears.

For instance, in JSP, `\#{}` expressions
are only allowed for tag attributes that accept deferred expressions.
`#{expr}` will generate an error if used anywhere else.

=== Literals

There are literals for boolean, integer,
floating point, string, and null in an eval-expression.

* Boolean - `true` and `false`

* Integer - As defined by the
`IntegerLiteral` construct in <<Collected Syntax>>

* Floating point - As defined by the
`FloatingPointLiteral` construct in <<Collected Syntax>>

* String - Enclosed with single or double quotes with the following rules
for escaping the enclosed string:
** `\` must be escaped as `\\`
** `"` must be escaped as `\"` when the string is enclosed with `"`
** `"` may be escaped as `\"` when the string is enclosed with `'`
** `'` must be escaped as `\'` when the string is enclosed with `'`
** `'` may be escaped as `\'` when the string is enclosed with `"`
** no other escaping is permitted

* Null - `null`

=== Errors, Warnings and Default Values

The Expression Language has been designed with
the presentation layer of web applications in mind. In that usage,
experience suggests that it is most important to be able to provide as
good a presentation as possible, even when there are simple errors in
the page. To meet this requirement, the EL does not provide warnings,
just default values and errors. Default values are type-correct values
that are assigned to a subexpression when there is some problem. An
error is an exception thrown (to be handled by the environment where the
EL is used).

=== Resolution of Model Objects and their Properties or Methods

A core concept in the EL is the evaluation of
a model object name into an object, and the resolution of properties or
methods applied to objects in an expression (operators `.` and `[]`).

The EL API provides a generalized mechanism,
an `ELResolver`, implemented by the underlying technology and which
defines the rules that govern the resolution of model object names and
their associated properties.

The resolution of names and properties is
further affected by the presence of:

* Functions. See <<Functions>>.

* Variables. See <<Variables>>.

* Imported names (classes, fields, and
methods). See <<Static Field and Method Reference>>.

* Lambda expressions and arguments. See <<Lambda Expressions>>.

The rules described below are used in
resolving names and properties when evaluating identifiers, function
calls, and object properties and method calls.

==== Evaluating Identifiers

These steps are used for evaluating an identifier:

* If the identifier is a lambda argument passed
to a lambda expression invocation, its value is returned.

* Else if the identifier is a variable, the
associated expression is evaluated and returned.

* Else if the identifier is resolved by the
``ELResolver``s, the value returned from the ``ELResolver``s is returned.

* Else if the identifier is an imported static
field, its value is returned.

* Else return not resolved.

One implication of the explicit search order
of the identifiers is that an identifier hides other identifiers (of the
same name) that come after it in the list.

===== Resolving Identifiers with ``ELResolver``s

Resolution of identifiers with ``ELResolver``s can be optimised when a single,
stand-alone identifier (e.g. `identifier-a`) needs to be resolved compared to
the resolution of multiple identifiers (e.g. `identifier-a.identifier-b`). This
is particularly important for applications using Jakarta Pages as optimisation
enables measurable performance improvements for instances of
`jakarta.servlet.jsp.el.ImportELResolver`.

To support such optimisations, when resolving a single, stand-alone identifier,
the implementation is required to put an instance of `Boolean.TRUE` into the
`ELContext` under the key `jakarta.el.ELResolver.StandaloneIdentifierMarker` for
the duration of the attempt to resolve the identifier.

==== Evaluating functions

The expression with the syntax
_func(args...)(args...)..._ can mean any of the following:

* A call to an EL function with empty namespace.

* A call to a lambda expression.

* A call to the constructor of an imported class.

* A call to a static method that has been imported statically.

Note the above syntax allows the invocation
of a lambda expression that returns another lambda expression, which is
then invoked.

The following steps are used to evaluate the
above expression:

* Evaluate the name of the function as an identifier:

** If the identifier is a lambda argument passed
to a lambda expression invocation, its value is returned.

** Else if the identifier is a variable, the
associated expression is evaluated and returned.

** Else if the identifier is resolved by the ``ELResolver``s,
the value returned from the ``ELResolver``s is returned.

* If the result of evaluating the function name is a `LambdaExpression`,
the `LambdaExpression` is invoked with the supplied
arguments. If the result of evaluating the `LambdaExpression` is another
`LambdaExpression`, and the syntax contains repeated function invocations,
such as _func()()..._, then the resultant `LambdaExpression` is in turn
evaluated, and so on.

* Else if the function has been mapped
previously in a `FunctionMapper`, the mapped method is invoked with the
supplied arguments.

* Else if the function name is the name of an
imported class, the constructor for this class is invoked with the
supplied arguments.

* Else if the function name is the name of an
imported static method, the method is invoked with the supplied
arguments.

* Else error.

==== Evaluating objects with properties

The steps for evaluating an expression with
`[]` or `.` operators (property reference and method call) are described in
<<Operators `[]` and `.`>>. However, the
syntax for `.` operator is also used to reference a static field, or to
invoke a static method. Therefore if the expression with a `.` operator is
not resolved by the ``ELResolver``s, and if the identifier for the base
object is the name of an imported class, the expression becomes a
reference to a static field, or an invocation of a static method, of the
imported class.

==== Invoking method expressions

A method expression can consist of either a
single variable (e.g. `${name}`) or a property resolution on some
object, via the `.` or `[]` operator (e.g. `${employee.getName}`).
<<Operators `[]` and `.`>> describes how to
invoke a method of an object. This form of method expressions allows
arguments to the method to be specified in the EL expression (e.g.
`${employee.getName()}`).

To invoke a method expression of a single
variable, the identifier is first evaluated, as described in
<<Evaluating Identifiers>>. If the
identifier evaluates to a `jakarta.el.MethodExpression`, the method
expression is invoked and the result returned, otherwise an error is
raised. This form of method expression does not allow arguments to be
specified in the EL expression.

=== Operators `[]` and `.`

The EL follows ECMAScript in unifying the
treatment of the `.` and `[]` operators.

`expr-a.identifier-b` is equivalent to
`expr-a["identifier-b"]`; that is, the identifier `identifier-b` is
used to construct a literal whose value is the identifier, and then the
`[]` operator is used with that value.

Similarly, `expr-a.identifier-b(params)` is
equivalent to `expr-a["identifier-b"](params).`

The expression
`expr-a["identifier-b"](params)` denotes a method invocation
with parameters, where `params` is a comma-separated list of expressions
denoting the parameters for the method call.

To evaluate `expr-a[expr-b] or expr-a[expr-b](params)`:

* Evaluate `expr-a` into `value-a`.

* If `value-a` is `null`:

** If `expr-a[expr-b]` is the last property being resolved:

*** If the expression is a value expression and
`ValueExpression.getValue(context)` was called to initiate this
expression evaluation, return `null`.

*** Otherwise, throw `PropertyNotFoundException`. +
_[trying to de-reference null for an lvalue]_

** Otherwise, return `null`.

* Evaluate `expr-b` into `value-b`.

* If `value-b` is `null`:

** If `expr-a[expr-b]` is the last property being resolved:

*** If the expression is a value expression and
`ValueExpression.getValue(context)` was called to initiate this
expression evaluation, return `null`.

*** Otherwise, throw `PropertyNotFoundException`. +
_[trying to de-reference null for an lvalue]_

** Otherwise, return `null`.

* If the expression is a value expression:

** If `expr-a[expr-b]` is the last property being resolved:

*** If `ValueExpression.getValue(context)` was
called to initiate this expression evaluation:

**** If the expression is a parametered method
call, evaluate `params` into `param-values`, and invoke
`elResolver.invoke(context, value-a, value-b, null, param-values)`.

**** Otherwise, invoke `elResolver.getValue(value-a, value-b)`.

*** If `ValueExpression.getType(context)` was
called, invoke `elResolver.getType(context, value-a, value-b)`.

*** If `ValueExpression.isReadOnly(context)` was
called, invoke `elResolver.isReadOnly(context, value-a, value-b)`.

*** If `ValueExpression.setValue(context, val)` was called,
invoke `elResolver.setValue(context, value-a, value-b, val)`.

** Otherwise:

*** If the expression is a parametered method
call, evaluate `params` into `param-values`, and invoke
`elResolver.invoke(context, value-a, value-b, null, params)`.

*** Otherwise, invoke `elResolver.getValue(value-a, value-b)`.

* Otherwise, the expression is a method expression:

** If `expr-a[expr-b]` is the last property being resolved:

*** Coerce `value-b` to `String`.

*** If the expression is not a parametered method
call, find the method on object `value-a` with name `value-b` and with
the set of expected parameter types provided at parse time. If the
method does not exist, or the return type does not match the expected
return type provided at parse time, throw `MethodNotFoundException`.

*** If `MethodExpression.invoke(context, params)` was called:

**** If the expression is a parametered method
call, evaluate `params` into `param-values`, and invoke
`elResolver.invoke(context, value-a, value-b, paramTypes, param-values)`
where `paramTypes` is the parameter types, if provided at parse time, and
is `null` otherwise.

**** Otherwise, invoke the found method with the parameters passed to
the invoke method.

*** If `MethodExpression.getMethodInfo(context)`
was called, construct and return a new `MethodInfo` object.

** Otherwise:

*** If the expression is a parametered method call, evaluate `params`
into `param-values`, and invoke
`elResolver.invoke(context, value-a, value-b, null, params)`.

*** Otherwise, invoke `elResolver.getValue(value-a, value-b)`.


=== Arithmetic Operators

Arithmetic is provided to act on integer (`BigInteger` and `Long`) and
floating point (`BigDecimal` and `Double`) values. There are 5 operators:

* Addition: `+`

* Substraction: `-`

* Multiplication: `*`

* Division: `/` and `div`

* Remainder (modulo): `%` and `mod`

The last two operators are available in both
syntaxes to be consistent with XPath and ECMAScript.

The evaluation of arithmetic operators is
described in the following sections. `A` and `B` are the evaluation of
subexpressions.

==== Binary operators - `A {+,-,*} B`

* If `A` and `B` are `null`, return `(Long) 0`

* If `A` or `B` is a `BigDecimal`, coerce both to `BigDecimal` and
then:

** If operator is `+`, return `A.add(B)`

** If operator is `-`, return `A.subtract(B)`

** If operator is `*`, return `A.multiply(B)`

* If `A` or `B` is a `Float`, `Double`, or `String` containing `.`,
`e`, or `E`:

** If `A` or `B` is `BigInteger`, coerce both `A` and `B` to `BigDecimal`
and apply operator

** Otherwise, coerce both `A` and `B` to `Double` and apply operator

* If `A` or `B` is `BigInteger`, coerce both to `BigInteger` and then:

** If operator is `+`, return `A.add(B)`

** If operator is `-`, return `A.subtract(B)`

** If operator is `*`, return `A.multiply(B)`

* Otherwise coerce both `A` and `B` to `Long` and apply operator

* If operator results in exception, error

==== Binary operator - `A {/,div} B`

* If `A` and `B` are `null`, return `(Long) 0`

* If `A` or `B` is a `BigDecimal` or a `BigInteger`, coerce both to
`BigDecimal` and return `A.divide(B, BigDecimal.ROUND_HALF_UP)`

* Otherwise, coerce both `A` and `B` to `Double` and apply operator

* If operator results in exception, error

==== Binary operator - `A {%,mod} B`

* If `A` and `B` are `null`, return `(Long) 0`

* If `A` or `B` is a `BigDecimal`, `Float`, `Double`, or `String`
containing `.`, `e`, or `E`, coerce both `A` and `B` to `Double`
and apply operator

* If `A` or `B` is a `BigInteger`, coerce both to `BigInteger` and
return `A.remainder(B)`

* Otherwise coerce both `A` and `B` to `Long` and apply operator

* If operator results in exception, error

==== Unary minus operator - `-A`

* If `A` is `null`, return `(Long) 0`

* If `A` is a `BigDecimal` or `BigInteger`, return `A.negate()`

* If `A` is a `String`:

** If `A` contains `.`, `e`, or `E`, coerce to a `Double` and apply
operator

** Otherwise, coerce to a `Long` and apply operator

** If operator results in exception, error

* If `A` is `Byte`, `Short`, `Integer`, `Long`, `Float`, `Double`

** Retain type, apply operator

** If operator results in exception, error

* Otherwise, error

=== Concatenation Operator - `A += B`

To evaluate `A += B`:

* If `A` and `B` are ``Map``s

** return `A.putAll(B)`

** If method call results in exception, error

* If `A` is a `Set` and `B` is a collection

** return `A.addAll(B)`

** If method call results in exception, error

* If `A` is a `List` and `B` is a collection

** return `A.addAll(B)`

** If method call results in exception, error

* Otherwise, Coerce `A` and `B` to String

** return the concatenated string of `A` and `B`

=== Relational Operators

The relational operators are:

* `==` and `eq`

* `!=` and `ne`

* `<` and `lt`

* `>` and `gt`

* `\<=` and `le`

* `>=` and `ge`

The second versions of the last 4 operators
are made available to avoid having to use entity references in XML
syntax and have the exact same behavior, i.e. `<` behaves the same as
`lt` and so on.

The evaluation of relational operators is
described in the following sections.

==== `A {<,>,\<=,>=,lt,gt,le,ge} B`

* If `A==B`, if operator is `\<=`, `le`, `>=`, or `ge` return `true`

* If `A` is `null` or `B` is `null`, return `false`

* If `A` or `B` is `BigDecimal`, coerce both `A` and `B` to `BigDecimal`
and use the return value of `A.compareTo(B)`

* If `A` or `B` is `Float` or `Double` coerce both `A` and `B` to
`Double` apply operator

* If `A` or `B` is `BigInteger`, coerce both `A` and `B` to `BigInteger`
and use the return value of `A.compareTo(B)`

* If `A` or `B` is `Byte`, `Short`, `Character`, `Integer`, or `Long`
coerce both `A` and `B` to `Long` and apply operator

* If `A` or `B` is `String` coerce both `A` and `B` to `String`, compare
lexically

* If `A` is `Comparable`, then:

** If `A.compareTo(B)` throws exception, error

** Otherwise use result of `A.compareTo(B)`

* If `B` is `Comparable`, then:

** If `B.compareTo(A)` throws exception, error

** Otherwise use result of `B.compareTo(A)`

* Otherwise, error

==== `A {==,!=,eq,ne} B`

* If `A==B`, apply operator

* If `A` is `null` or `B` is `null` return `false` for `==` or `eq`, `true`
for `!=` or `ne`

* If `A` or `B` is `BigDecimal`, coerce both `A` and `B` to
`BigDecimal` and then:

** If operator is `==` or `eq`, return `A.equals(B)`

** If operator is `!=` or `ne`, retur `!A.equals(B)`

* If `A` or `B` is `Float` or `Double` coerce both `A` and `B` to
`Double`, apply operator

* If `A` or `B` is `BigInteger`, coerce both `A` and `B` to
`BigInteger` and then:

** If operator is `==` or `eq`, return `A.equals(B)`

** If operator is `!=` or `ne`, return `!A.equals(B)`

* If `A` or `B` is `Byte`, `Short`, `Character`, `Integer`, or `Long`
coerce both `A` and `B` to `Long`, apply operator

* If `A` or `B` is `Boolean` coerce both `A` and `B` to `Boolean`,
apply operator

* If `A` or `B` is an enum, coerce both `A` and `B` to enum, apply
operator

* If `A` or `B` is `String` coerce both `A` and `B` to `String`, compare
lexically

* Otherwise if an error occurs while calling `A.equals(B)`, error

* Otherwise, apply operator to result of `A.equals(B)`

=== Logical Operators

The logical operators are:

* `&&` and `and`

* `||` and `or`

* `!` and `not`

The evaluation of logical operators is described in the following
sections.

==== Binary operators - `A {&&,||,and,or} B`

* Coerce both `A` and `B` to `Boolean`, apply operator

The operator stops as soon as the expression can be determined, i.e.,
`A and B and C and D` – if `B` is false, then only `A and B` is
evaluated.

==== Unary not operator - `{!,not} A`

* Coerce `A` to `Boolean`, apply operator


=== Empty Operator - `empty A`

The `empty` operator is a prefix operator that can be used to determine
if a value is `null` or empty.

To evaluate `empty A`:

* If `A` is `null`, return `true`

* Otherwise, if `A` is the empty string, then return `true`

* Otherwise, if `A` is an empty array, then return `true`

* Otherwise, if `A` is an empty `Map`, return `true`

* Otherwise, if `A` is an empty `Collection`, return `true`

* Otherwise return `false`

=== Conditional Operator - `A ? B : C`

Evaluate `B` or `C`, depending on the result of the evaluation of `A`.

Coerce `A` to `Boolean`:

* If `A` is `true`, evaluate and return `B`

* If `A` is `false`, evaluate and return `C`

== Elvis Operator - `A ?: B`

The Elivs operator returns the first operand if that operand is `true`.
Otherwise, the second operand is returned.

Coerce `A` to `boolean` (primitive):

* If `A` is `true`, evaluate and return `A` (without coercion)

* If `A` is `false`, evaluate and return `B`

== Null coalescing Operator - `A ?? B`

The Null coalescing operator returns the first operand if that operand is non-null.
Otherwise, the second operand is returned.

To evaluate `A ?? B`:

* If `A` is `null`, return `B`

* Otherwise return `A`

=== Assignment Operator - `A = B`

Assign the value of `B` to `A`. `A` must be an _lvalue_, otherwise, a
`PropertyNotWritableException` will be thrown.

The assignment operator is right-associative. For instance, `A=B=C` is
the same as `A=(B=C)`.

To evaluate `expr-a` = `expr-b`:

* Evaluate `expr-a`, up to the last property resolution, to (`base-a`,
`prop-a`)

* If `base-a` is `null`, and `prop-a` is a `String`:

** If `prop-a` is a Lambda parameter, throw a
`PropertyNotWritableException`

** If prop-a is an EL variable (see <<Variables>>),
evaluate the `ValueExpression` the variable was set to, to obtain the
new (`base-a`, `prop-a`)

* Evaluate `expr-b`, to `value-b`

* Invoke `ELResolver.setValue(base-a, prop-a, value-b)`

* Return `value-b`

The behavior of the assignment operator is determined by the
`ELResolver`. For instance, in a stand-alone environment, the class
`StandardELContext` contains a default `ELResolver` that allows the
assignment of an expression to a non-existing name, resulting in the
creation of a bean with the given name in the local bean repository. A
JSP container may use the `ScopeAttributeELResolver` to assign values
to scope attributes, or to create attributes in the page scope.

=== Semicolon Operator - `A ; B`

The semicolon operator behaves like the comma operator in C.

To evaluate `A;B`, `A` is first evaluated, and its value is discarded.
`B` is then evaluated and its value is returned.

=== Parentheses

Parentheses can be used to change precedence,
as in: `${(a*(b+c))}`

=== Operator Precedence

Highest to lowest, left-to-right.

* `[] .`

* `()`

* `-` (unary) `not ! empty`

* `* / div % mod`

* `+ -` (binary)

* `+=`

* `< > \<= >= lt gt le ge`

* `== != eq ne`

* `&& and`

* `|| or`

* `? :`

* `\->` (Lambda Expression)

* `=`

* `;`

Qualified functions with a namespace prefix have precedence over the
operators. Thus the expression `${c?b:f()}` is illegal because `b:f()`
is being parsed as a qualified function instead of part of a
conditional expression. As usual, `()` can be used to make
the precedence explicit, e.g `${c?b:(f())}`.

The symbol `\->` in a Lambda Expression behaves like an operator for
the purpose of ordering the operator precedence, and it has a higher
precedence than the assignment and semicolon operators. The following
examples illustrates when `()` is and is not needed.

* `v = x\->x+1`

* `x\-> (a=x)`

* `x\-> c?x+1:x+2`

All operators are left associative except for the `?:`, `=`, and `\->`
operators, which are right associative. For instance, `a=b=c` is the
parsed as `a=(b=c)`, and `x\->y\->x+y` is parsed as `x\->(y\->x+y)`.

=== Reserved Words

The following words are reserved for the
language and must not be used as identifiers.

[cols=5]
|===
|and
|eq
|gt
|true
|instanceof
|or
|ne
|le
|false
|empty
|not
|lt
|ge
|null
|div
|mod||||
|===

Note that many of these words are not in the
language now, but they may be in the future, so developers must avoid
using these words.

=== Functions

The EL has qualified functions, reusing the
notion of qualification from XML namespaces (and attributes), XSL
functions, and JSP custom actions. Functions are mapped to public static
methods in Java classes.

The full syntax is that of qualified n-ary
functions:

[subs="normal"]
----
 [ns:]f([a~1~[,a~2~[,...[,a~n~]]]])
----

Where `ns` is the namespace prefix, `f` is
the name of the function, and `a` is an argument.

EL functions are mapped, resolved and bound
at parse time. It is the responsibility of the `FunctionMapper` class to
provide the mapping of namespace-qualified functions to static methods
of specific classes when expressions are created. If no `FunctionMapper`
is provided (by passing in `null`), functions are disabled.

=== Variables

Just like `FunctionMapper` provides a
flexible mechanism to add functions to the EL, `VariableMapper` provides
a flexible mechanism to support the notion of EL variables. An EL
variable does not directly refer to a model object that can then be
resolved by an `ELResolver`. Instead, an EL variable refers to an EL
expression. The evaluation of that EL expression yields the value
associated with the EL variable.

EL variables are mapped, resolved and bound
at parse time. It is the responsibility of the `VariableMapper` class to
provide the mapping of EL variables to ``ValueExpression``s when
expressions are created. If no `VariableMapper` is provided (by passing
in `null`), variable mapping is disabled.

See the `jakarta.el` package description for more details.

=== Lambda Expressions

A lambda expression is a `ValueExpression` with
parameters. The syntax is similar to the lambda expression in the Java
Language, except that in EL, the body of the lambda expression is an EL
expression. These are some examples:

* `x\->x+1`

* `(x,y)\->x+y`

* `()\->64`

The identifiers to the left of `\->` are lambda
parameters. The parenthesis is optional if and only if there is one
parameter.

A lambda expression behaves like a function.
It can be invoked immediately:

* `((x,y)\->x+y)(3,4)` evaluates to `7`

When a lambda expression is assigned, it can
be referenced and invoked indirectly:

* `v = (x,y)\->x+y; v(3,4)` evaluates to `7`

* `fact = n \-> n==0? 1: n*fact(n-1); fact(5)` evaluates to `120`

It can also be passed as an argument to a
method, and be invoked in the method, by invoking
`jakarta.el.LambdaExpression.invoke()`, such as:

* `employees.where(e\->e.firstName == 'Bob')`

When a lambda expression is invoked, the
expression in the body is evaluated, with its formal parameters replaced
by the arguments supplied at the invocation. The number of arguments
must be equal to or more than the number the formal parameters. Any
extra arguments are ignored.

A lambda expression can be nested within another lambda expression, like:

* `customers.select(c\->[c.name, c.orders.sum(o\->o.total)])`

The scope of a lambda argument is the body of
the lambda expression. A lambda argument hides other EL variables,
identifiers or arguments of the nesting lambda expressions, of the same
name.

Note that in the case of nested lambda
expressions where the body of the inner lambda expression contains
references to parameters of outer lambda expressions, such as:

* `x\->y\->x+y`

the scope of the outer lambda parameters
extends to cover the inner body. For instance, with the above example,
the argument `x` must be in scope when `x+y` is evaluated, even though the
body of the outer lambda expression has already been executed.

=== Enums

The Unified EL supports Java enumerated
types. Coercion rules for dealing with enumerated types are included in
the following section. Also, when referring to values that are instances
of an enumerated type from within an EL expression, use the literal
string value to cause coercion to happen via the below rules. For
example, let’s say we have an enum called `Suit` that has members `Heart`,
`Diamond`, `Club`, and `Spade`. Furthermore, let’s say we have a reference in
the EL, `mySuit`, that is a `Spade`. If you want to test for equality with
the `Spade` enum, you would say `${mySuit == 'Spade'}`. The type of the
`mySuit` will trigger the invocation of `Enum.valueOf(Suit.class, 'Spade')`.


=== Arrays

In addition to accessing array elements by index, the unified EL also supports
the property `length` for arrays which will return the length of the array as an
integer.


=== `java.util.Optional`

Note: Unified EL implementations have their own `Optional` class (see
<<Optional>>) that is separate to `java.util.Optional` and the behaviour
described in this section.

By default, instances of `java.util.Optional` are treated by the unified EL in 
an identical manner to any other Java object. This means, for example, that
resolving the expression `someObject.optionalProperty.otherProperty` would
result in a `PropertyNotFoundException` if `optionalProperty` was an instance of
`java.util.Optional`.

The EL API provides an additional `ELResolver` implementation,
`OptionalELResolver` that provides alternative handling for
`java.util.Optional`. With this resolver added to the set of standard resolvers,
the expression `someObject.optionalProperty.otherProperty` would result in
`null` if the `optionalProperty` was an empty `java.util.Optional` and the value
of `otherProperty` if the `java.util.Optional` was populated with an object that
had a property named `otherProperty`. The behaviour of the `OptionalELResolver`
is fully documented in its Javadoc. 

  
=== Static Field and Method Reference

A static field or static method of a Java
class can be referenced with the syntax _classname.field_, such as:

* `Boolean.TRUE`

the classname is the name of a class, without
the package name.

An enum constant is a public static field, so
the same syntax can be used to refer to an enum constant, like the
following:

* `RoundingMode.FLOOR`

==== Access Restrictions and Imports

For security, the following restrictions are
enforced.

. Only the public static fields and methods can be referenced.

. The fields and/or methods must be in an accessible module.

. Static fields cannot be modified.

. Except for classes with `java.lang.*` package
names, a class has to be explicitly imported before its static fields or
methods can be referenced.

==== Imports of Packages, Classes, and Static Fields

Either a class or a package can be explicitly
imported into the EL evaluation environment. Importing a package imports
all the public, concrete classes in the package. The classes that can be
imported are restricted to the classes that can be loaded by the current
class loader.

By default, the following packages are
imported by the EL environment:

* `java.lang.*`

A static field can also be imported
statically. A statically imported static field can be referenced by the
field name, without the class name.

The imports of packages, classes, and static
fields are handled by the `ImportHandler` in the `ELContext`.

==== Constructor Reference

A class name reference, followed by arguments in parenthesis, such as:

* `Boolean(true)`

denotes the invocation of the constructor of
the class with the supplied arguments. The same restrictions (the class
must be public and has already been imported) for static methods apply
to the constructor calls.

==== Additional Default Imports

When EL is used in conjunction with another specification, for example Jakarta
Server Pages, that specification may define further packages, in addition to
`java.lang.*`, that are imported by default into the EL environment.

=== Type Conversion

Every expression is evaluated in the context
of an expected type. The result of the expression evaluation may not
match the expected type exactly, so the rules described in the following
sections are applied.

Custom type conversions can be specified in
an `ELResolver` by implementing the method `convertToType`. More than one
`ELResolver` can be specified for performing custom conversions, and they
are selected and applied in the order of their positions in the
`ELResolver` chain, as usual.

During expression evaluations, the custom
type converters are first selected and applied. If there is no custom
type converter for the conversion, the default conversions specified in
the following sections are used.

==== To Coerce a Value `X` to Type `Y`

* If `X` is `null` and `Y` is not a primitive type and also not a `String`,
return `null`

* If `X` is of a primitive type, Let `X’` be the equivalent "boxed form"
of `X` +
Otherwise, Let `X’` be the same as `X`

* If `Y` is of a primitive type, Let `Y’` be the equivalent "boxed form"
of `Y` +
Otherwise, Let `Y’` be the same as `Y`

* Apply the rules in Sections
<<Coerce `A` to `String`>> to <<Coerce `A` to Any Other Type `T`>>
for coercing `X’` to `Y’`

* If `Y` is a primitive type, then the result
is found by "unboxing" the result of the coercion. If the result of the
coercion is `null`, then error

* If `Y` is not a primitive type, then the
result is the result of the coercion

For example, if coercing an `int` to a `String`, "box" the `int` into an
`Integer` and apply the rule for coercing an `Integer` to a `String`. Or
if coercing a `String` to a `double`, apply the rule for coercing a
`String` to a `Double`, then "unbox" the resulting `Double`, making sure
the resulting `Double` isn’t actually `null`.

==== Coerce `A` to `String`

* If `A` is `null` , return `""`

* Otherwise, if `A` is `String`, return `A`

* Otherwise, if `A` is `Enum`, return `A.name()`

* Otherwise, if `A.toString()` throws an exception, error

* Otherwise, return `A.toString()`

==== Coerce `A` to `Number` type `N`

* If `A` is `null` and `N` is not a primitive type, return `null`

* If `A` is `null` or `""`, return `0`

* If `A` is `Character`, convert `A` to
`new Short\((short)a.charValue())`, and apply the following rules

* If `A` is `Boolean`, then error

* If `A` is `Number` type `N`, return `A`

* If `A` is `Number`, coerce quietly to type
`N` using the following algorithm:

** If `N` is `BigInteger`:

*** If `A` is a `BigDecimal`, return `A.toBigInteger()`

*** Otherwise, return `BigInteger.valueOf(A.longValue())`

** If `N` is `BigDecimal`:

*** If `A` is a `BigInteger`, return `new BigDecimal(A)`

*** Otherwise, return `new BigDecimal(A.doubleValue())`

** If `N` is `Byte`, return `new Byte(A.byteValue())`

** If `N` is `Short`, return `new Short(A.shortValue())`

** If `N` is `Integer`, return `new Integer(A.intValue())`

** If `N` is `Long`, return `new Long(A.longValue())`

** If `N` is `Float`, return `new Float(A.floatValue())`

** If `N` is `Double`, return `new Double(A.doubleValue())`

** Otherwise, error

* If `A` is `String`, then:

** If `N` is `BigDecimal` then:

*** If `new BigDecimal(A)` throws an exception then error

*** Otherwise, return `new BigDecimal(A)`

** If `N` is `BigInteger` then:

*** If `new BigInteger(A)` throws an exception then error

*** Otherwise, return `new BigInteger(A)`

** If `N.valueOf(A)` throws an exception, then error

** Otherwise, return `N.valueOf(A)`

* Otherwise, error

==== Coerce `A` to `Character` or `char`

* If `A` is `null` and the target type is not the primitive type `char`,
return `null`

* If `A` is `null` or `""`, return `(char)0`

* If `A` is `Character`, return `A`

* If `A` is `Boolean`, error

* If `A` is `Number`, coerce quietly to type
`Short`, then return a `Character` whose numeric value is equivalent to
that of a `Short`

* If `A` is `String`, return `A.charAt(0)`

* Otherwise, error

==== Coerce `A` to `Boolean` or `boolean`

* If `A` is `null` and the target type is not the
primitive type `boolean`, return `null`

* If `A` is `null` or `""`, return `false`

* Otherwise, if `A` is a `Boolean`, return `A`

* Otherwise, if `A` is a `String`, and
`Boolean.valueOf(A)` does not throw an exception, return it

* Otherwise, error

====  Coerce `A` to an `Enum` Type `T`

* If `A` is `null`, return `null`

* If `A` is assignable to `T`, coerce quietly

* If `A` is `""`, return `null`

* If `A` is a `String` call `Enum.valueOf(T.getClass(), A)` and return
the result

* Otherwise, error

==== Coerce `A` to an array of Type `T`

* If `A` is `null`, return `null`

* If `A` is assignable to an array of type `T`, coerce quietly

* If `A` is an array, return a copy of the array where each element has been coerced to type `T` using these rules

* Otherwise, error

Note: If coercion of any element in the array fails, the coercion of the array fails

==== Coerce `A` to functional interface method invocation

 * If `A` is a `LambdaExpression` then:
 
 ** Return the result of invoking the `LambdaExpression` with the parameters
    (coerced if necessary) that were passed to the Functional Interface method
    invocation
 
 * Otherwise, apply the rules in <<Coerce `A` to Any Other Type `T`>>

Note: A Type is only considered to be a functional interface it it is annotated
with `java.lang.FunctionalInterface`.

==== Coerce `A` to Any Other Type `T`

* If `A` is `null`, return `null`

* If `A` is assignable to `T`, coerce quietly

* If `A` is a `String`, and `T` has no `PropertyEditor:`

** If `A` is `""`, return `null`

** Otherwise error

* If `A` is a `String` and ``T``'s `PropertyEditor` throws an exception:

** If `A` is `""`, return `null`

** Otherwise, error

* Otherwise, apply ``T``'s `PropertyEditor`

* Otherwise, error


=== Collected Syntax

The following is a javaCC grammar with syntax
tree generation. It is meant to be used as a guide and reference only.

[source]
----
/* == Option Declaration == */
options
{
    STATIC=false;
    NODE_PREFIX="Ast";
    VISITOR_EXCEPTION="ELException";
    VISITOR=false;
    MULTI=true;
    NODE_DEFAULT_VOID=true;
    JAVA_UNICODE_ESCAPE=false;
    UNICODE_INPUT=true;
    BUILD_NODE_FILES=true;
}

/* == Parser Declaration == */
PARSER_BEGIN( ELParser )
package com.sun.el.parser;
import java.io.StringReader;
import ELException;
public class ELParser
{
    public static Node parse(String ref) throws ELException
    {
        try {
            return (new ELParser(new StringReader(ref))).CompositeExpression();
        } catch (ParseException pe) {
            throw new ELException(pe.getMessage());
        }
    }
}
PARSER_END( ELParser )

/*
 * CompositeExpression
 * Allow most flexible parsing, restrict by examining
 * type of returned node
 */
AstCompositeExpression CompositeExpression() #CompositeExpression : {}
{
    (DeferredExpression() | DynamicExpression() | LiteralExpression())* <EOF>
    {
        return jjtThis;
    }
}

/*
 * LiteralExpression
 * Non-EL Expression blocks
 */
void LiteralExpression() #LiteralExpression : { Token t = null; }
{
    t=<LITERAL_EXPRESSION> { jjtThis.setImage(t.image); }
}

/*
 * DeferredExpression
 * #{...} Expressions
 */
void DeferredExpression() #DeferredExpression : {}
{
    <START_DEFERRED_EXPRESSION> Expression() <RCURL> 
}

/*
 * DynamicExpression
 * ${...} Expressions
 */
void DynamicExpression() #DynamicExpression : {}
{
    <START_DYNAMIC_EXPRESSION> Expression() <RCURL> 
}

/*
 * Expression
 * EL Expression Language Root
 */
void Expression() : {}
{
        SemiColon()
}

/*
 * SemiColon
 */
void SemiColon() : {}
{
        Assignment() (<SEMICOLON> Assignment() #SemiColon(2) )*
}

/*
 * Assignment
 * For '=', right associatve, then LambdaExpression or Choice or Assignment
 */
void Assignment() : {}
{
        LOOKAHEAD(4) LambdaExpression() |
        Ternary() ( LOOKAHEAD(2) <ASSIGN> Assignment() #Assign(2) )*
}

/*
 * LambdaExpression
 */
void LambdaExpression() #LambdaExpression : {}
{
       LambdaParameters() <ARROW>
       (LOOKAHEAD(3) LambdaExpression() | Ternary() )
}

void LambdaParameters() #LambdaParameters: {}
{
       Identifier()
       | <LPAREN>
            (Identifier() (<COMMA> Identifier())*)?
         <RPAREN>
}

/*
 * Ternary
 * For '??' '?:' '? :' then Or
 */
void Ternary() : {}
{
    Or()
    (
        LOOKAHEAD(2) (<QUESTIONMARK><QUESTIONMARK> Ternary() #NullCoalescing(2))
    |
        LOOKAHEAD(2) (<QUESTIONMARK><COLON> Ternary() #Elvis(2))
    |
        (<QUESTIONMARK> Ternary() <COLON> Ternary() #Choice(3))
    )?
}

/*
 * Or
 * For 'or' '||', then And
 */
void Or() : {}
{
    And() ((<OR0>|<OR1>) And() #Or(2))*
}

/*
 * And
 * For 'and' '&&', then Equality
 */
void And() : {}
{
    Equality() ((<AND0>|<AND1>) Equality() #And(2))*
}

/*
 * Equality
 * For '==' 'eq' '!=' 'ne', then Compare
 */
void Equality() : {}
{
    Compare()
    (
        ((<EQ0>|<EQ1>) Compare() #Equal(2))
    |
        ((<NE0>|<NE1>) Compare() #NotEqual(2))
    )*
}

/*
 * Compare
 * For a bunch of them, then Math
 */
void Compare() : {}
{
    Concatenation()
    (
        ((<LT0>|<LT1>) Concatenation() #LessThan(2))
    |
        ((<GT0>|<GT1>) Concatenation() #GreaterThan(2))
    |
        ((<LE0>|<LE1>) Concatenation() #LessThanEqual(2))
    |
        ((<GE0>|<GE1>) Concatenation() #GreaterThanEqual(2))
    )*
}

/*
 * Concatenation
 * For '&', then Math()
 */
void Concatenation() : {}
{
        Math() ( <CONCAT> Math() #Concat(2) )*
}

/*
 * Math
 * For '+' '-', then Multiplication
 */
void Math() : {}
{
    Multiplication()
    (
        (<PLUS> Multiplication() #Plus(2))
    |
        (<MINUS> Multiplication() #Minus(2))
    )*
}

/*
 * Multiplication
 * For a bunch of them, then Unary
 */
void Multiplication() : {}
{
    Unary()
    (
        (<MULT> Unary() #Mult(2))
    |
        ((<DIV0>|<DIV1>) Unary() #Div(2))
    |
        ((<MOD0>|<MOD1>) Unary() #Mod(2))   
    )*  
}

/*
 * Unary
 * For '-' '!' 'not' 'empty', then Value
 */
void Unary() : {}
{
        <MINUS> Unary() #Negative 
    |
        (<NOT0>|<NOT1>) Unary() #Not 
    |
        <EMPTY> Unary() #Empty
    |   
        Value()
}

/*
 * Value
 * Defines Prefix plus zero or more Suffixes
 */
void Value() : {}
{
    (ValuePrefix() (ValueSuffix())*) #Value(>1)
}

/*
 * ValuePrefix
 * For Literals, Variables, and Functions
 */
void ValuePrefix() : {}
{
    Literal() | NonLiteral()
}

/*
 * ValueSuffix
 * Either dot or bracket notation
 */
void ValueSuffix() : {}
{
    DotSuffix() | BracketSuffix()
}

/*
 * DotSuffix
 * Dot Property and Dot Method
 */
void DotSuffix() #DotSuffix : {  Token t = null; }
{
        <DOT> t=<IDENTIFIER> { jjtThis.setImage(t.image); }
        (MethodArguments())?
}

/*
 * BracketSuffix
 * Sub Expression Suffix
 */
void BracketSuffix() #BracketSuffix : {}
{
    <LBRACK> Expression() <RBRACK>
        (MethodArguments())?
}


/*
 * MethodArguments
 */
void MethodArguments() #MethodArguments : {}
{
        <LPAREN> (Expression() (<COMMA> Expression())*)? <RPAREN>
}

/*
 * Parenthesized Lambda Expression, with optional invokation
 */
void LambdaExpressionOrCall() #LambdaExpression : {}

{
    <LPAREN>
        LambdaParameters() <ARROW>
        (LOOKAHEAD(3) LambdaExpression() | Ternary() )
    <RPAREN>
    (MethodArguments())*
}

/*
 * NonLiteral
 * For Grouped Operations, Identifiers, and Functions
 */
void NonLiteral() : {}
{
        LOOKAHEAD(5) LambdaExpressionOrCall() // check beyond the arrow 
        | <LPAREN> Expression() <RPAREN>
        | LOOKAHEAD(4) Function()
        | Identifier()
        | MapData()
        | ListData()
}

void MapData() #MapData: {}
{
    <START_MAP>
        ( MapEntry() ( <COMMA> MapEntry() )* )?
    <RCURL>
}

void MapEntry() #MapEntry: {}
{
    Expression() (<COLON> Expression())?
}

void ListData() #ListData: {}
{
    <LBRACK>
        ( Expression() ( <COMMA> Expression() )* )?
    <RBRACK>
}

/*
 * Identifier
 * Java Language Identifier
 */
void Identifier() #Identifier : { Token t = null; }
{
    t=<IDENTIFIER> { jjtThis.setImage(t.image); }
}

/*
 * Function
 * Namespace:Name(a,b,c)
 */
void Function() #Function :
{
    Token t0 = null;
    Token t1 = null;
}
{
    t0=<IDENTIFIER> (<COLON> t1=<IDENTIFIER>)?
    {
        if (t1 != null) {
            jjtThis.setPrefix(t0.image);
            jjtThis.setLocalName(t1.image);
        } else {
            jjtThis.setLocalName(t0.image);
        }
    }
        (MethodArguments())+
}


/*
 * Literal
 * Reserved Keywords
 */
void Literal() : {}
{
    Boolean()
    | FloatingPoint()
    | Integer()
    | String()
    | Null()
}

/*
 * Boolean
 * For 'true' 'false'
 */
void Boolean() : {}
{
    <TRUE> #True
    | <FALSE> #False
}

/*
 * FloatingPoint
 * For Decimal and Floating Point Literals
 */
void FloatingPoint() #FloatingPoint : { Token t = null; }
{
    t=<FLOATING_POINT_LITERAL> { jjtThis.setImage(t.image); }
}

/*
 * Integer
 * For Simple Numeric Literals
 */
void Integer() #Integer : { Token t = null; }
{
    t=<INTEGER_LITERAL> { jjtThis.setImage(t.image); }
}

/*
 * String
 * For Quoted Literals
 */
void String() #String : { Token t = null; }
{
    t=<STRING_LITERAL> { jjtThis.setImage(t.image); }
}

/*
 * Null
 * For 'null'
 */
void Null() #Null : {}
{
    <NULL>
}


/* ========================================================================== */
TOKEN_MGR_DECLS:
{
    java.util.Stack<Integer> stack = new java.util.Stack<Integer>();
}

<DEFAULT> TOKEN :
{
  < LITERAL_EXPRESSION:
    ((~["\\", "$", "#"])
      | ("\\" ("\\" | "$" | "#"))
      | ("$" ~["{", "$", "#"])
      | ("#" ~["{", "$", "#"])
    )+
    | "$"
    | "#"
  >
|
  < START_DYNAMIC_EXPRESSION: "${" > {stack.push(DEFAULT);}: IN_EXPRESSION
|
  < START_DEFERRED_EXPRESSION: "#{" > {stack.push(DEFAULT);}: IN_EXPRESSION
}

<DEFAULT> SKIP : { "\\" }

<IN_EXPRESSION, IN_MAP> SKIP:
{ " " | "\t" | "\n" | "\r" }

<IN_EXPRESSION, IN_MAP> TOKEN :
{
    < START_MAP : "{" > {stack.push(curLexState);}: IN_MAP
|   < RCURL: "}" > {SwitchTo(stack.pop());}
|   < INTEGER_LITERAL: ["0"-"9"] (["0"-"9"])* >
|   < FLOATING_POINT_LITERAL: (["0"-"9"])+ "." (["0"-"9"])* (<EXPONENT>)? 
        | "." (["0"-"9"])+ (<EXPONENT>)?
        | (["0"-"9"])+ <EXPONENT>
    >
|   < #EXPONENT: ["e","E"] (["+","-"])? (["0"-"9"])+ >
|   < STRING_LITERAL: ("\"" ((~["\"","\\"])
        | ("\\" ( ["\\","\""] )))* "\"")
        | ("\'" ((~["\'","\\"])
        | ("\\" ( ["\\","\'"] )))* "\'")
    >
|   < BADLY_ESCAPED_STRING_LITERAL: ("\"" (~["\"","\\"])* ("\\" ( ~["\\","\""] )))
        | ("\'" (~["\'","\\"])* ("\\" ( ~["\\","\'"] )))
    >
|   < TRUE : "true" >
|   < FALSE : "false" >
|   < NULL : "null" >
|   < DOT : "." >
|   < LPAREN : "(" >
|   < RPAREN : ")" >
|   < LBRACK : "[" >
|   < RBRACK : "]" >
|   < COLON : ":" >
|   < COMMA : "," >
|   < SEMICOLON : ";" >
|   < GT0 : ">" >
|   < GT1 : "gt" >
|   < LT0 : "<" >
|   < LT1 : "lt" >
|   < GE0 : ">=" >
|   < GE1 : "ge" >
|   < LE0 : "\<=" >
|   < LE1 : "le" >
|   < EQ0 : "==" >
|   < EQ1 : "eq" >
|   < NE0 : "!=" >
|   < NE1 : "ne" >
|   < NOT0 : "!" >
|   < NOT1 : "not" >
|   < AND0 : "&&" >
|   < AND1 : "and" >
|   < OR0 : "||" >
|   < OR1 : "or" >
|   < EMPTY : "empty" >
|   < INSTANCEOF : "instanceof" >
|   < MULT : "*" >
|   < PLUS : "+" >
|   < MINUS : "-" >
|   < QUESTIONMARK : "?" >
|   < DIV0 : "/" >
|   < DIV1 : "div" >
|   < MOD0 : "%" >
|   < MOD1 : "mod" >
|   < CONCAT : "+=" >
|   < ASSIGN : "=" >
|   < ARROW : "->" >
|   < IDENTIFIER : (<LETTER>|<IMPL_OBJ_START>) (<LETTER>|<DIGIT>)* >
|   < #IMPL_OBJ_START: "#" >
|   < #LETTER:
        [
        "\u0024",
        "\u0041"-"\u005a",
        "\u005f",
        "\u0061"-"\u007a",
        "\u00c0"-"\u00d6",
        "\u00d8"-"\u00f6",
        "\u00f8"-"\u00ff",
        "\u0100"-"\u1fff",
        "\u3040"-"\u318f",
        "\u3300"-"\u337f",
        "\u3400"-"\u3d2d",
        "\u4e00"-"\u9fff",
        "\uf900"-"\ufaff"
        ]
    >
|   < #DIGIT:
        [
        "\u0030"-"\u0039",
        "\u0660"-"\u0669",
        "\u06f0"-"\u06f9",
        "\u0966"-"\u096f",
        "\u09e6"-"\u09ef",
        "\u0a66"-"\u0a6f",
        "\u0ae6"-"\u0aef",
        "\u0b66"-"\u0b6f",
        "\u0be7"-"\u0bef",
        "\u0c66"-"\u0c6f",
        "\u0ce6"-"\u0cef",
        "\u0d66"-"\u0d6f",
        "\u0e50"-"\u0e59",
        "\u0ed0"-"\u0ed9",
        "\u1040"-"\u1049"
        ]
    >
|   < ILLEGAL_CHARACTER: (~[]) >
}
----

Notes

* `*` = 0 or more, `+` = 1 or more, `?` = 0 or 1

* An identifier is constrained to be a Java identifier - e.g., no `-`,
no `/`, etc.

* A `String` only recognizes a limited set of escape sequences, and `\`
may not appear unescaped

* The relational operator for equality is `==` (double equals)

* The value of an `IntegerLiteral` ranges from `Long.MIN_VALUE` to
`Long.MAX_VALUE`

* The value of a `FloatingPointLiteral` ranges from
`Double.MIN_VALUE` to `Double.MAX_VALUE`

* It is illegal to nest `${` or `\#{` inside
an outer `${` or `#{`

== Operations on Collection Objects

This chapter describes how collection objects
and literals can be constructed in the EL expression, and how collection
objects can be manipulated and processed by applying operations in a
pipeline.

=== Overview

To provide support for collection objects, EL
includes syntaxes for constructing sets, lists, and maps dynamically.
Any EL expressions, not just literals, can be used in the construction.

EL also includes a set of operations that can
be applied on collections. By design, the methods supporting these
operations have names and semantics very similar to those in Java SE 8
libraries. Since EL and Java have different syntaxes and capabilities,
they are not identical, but they are similar enough that users should
have no problem switching from one to the other.

Since the methods supporting the collection
operations did not exist in Java SE 7, they were implemented in the
Expression Language with ``ELResolver``s. In an EL expression, collection
operations are carried out by invoking methods, and no special syntaxes
are introduced for them. Strictly speaking, these operations are not
part of the expression language, and can be taken as examples of what
can be achieved with the expression language. The specification
specifies the syntaxes and behaviors of a standard set of collection
operations. However, an user can easily add, extend and modify the
behavior of the operations by providing customized ``ELResolver``s.

Compared to Java SE 8, the collection support
in EL has a much smaller and simpler scope. Although EL does not
disallow collections of infinite size, it works best when the collection
objects are created in memory, with known sizes. It also does not
address the performance issue in a multi-threaded environment, and does
not provide explicit controls for evaluating collection operations in
parallel. A future version of EL will likely include functionalities
from Java SE 8.

Central to the implementation is the use of
lambda expressions, now supported in EL. A lambda expression in the Java
language is used to specify a method in an anonymous implementation of a
functional interface. The concept of a lambda expression in EL is much
simpler: it is just an anonymous function that can be passed as an
argument to a method, to be evaluated in the method when needed. In the
collection operations, lambda expressions are specified as arguments to
the methods supporting the operations. Usually when the lambda
expressions are invoked, an element from the stream of the collection is
passed as an argument to the lambda expression. For instance, the
argument to the `filter` method is a lambda expression which acts as a
predicate function to determine if an element should be included in the
resulting stream.

=== Construction of Collection Objects

EL allows the construction of sets, lists,
and maps dynamically. Any EL expressions, including nested collection
constructions, can be used in the construction. These expressions are
evaluated at the time of the construction.

==== Set Construction

Construct an instance of `java.util.Set<Object>`.

===== Syntax

`SetData := '{' DataList '}'`

`DataList := (expression (',' expression)* )?`

===== Example

`{1, 2, 3}`

==== List Construction

Construct an instance of `java.util.List<Object>`.

===== Syntax

`ListData := '[' DataList ']'`

`DataList := (expression (',' expression)* )?`

===== Example

`[1, "two", [foo, bar]]`

==== Map Construction

Construct an instance of `java.util.Map<Object,Object>`.

===== Syntax

`Map := '{' MapEntries '}'`

`MapEntries := (MapEntry (',' MapEntry)* )?`

`MapEntry := expression ':' expression`

===== Example

`{"one":1, "two":2, "three":3}`

=== Collection Operations

==== Streams and Pipelines

The operations on a collection object are
realized as method calls to the stream of elements derived from the
collection. The method `stream` can be used to obtain a `Stream` from a
`java.util.Collection` or a Java array.

To obtain a `Stream` from a `Map`, the collection view of a `Map`,
such as `MapEntry` can be used as the source of `Stream`.

Some operations return another `Stream`, which
allows other operations. Therefore the operations can be chained
together to form a pipeline. For example, to get a list of titles of
history books, one can write in EL:

----
books.stream().filter(b->b.category == 'history’)
              .map(b->b.title)
              .toList()
----

A stream pipeline consists of:

* the source;

* intermediate operations; and

* a terminal operation.

The source of a pipeline is the `Stream` object.

An intermediate operation is a method in
`Stream` that returns a `Stream`. A pipeline may contain zero or more
intermediate operations.

A pipeline ends in a terminal operation. A terminal operation is a
method in `Stream` that does not return a `Stream`.

The execution of a pipeline only begins when
the terminal operation starts its execution. Most of the intermediate
operations are evaluated lazily: they only yield as many elements in the
stream as are required by the downstream operations. Because of this,
they need not keep intermediate results of the operations. For instance,
the filter operation does not keep a collection of the filtered
elements.

A notable exception is the sorted operation,
since all elements are needed for sorting.

The specification specifies the behavior of
the operations in a pipeline, and does not specify the implementation of
a pipeline. The operations must not modify the source collection. The
user must also make sure that the source collection is not modified
externally during the execution of the pipeline, otherwise the behavior
of the collection operations will be undefined.

The behavior of the operations are undefined
if the collection contains null elements. Null elements in a collection
should be removed by a filter to obtain consistent results.

The source stream in a pipeline that has
already started its execution cannot be used in another pipeline,
otherwise the behavior is undefined.

==== Operation Syntax Description

The implementation of `Stream` that contains
the methods supporting the operations are not part of the API. The
syntax and the behavior of the operations are described in this chapter.

For documentation purposes, pseudo method
declarations are used in this chapter for the operations. A method
includes:

* The return type

* The type of the source stream

* The method name

* The method parameters

A typical method declaration would looks like:

* `returnT Stream<T>.method(T1 arg1, T2 arg2)`

Some methods have optional parameters. The
declarations of the methods with all possible combinations of the
parameters are listed in the syntax sections, as if they are overloaded.
Any `null` parameter will result in a `NullPointerException` at run-time.

Some of the parameters are lambda
expressions, also known as functions. A lambda expression can have its
own parameters and can return a value. To describe the parameter types
and the return type of a lambda expression, the following is an example
of the notation that is used:

* `(p1,p2)\->returnT`

For instance, the declaration for the operation filter is:

* `Stream<S> Stream<S>.filter((S\->boolean) predicate)`

From this we know that the source object is a `Stream` of `S`, and the
return object is also a `Stream`, of the same type.
The operator takes a predicate function (lambda expression) as an
argument. The argument of the function is an element of the source, and
the function returns a boolean.

The generic types in the declaration are used
only to help the readers to identify the type relationships among
various parts of the declaration, and do not have the same meaning as
used in the Java language. At runtime, EL deals with Objects, and does
not track generic types.

==== Implementation Classes

The specification makes references to some
implementation classes that are not in the API. They contains methods
whose behaviors are specified in this section.

===== Stream

An instance of `Stream` is obtained by calling the method `stream()` of
a `java.util.Collection` object or a Java array.
The methods in this class support the stream operations and are
described in <<filter,2.3.5>> to <<findFirst,2.3.26>>.

===== Optional

An `Optional` is used to represent a value that
may not exist. Instead of using `null` as a default value, the use of
`Optional` allows the user to specify a default.

A non-existing or empty value is represented
by an empty `Optional`.

An `Optional` is usually the result of a computation over the elements
of a `Stream`, where an empty `Stream` results
in an empty `Optional`. See for example, <<max>>.

The following are methods in `Optional<T>`.

* `T get()` +
Returns the value held by the `Optional`, or
throws an `ELException` if the `Optional` is empty.

* `void ifPresent((x)\->void) consumer)` +
The value held by the `Optional` is processed
by the function consumer if it is not empty. See also
<<consumer>>.

* `T orElse(T other)` +
Returns the value held by the `Optional`, or
the value other if the `Optional` is empty.

* `T orElseGet((()\->T) other)` +
Returns the value held by the `Optional`, or the value returned by the
lambda expression `other` if the `Optional` is empty.

==== Functions

Some operations takes functions (lambda
expressions) as parameters. Again, we use the notation:

`(arg1Type, ...)\->returnType`

to describe the argument types and the return type of a function.

===== predicate

`S \-> boolean`

This function takes the input argument, usually the element of the
source stream, and determines if it satisfies some criteria.

===== mapper

`S \-> R`

This function maps, or transforms the input
argument, usually the element of the source stream, to the result.

===== comparator

`(S, S) \-> int`

This function compares two arguments, usually
the elements of the source stream, and returns a negative integer, zero,
or a positive integer, if the first argument is respectively less than,
equal to, or greater than the second argument.

===== consumer

`S \-> void`

This function processes the input argument,
usually the element of the source stream, and returns nothing.

===== binaryOperator

`(S, S) \-> S`

This function applies a binary operation to
the input arguments, and returns the result. The first argument is
usually an internal accumulator value, and the second argument is
usually the element of the source stream.

The arguments and the result are of the same
type.

==== filter

===== Syntax

`Stream<S> Stream<S>.filter((S\->boolean) predicate)`

===== Description

This method produces a stream containing the
source stream elements for which the `predicate` function returns `true`.
The argument of `predicate` function represents the element to test.

===== See

<<predicate>>

===== Example

To find the products whose price is greater than or equal to 10:

`products.stream().filter(p\->p.unitPrice >= 10).toList()`

==== map

===== Syntax

`Stream<R> Stream<S>.map((S\->R) mapper)`

===== Description

This method produces a stream by applying the
`mapper` function to the elements of the source stream. The argument of
`mapper` function represents the element to process, and the result of the
`mapper` function represents the element of the resulting `Stream`.

===== See

<<mapper>>

===== Examples

To get the list of the names of all products:

`products.stream().map(p\->p.name).toList()`

To create a list of product names and prices
for products with a price greater than or equal to 10:

----
products.stream().filter(p->p.unitPrice >= 10)
                 .map(p->[p.name, p.unitPrice])
                 .toList()
----

==== flatMap

===== Syntax

`Stream<R> Stream<S>.flatMap((S\->Stream<R>) mapper)`

===== Description

This method produces a stream by mapping each
of the source elements to another stream and then concatenating the
mapped streams. If the mapper function does not return a `Stream`, the
behavior is undefined.

===== See

<<mapper>>

===== Examples

To list all orders of US customers:
----
customers.stream().filter(c->c.country == 'USA')
                  .flatMap(c->c.orders.stream())
                  .toList()
----

To obtain an alphabetical list of characters used in a list
of words:
----
words.stream().flatMap(w->w.toCharArray().stream())
              .sorted()
              .distinct()
              .toList()
----

==== distinct

===== Syntax

`Stream<S> Stream<S>.distinct()`

===== Description

This method produces a stream containing the
elements of the source stream that are distinct, according to
`Object.equals`.

===== Example

To remove the duplicate element b:

`['a', 'b', 'b', 'c'].stream().distinct().toArray()`

==== sorted

===== Syntax

`Stream<S> Stream<S>.sorted()`

`Stream<S> Stream<S>.sorted(((p,q)\->int) comparator)`

===== Description

This method produces a stream containing the
elements of the source stream in sorted order. If no `comparator` is
specified, the elements are sorted in natural order. The behavior is
undefined if no `comparator` is specified, and the elements do not
implement `java.lang.Comparable`. If a `comparator` is specified, the
elements are sorted with the provided comparator.

The source collection is unaffected by this
operation.

===== See

<<comparator>>

===== Examples

To sort a list of integers

`[1,3,2,4].stream().sorted().toList()`

To sort a list of integers in reversed order

`[1,3,2,4].stream().sorted((i,j)\->j-i).List()`

To sort a list of words in the order of word
length; and then for words of the same length, in alphabetical order:
----
words.stream().sorted(
        (s,t)->(s.length()==t.length() ? s.compareTo(t)
                                       : s.length() - t.length()))
                .toList()
----

To sort the products by name:

`products.stream().sorted\((p,q)\->p.name.compareTo(q.name)).toList()`

Or by defining a comparing function, this can
be rewritten as:
----
comparing = map->(x,y)->map(x).compareTo(map(y));
products.stream().sorted(comparing(p->p.name)).toList()
----

==== forEach

===== Syntax

`Object stream<S>.forEach(((S)\->void)consumer)`

===== Description

This method invokes the `consumer` function for
each element in the source stream.

This method always returns `null`.

===== See

<<consumer>>

===== Example

To print a list of customer names:

`customers.stream().forEach(c\->printer.print(c.name))`

==== peek

===== Syntax

`Stream<S> Stream<S>.peek(((S)\->void)consumer)`

===== Description

This method produces a stream containing the
elements of the source stream, and invokes the `consumer` function for
each element in the stream. The primary purpose of this method is for
debugging, where one can take a peek at the elements in the stream at
the place where this method is inserted.

===== See

<<consumer>>

===== Example

To print a list of integers before and after a filter:
----
[1,2,3,4,5].stream().peek(i->print(i))
                    .filter(i-> i%2 == 0)
                    .peek(i->print(i))
                    .toList()
----

==== iterator

===== Syntax

`Iterator<S> Stream<S>.iterator()`

===== Description

This method returns an iterator for the
source stream, suitable for use in Java codes.

==== limit

===== Syntax

`Stream<S> Stream<S>.limit(Number count)`

===== Description

This method produces a stream containing the
first `count` number of elements of the source stream.

If `count` is greater than the number of source
elements, all the elements are included in the returned stream. If the
`count` is less than or equal to zero, an empty stream is returned.

===== Example

To list the 3 least expensive products:
----
products.stream().sorted(p->p.unitPrice)
                 .limit(3)
                 .toList()
----

==== substream

===== Syntax

`Stream<S> Stream<S>.substream(Number start)`

`Stream<S> Stream<S>.substream(Number start, Number end)`

===== Description

This method produces a stream containing the
source elements, skipping the first `start` elements, and including the
rest of the elements in the stream if `end` is not specified, or the next
`(end - start)` elements in the stream if end is specified.

If the elements in the source stream has
fewer than `start` elements, nothing is included. If `start` is less than or
equal to zero, no elements are skipped.

===== Example

The example

`[1,2,3,4,5].stream().substream(2,4).toArray()`

produces the array `[3,4]`.

==== toArray

===== Syntax

`S[] Stream<S>.toArray()`

===== Description

This method returns an array containing the
elements of the source stream.

==== toList

===== Syntax

`List Stream<S>.toList()`

===== Description

This method returns a List containing the
elements of the source stream.


==== reduce

===== Syntax

`Optional<S> Stream<S>.reduce(((S,S)\->S) binaryOperator)`

`S Stream<S>.reduce(S seed, ((S,S)\->S) binaryOperator))`

===== Description

The method with a `seed` value starts by
assigning the `seed` value to an internal accumulator. Then for each of
the elements in the source stream, the next accumulator value is
computed, by invoking the `binaryOperator` function, with the current
accumulator value as the first argument and the current element as the
second argument. The final accumulator value is returned.

The method without a `seed` value uses the
first element of the source elements as the `seed` value. If the source
stream is empty, an empty `Optional` is returned, otherwise an `Optional`
with the final accumulator value is returned.

===== See

<<Optional>>

<<binaryOperator>>

===== Example

To find tallest student in a class:

`students.stream().reduce((p,q)\->(p.height>q.height? p: q).get()`

==== max

===== Syntax

`Optional<S> Stream<S>.max()`

`Optional<S> Stream<S>.max(((p,q)\->int) comparator)`

===== Description

This method computes the maximum of the
elements in the source stream. If the `comparator` function is specified,
it is used for comparisons. If no `comparator` function is specified, the
elements themselves are compared, and must implement `Comparable`,
otherwise an `ELException` is thrown.

This method returns an empty `Optional` for an
empty stream.

===== See

<<comparator>>

===== Examples

To find tallest student in a class:

`students.stream().max((p,q)\->p.height-q.height)`

To find the maximum height of the students in a class:

`students.stream().map(s\->s.height).max()`

==== min

===== Syntax

`Optional<S> Stream<S>.min()`

`Optional<S> Stream<S>.min(((p,q)\->int) comparator)`

===== Description

This method computes the minimum of the
elements in the source stream. If the `comparator` function is specified,
it is used for comparisons. If no `comparator` function is specified, the
elements themselves are compared, and must implement `Comparable`,
otherwise an `ELException` is thrown.

This method returns an empty `Optional` for an
empty stream.

===== See

<<comparator>>

==== average

===== Syntax

`Optional<S> Stream<S>.average()`

===== Description

This method computes the average of all
elements in the source stream by first computes the sum of the elements
and then divides the sum by the number of elements. The elements are
coerced to Number types according to <<Coerce `A` to `Number` type `N`>>
during the computation.

This method returns an empty `Optional` for an
empty stream.

==== sum

===== Syntax

`Number Stream<S>.sum()`

===== Description

This method computes the sum of all elements
in the source stream. The elements are coerced to Number types according
to <<Coerce `A` to `Number` type `N`>> during
the computation.

This method returns zero for an empty stream.

==== count

===== Syntax

`Long Stream<S>.count()`

===== Description

This method returns the count of elements in
the source stream.

==== anyMatch

===== Syntax

`Optional<boolean> Stream<S>.anyMatch((S\->boolean) predicate)`

===== Description

This method returns an `Optional` of `true` if
any element in the source stream satisfies the test given by the
`predicate`. It returns an empty `Optional` if the stream is empty.

===== See

<<predicate>>

===== Example

To determine if the list of integers contains
any negative numbers:

`integers.stream().anyMatch(i\->i<0).orElse(false)`

Note the use of `orElse` to set a default value
for the empty list.

==== allMatch

===== Syntax

`Optional<boolean> Stream<S>.allMatch((S\->boolean) predicate)`

===== Description

This method returns an `Optional` of `true` if
all elements in the source stream satisfy the test given by the
`predicate`. It returns an empty `Optional` if the stream is empty.

===== See

<<predicate>>

==== noneMatch

===== Syntax

`Optional<boolean> Stream<S>.noneMatch((S\->boolean) predicate)`

===== Description

This method returns an `Optional` of `true` if
none of the elements in the source stream satisfies the test given by
the `predicate`. It returns an empty `Optional` if the stream is empty.

===== See

<<predicate>>


==== findFirst

===== Syntax

`Optional<S> Stream<S>.findFirst()`

===== Description

This method returns an `Optional` containing
the first element in the stream, or an empty `Optional` if the stream is
empty.

===== See

<<Optional>>


[appendix]
== Changes

This appendix lists the changes in the EL specification.
This appendix is non-normative.

=== Changes between 6.1 and 6.0

* https://github.com/jakartaee/expression-language/issues/211[#211]
  Provider a marker in the `ELContext` when resolving single, stand-alone
  identifiers that allows an `ELResolver` to optimise the resolution of the
  identifier. This change supports a performance improvement in
  `jakarta.servlet.jsp.el.ImportELResolver`.

* https://github.com/jakartaee/expression-language/issues/243[#243]
  Add support for the null coalescing operator (`??`) and the Elvis operator
  (`?:`).

* https://github.com/jakartaee/expression-language/issues/247[#247]
  Expand the definition of the concatenation operator (`+=`) to include
  collections.

* https://github.com/jakartaee/expression-language/issues/313[#313]
  Add support for inner classes when using the `ImportHandler` and clarify that
  the import handler expects canonical class names where full class names are
  required.

=== Changes between 6.0 and 5.0

* The EL API requires Java 17 as a minimum.

* Remove deprecated code.

* Update to reflect new GitHub organisation and project names.

* https://github.com/jakartaee/expression-language/issues/47[#47]
  Make the dependency on the `java.beans.\*` package (and hence the
  `java.desktop` module) optional. If the `java.beans.*` package is not
  available, an internal implementation is used to support the `BeanELResolver`
  that just provides getter/setter support as all other JavaBeans functionality
  requires classes from the `java.beans.*` package.

* https://github.com/jakartaee/expression-language/issues/175[#175]
  Add support for a new property, `length`, for arrays.

* https://github.com/jakartaee/expression-language/issues/176[#176]
  Add support for `java.util.Optional`. The support is not enabled by default.
  Users must add the new resolver `OptionalELResolver` to enable this support.

* https://github.com/jakartaee/expression-language/issues/188[#188]
  Allow for module visibility when accessing class methods.

* https://github.com/jakartaee/expression-language/issues/192[#192]
  Removal all references to the Java SecurityManager and associated APIs.

* https://github.com/jakartaee/expression-language/issues/201[#201]
  Add support for `java.lang.Record` instances. The support is enabled by
  default. Support is provided via the new `RecordELResolver`.

=== Changes between 5.0 and 4.0

* The EL API requires Java 11 as a minimum.

* https://github.com/jakartaee/expression-language/issues/43[#43]
  The `BeanELResolver` is updated to also consider default method
  implementations when looking for property getters, property setters and
  methods.

* https://github.com/jakartaee/expression-language/issues/45[#45]
  Add support for coercing a `LambdaExpression` instance to a functional
  interface method invocation

* https://github.com/jakartaee/expression-language/issues/47[#47]
  The `ELResolver` method `getFeatureDescriptors()` has been deprecated with
  removal planed for EL 6.0. This is to remove the dependency on the
  `java.desktop` module from the EL API.

* https://github.com/jakartaee/expression-language/issues/117[#117]
  Remove the deprecated, mis-spelt method
  `MethodExpression.isParmetersProvided()` from the API

* https://github.com/jakartaee/expression-language/issues/154[#154]
  Add the `MethodReference` class to provide access to details of the method to
  which a `MethodExpression` resolves, including any annotations present on the
  method.

* https://github.com/jakartaee/expression-language/issues/157[#157]
  Generics are now used where appropriate throughout the API

* https://github.com/jakartaee/expression-language/issues/159[#159]
  Clarify expected behaviour when matching expressions to methods based on
  parameter types

* https://github.com/jakartaee/expression-language/issues/167[#167]
  Add a default implementation that returns `null` for the `ELResolver` method
  `getFeatureDescriptors()` since it has been deprecated so that custom
  `ELResolver` implementations do not need to implement the method.

* https://github.com/jakartaee/expression-language/issues/168[#168]
  Explicitly document that `ELResolver.getType()` must return null if either the
  ELResolver or the resolved property is read-only. This changes the documented
  behaviour of the `StaticFieldELResolver`.

* Explicitly state that specifications that depend on this specification may
  define further packages, in additional to `java.lang.*` that are imported by
  default into the EL environment.

* https://github.com/jakartaee/expression-language/issues/172[#172]
  Add support for coercing arrays.

* Add a JPMS module descriptor that defines the module name as `jakarta.el`.

* https://github.com/jakartaee/expression-language/issues/175[#175]
  Add a special case for the property `length` to the `ArrayELResolver` that
  returns the length of the array.

* https://github.com/jakartaee/expression-language/issues/183[#183]
  Avoid a `NullPointerException` when passing `null` for varargs.

=== Changes between 4.0 and JSR 341

* The API has moved from the `javax.el` package to the `jakarta.el`
  package.