Expression (Java SE 17 & JDK 17)
Description: declaration: module: java.desktop, package: java.beans, class: Expression
Keywords: java.beans.Expression class

Module java.desktop
Package java.beans

# Class Expression

java.lang.Object
java.beans.Statement
java.beans.Expression

public class Expression extends Statement
An `Expression` object represents a primitive expression in which a single method is applied to a target and a set of arguments to return a result - as in `"a.getFoo()"`.

In addition to the properties of the super class, the `Expression` object provides a value which is the object returned when this expression is evaluated. The return value is typically not provided by the caller and is instead computed by dynamically finding the method and invoking it when the first call to `getValue` is made.

Since:
  1.4
See Also:
  - `getValue()`
  - `setValue(java.lang.Object)`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `Expression(Object value, Object target, String methodName, Object[] arguments)`
  Creates a new `Expression` object with the specified value for the specified target object to invoke the method specified by the name and by the array of arguments.
  `Expression(Object target, String methodName, Object[] arguments)`
  Creates a new `Expression` object for the specified target object to invoke the method specified by the name and by the array of arguments.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `execute()`
  The `execute` method finds a method whose name is the same as the `methodName` property, and invokes the method on the target.
  `Object`
  `getValue()`
  If the value property of this instance is not already set, this method dynamically finds the method with the specified methodName on this target with these arguments and calls it.
  `void`
  `setValue(Object value)`
  Sets the value of this expression to `value`.
  `String`
  `toString()`
  Prints the value of this expression using a Java-style syntax.

  ### Methods declared in class java.beans.Statement

  `getArguments, getMethodName, getTarget`

  ### Methods declared in class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- ## Constructor Details

  - ### Expression

    @ConstructorProperties({"target","methodName","arguments"}) public Expression(Object target, String methodName, Object[] arguments)
    Creates a new `Expression` object for the specified target object to invoke the method specified by the name and by the array of arguments.

    The `target` and the `methodName` values should not be `null`. Otherwise an attempt to execute this `Expression` will result in a `NullPointerException`. If the `arguments` value is `null`, an empty array is used as the value of the `arguments` property.

    Parameters:
      `target` - the target object of this expression
      `methodName` - the name of the method to invoke on the specified target
      `arguments` - the array of arguments to invoke the specified method
    See Also:
      - `getValue()`

  - ### Expression

    public Expression(Object value, Object target, String methodName, Object[] arguments)
    Creates a new `Expression` object with the specified value for the specified target object to invoke the method specified by the name and by the array of arguments. The `value` value is used as the value of the `value` property, so the `getValue()` method will return it without executing this `Expression`.

    The `target` and the `methodName` values should not be `null`. Otherwise an attempt to execute this `Expression` will result in a `NullPointerException`. If the `arguments` value is `null`, an empty array is used as the value of the `arguments` property.

    Parameters:
      `value` - the value of this expression
      `target` - the target object of this expression
      `methodName` - the name of the method to invoke on the specified target
      `arguments` - the array of arguments to invoke the specified method
    See Also:
      - `setValue(java.lang.Object)`

- ## Method Details

  - ### execute

    public void execute() throws Exception
    The `execute` method finds a method whose name is the same as the `methodName` property, and invokes the method on the target. When the target's class defines many methods with the given name the implementation should choose the most specific method using the algorithm specified in the Java Language Specification (15.11). The dynamic class of the target and arguments are used in place of the compile-time type information and, like the `Method` class itself, conversion between primitive values and their associated wrapper classes is handled internally.

    The following method types are handled as special cases:

    - Static methods may be called by using a class object as the target.
    - The reserved method name "new" may be used to call a class's constructor as if all classes defined static "new" methods. Constructor invocations are typically considered `Expression`s rather than `Statement`s as they return a value.
    - The method names "get" and "set" defined in the `List` interface may also be applied to array instances, mapping to the static methods of the same name in the `Array` class.

    If the invoked method completes normally, the value it returns is copied in the `value` property. Note that the `value` property is set to `null`, if the return type of the underlying method is `void`.

    Overrides:
      `execute` in class `Statement`
    Throws:
      `NullPointerException` - if the value of the `target` or `methodName` property is `null`
      `NoSuchMethodException` - if a matching method is not found
      `SecurityException` - if a security manager exists and it denies the method invocation
      `Exception` - that is thrown by the invoked method
    Since:
      1.7
    See Also:
      - `Method`

  - ### getValue

    public Object getValue() throws Exception
    If the value property of this instance is not already set, this method dynamically finds the method with the specified methodName on this target with these arguments and calls it. The result of the method invocation is first copied into the value property of this expression and then returned as the result of `getValue`. If the value property was already set, either by a call to `setValue` or a previous call to `getValue` then the value property is returned without either looking up or calling the method.

    The value property of an `Expression` is set to a unique private (non-`null`) value by default and this value is used as an internal indication that the method has not yet been called. A return value of `null` replaces this default value in the same way that any other value would, ensuring that expressions are never evaluated more than once.

    See the `execute` method for details on how methods are chosen using the dynamic types of the target and arguments.

    Returns:
      The result of applying this method to these arguments.
    Throws:
      `Exception` - if the method with the specified methodName throws an exception
    See Also:
      - `Statement.execute()`
      - `setValue(java.lang.Object)`

  - ### setValue

    public void setValue(Object value)
    Sets the value of this expression to `value`. This value will be returned by the getValue method without calling the method associated with this expression.
    Parameters:
      `value` - The value of this expression.
    See Also:
      - `getValue()`

  - ### toString

    public String toString()
    Prints the value of this expression using a Java-style syntax.
    Overrides:
      `toString` in class `Statement`
    Returns:
      a string representation of the object.