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Java Standard Edition (SE)

Expression


java.beans

Class 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 Detail

      • Expression

        @ConstructorProperties(value={"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 Detail

      • 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 Expressions rather than Statements 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.


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