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Java Enterprise Edition (EE)

Expression


javax.el
Class Expression

java.lang.Object
  javax.el.Expression
All Implemented Interfaces:
java.io.Serializable
Direct Known Subclasses:
MethodExpression, ValueExpression
public abstract class Expression
extends java.lang.Object
implements java.io.Serializable

Base class for the expression subclasses ValueExpression and MethodExpression, implementing characterstics common to both.

All expressions must implement the equals() and hashCode() methods so that two expressions can be compared for equality. They are redefined abstract in this class to force their implementation in subclasses.

All expressions must also be Serializable so that they can be saved and restored.

Expressions are also designed to be immutable so that only one instance needs to be created for any given expression String / FunctionMapper. This allows a container to pre-create expressions and not have to re-parse them each time they are evaluated.

Since:
JSP 2.1
See Also:
Serialized Form

Constructor Summary
Expression()
           
 
Method Summary
abstract  boolean equals(java.lang.Object obj)
          Determines whether the specified object is equal to this Expression.
abstract  java.lang.String getExpressionString()
          Returns the original String used to create this Expression, unmodified.
abstract  int hashCode()
          Returns the hash code for this Expression.
abstract  boolean isLiteralText()
          Returns whether this expression was created from only literal text.
 
Methods inherited from class java.lang.Object
clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

Expression

public Expression()
Method Detail

getExpressionString

public abstract java.lang.String getExpressionString()
Returns the original String used to create this Expression, unmodified.

This is used for debugging purposes but also for the purposes of comparison (e.g. to ensure the expression in a configuration file has not changed).

This method does not provide sufficient information to re-create an expression. Two different expressions can have exactly the same expression string but different function mappings. Serialization should be used to save and restore the state of an Expression.

Returns:
The original expression String.

equals

public abstract boolean equals(java.lang.Object obj)
Determines whether the specified object is equal to this Expression.

The result is true if and only if the argument is not null, is an Expression object that is the of the same type (ValueExpression or MethodExpression), and has an identical parsed representation.

Note that two expressions can be equal if their expression Strings are different. For example, ${fn1:foo()} and ${fn2:foo()} are equal if their corresponding FunctionMappers mapped fn1:foo and fn2:foo to the same method.

Overrides:
equals in class java.lang.Object
Parameters:
obj - the Object to test for equality.
Returns:
true if obj equals this Expression; false otherwise.
See Also:
Hashtable, Object.equals(java.lang.Object)

hashCode

public abstract int hashCode()
Returns the hash code for this Expression.

See the note in the equals(java.lang.Object) method on how two expressions can be equal if their expression Strings are different. Recall that if two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. Implementations must take special note and implement hashCode correctly.

Overrides:
hashCode in class java.lang.Object
Returns:
The hash code for this Expression.
See Also:
equals(java.lang.Object), Hashtable, Object.hashCode()

isLiteralText

public abstract boolean isLiteralText()
Returns whether this expression was created from only literal text.

This method must return true if and only if the expression string this expression was created from contained no unescaped EL delimeters (${...} or #{...}).

Returns:
true if this expression was created from only literal text; false otherwise.


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