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package javax.el;

import java.io.Serializable;

/**
 * Base class for the expression subclasses {@link ValueExpression} and
 * {@link MethodExpression}, implementing characteristics 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 / {@link 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 */ public abstract class Expression implements Serializable { // Debugging /** * 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.

* * @return The original expression String. */ public abstract String getExpressionString(); // Comparison /** * 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.

* * @param obj the Object to test for equality. * @return true if obj equals this * Expression; false otherwise. * @see java.util.Hashtable * @see java.lang.Object#equals(java.lang.Object) */ public abstract boolean equals(Object obj); /** * Returns the hash code for this Expression. * *

See the note in the {@link #equals} 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.

* * @return The hash code for this Expression. * @see #equals * @see java.util.Hashtable * @see java.lang.Object#hashCode() */ public abstract int hashCode(); /** * 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 * #{...}).

* * @return true if this expression was created from only * literal text; false otherwise. */ public abstract boolean isLiteralText(); }




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