src/java/org/apache/commons/functor/core/composite/TransposedFunction.java

/*
 * Copyright 2003,2004 The Apache Software Foundation.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.functor.core.composite;

import java.io.Serializable;

import org.apache.commons.functor.BinaryFunction;

/**
 * Transposes (swaps) the arguments to some other
 * {@link BinaryFunction function}.  
 * For example, given a function <i>f</i>
 * and the ordered pair of arguments <i>a</i>,
 * <i>b</i>.
 * {@link #evaluate evaluates} to 
 * <code>f.evaluate(b,a)</code>.
 * <p>
 * Note that although this class implements 
 * {@link Serializable}, a given instance will
 * only be truly <code>Serializable</code> if the
 * underlying functor is.  Attempts to serialize
 * an instance whose delegate is not 
 * <code>Serializable</code> will result in an exception.
 * </p>
 * @version $Revision$ $Date$
 * @author Rodney Waldhoff
 */
public class TransposedFunction implements BinaryFunction, Serializable {

    // constructor
    // ------------------------------------------------------------------------
    public TransposedFunction(BinaryFunction f) {
        function = f;
    }

    // functor interface
    // ------------------------------------------------------------------------
    public Object evaluate(Object left, Object right) {        
        return function.evaluate(right,left);
    }

    public boolean equals(Object that) {
        if(that instanceof TransposedFunction) {
            return equals((TransposedFunction)that);
        } else {
            return false;
        }
    }
    
    public boolean equals(TransposedFunction that) {
        return null != that && (null == function ? null == that.function : function.equals(that.function));
    }
    
    public int hashCode() {
        int hash = "TransposedFunction".hashCode();
        if(null != function) {
            hash ^= function.hashCode();
        }
        return hash;
    }
    
    public String toString() {
        return "TransposedFunction<" + function + ">";
    }
        
    // static
    // ------------------------------------------------------------------------
    public static TransposedFunction transpose(BinaryFunction f) {
        return null == f ? null : new TransposedFunction(f);
    }
        
    // attributes
    // ------------------------------------------------------------------------
    private BinaryFunction function = null;

}