transformerutils.java

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 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You 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.collections;

import java.util.Collection;
import java.util.Iterator;
import java.util.Map;

import org.apache.commons.collections.functors.ChainedTransformer;
import org.apache.commons.collections.functors.CloneTransformer;
import org.apache.commons.collections.functors.ClosureTransformer;
import org.apache.commons.collections.functors.ConstantTransformer;
import org.apache.commons.collections.functors.EqualPredicate;
import org.apache.commons.collections.functors.ExceptionTransformer;
import org.apache.commons.collections.functors.FactoryTransformer;
import org.apache.commons.collections.functors.InstantiateTransformer;
import org.apache.commons.collections.functors.InvokerTransformer;
import org.apache.commons.collections.functors.MapTransformer;
import org.apache.commons.collections.functors.NOPTransformer;
import org.apache.commons.collections.functors.PredicateTransformer;
import org.apache.commons.collections.functors.StringValueTransformer;
import org.apache.commons.collections.functors.SwitchTransformer;

/**
 * <code>TransformerUtils</code> provides reference implementations and 
 * utilities for the Transformer functor interface. The supplied transformers are:
 * <ul>
 * <li>Invoker - returns the result of a method call on the input object
 * <li>Clone - returns a clone of the input object
 * <li>Constant - always returns the same object
 * <li>Closure - performs a Closure and returns the input object
 * <li>Predicate - returns the result of the predicate as a Boolean
 * <li>Factory - returns a new object from a factory
 * <li>Chained - chains two or more transformers together
 * <li>Switch - calls one transformer based on one or more predicates
 * <li>SwitchMap - calls one transformer looked up from a Map
 * <li>Instantiate - the Class input object is instantiated
 * <li>Map - returns an object from a supplied Map
 * <li>Null - always returns null
 * <li>NOP - returns the input object, which should be immutable
 * <li>Exception - always throws an exception
 * <li>StringValue - returns a <code>java.lang.String</code> representation of the input object
 * </ul>
 * All the supplied transformers are Serializable.
 *
 * @since Commons Collections 3.0
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author Stephen Colebourne
 * @author James Carman
 */
public class TransformerUtils {

    /**
     * This class is not normally instantiated.
     */
    public TransformerUtils() {
        super();
    }

    /**
     * Gets a transformer that always throws an exception.
     * This could be useful during testing as a placeholder.
     * 
     * @see org.apache.commons.collections.functors.ExceptionTransformer
     * 
     * @return the transformer
     */
    public static Transformer exceptionTransformer() {
        return ExceptionTransformer.INSTANCE;
    }

    /**
     * Gets a transformer that always returns null.
     * 
     * @see org.apache.commons.collections.functors.ConstantTransformer
     * 
     * @return the transformer
     */
    public static Transformer nullTransformer() {
        return ConstantTransformer.NULL_INSTANCE;
    }

    /**
     * Gets a transformer that returns the input object.
     * The input object should be immutable to maintain the
     * contract of Transformer (although this is not checked).
     * 
     * @see org.apache.commons.collections.functors.NOPTransformer
     * 
     * @return the transformer
     */
    public static Transformer nopTransformer() {
        return NOPTransformer.INSTANCE;
    }

    /**
     * Gets a transformer that returns a clone of the input
     * object. The input object will be cloned using one of these
     * techniques (in order):
     * <ul>
     * <li>public clone method
     * <li>public copy constructor
     * <li>serialization clone
     * <ul>
     * 
     * @see org.apache.commons.collections.functors.CloneTransformer
     * 
     * @return the transformer
     */
    public static Transformer cloneTransformer() {
        return CloneTransformer.INSTANCE;
    }

    /**
     * Creates a Transformer that will return the same object each time the 
     * transformer is used.
     *
     * @see org.apache.commons.collections.functors.ConstantTransformer
     * 
     * @param constantToReturn  the constant object to return each time in the transformer
     * @return the transformer.
     */
    public static Transformer constantTransformer(Object constantToReturn) {
        return ConstantTransformer.getInstance(constantToReturn);
    }

    /**
     * Creates a Transformer that calls a Closure each time the transformer is used.
     * The transformer returns the input object.
     *
     * @see org.apache.commons.collections.functors.ClosureTransformer
     * 
     * @param closure  the closure to run each time in the transformer, not null
     * @return the transformer
     * @throws IllegalArgumentException if the closure is null
     */
    public static Transformer asTransformer(Closure closure) {
        return ClosureTransformer.getInstance(closure);
    }

    /**
     * Creates a Transformer that calls a Predicate each time the transformer is used.
     * The transformer will return either Boolean.TRUE or Boolean.FALSE.
     *
     * @see org.apache.commons.collections.functors.PredicateTransformer
     * 
     * @param predicate  the predicate to run each time in the transformer, not null
     * @return the transformer
     * @throws IllegalArgumentException if the predicate is null
     */
    public static Transformer asTransformer(Predicate predicate) {
        return PredicateTransformer.getInstance(predicate);
    }

    /**
     * Creates a Transformer that calls a Factory each time the transformer is used.
     * The transformer will return the value returned by the factory.
     *
     * @see org.apache.commons.collections.functors.FactoryTransformer
     * 
     * @param factory  the factory to run each time in the transformer, not null
     * @return the transformer
     * @throws IllegalArgumentException if the factory is null
     */
    public static Transformer asTransformer(Factory factory) {
        return FactoryTransformer.getInstance(factory);
    }

    /**
     * Create a new Transformer that calls two transformers, passing the result of
     * the first into the second.
     * 
     * @see org.apache.commons.collections.functors.ChainedTransformer
     * 
     * @param transformer1  the first transformer
     * @param transformer2  the second transformer
     * @return the transformer
     * @throws IllegalArgumentException if either transformer is null
     */
    public static Transformer chainedTransformer(Transformer transformer1, Transformer transformer2) {
        return ChainedTransformer.getInstance(transformer1, transformer2);
    }

    /**
     * Create a new Transformer that calls each transformer in turn, passing the 
     * result into the next transformer.
     * 
     * @see org.apache.commons.collections.functors.ChainedTransformer
     * 
     * @param transformers  an array of transformers to chain
     * @return the transformer
     * @throws IllegalArgumentException if the transformers array is null
     * @throws IllegalArgumentException if any transformer in the array is null
     */
    public static Transformer chainedTransformer(Transformer[] transformers) {
        return ChainedTransformer.getInstance(transformers);
    }

    /**
     * Create a new Transformer that calls each transformer in turn, passing the 
     * result into the next transformer. The ordering is that of the iterator()
     * method on the collection.
     * 
     * @see org.apache.commons.collections.functors.ChainedTransformer
     * 
     * @param transformers  a collection of transformers to chain
     * @return the transformer