Java tutorial
/* * Copyright 2002-2004 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.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: 1.13 $ $Date: 2004/05/26 21:50:52 $ * * @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 * @throws IllegalArgumentException if the transformers collection is null * @throws IllegalArgumentException if any transformer in the collection is null */ public static Transformer chainedTransformer(Collection transformers) { return ChainedTransformer.getInstance(transformers); } /** * Create a new Transformer that calls one of two transformers depending * on the specified predicate. * * @see org.apache.commons.collections.functors.SwitchTransformer * * @param predicate the predicate to switch on * @param trueTransformer the transformer called if the predicate is true * @param falseTransformer the transformer called if the predicate is false * @return the transformer * @throws IllegalArgumentException if the predicate is null * @throws IllegalArgumentException if either transformer is null */ public static Transformer switchTransformer(Predicate predicate, Transformer trueTransformer, Transformer falseTransformer) { return SwitchTransformer.getInstance(new Predicate[] { predicate }, new Transformer[] { trueTransformer }, falseTransformer); } /** * Create a new Transformer that calls one of the transformers depending * on the predicates. The transformer at array location 0 is called if the * predicate at array location 0 returned true. Each predicate is evaluated * until one returns true. If no predicates evaluate to true, null is returned. * * @see org.apache.commons.collections.functors.SwitchTransformer * * @param predicates an array of predicates to check * @param transformers an array of transformers to call * @return the transformer * @throws IllegalArgumentException if the either array is null * @throws IllegalArgumentException if the either array has 0 elements * @throws IllegalArgumentException if any element in the arrays is null * @throws IllegalArgumentException if the arrays are different sizes */ public static Transformer switchTransformer(Predicate[] predicates, Transformer[] transformers) { return SwitchTransformer.getInstance(predicates, transformers, null); } /** * Create a new Transformer that calls one of the transformers depending * on the predicates. The transformer at array location 0 is called if the * predicate at array location 0 returned true. Each predicate is evaluated * until one returns true. If no predicates evaluate to true, the default * transformer is called. If the default transformer is null, null is returned. * * @see org.apache.commons.collections.functors.SwitchTransformer * * @param predicates an array of predicates to check * @param transformers an array of transformers to call * @param defaultTransformer the default to call if no predicate matches, null means return null * @return the transformer * @throws IllegalArgumentException if the either array is null * @throws IllegalArgumentException if the either array has 0 elements * @throws IllegalArgumentException if any element in the arrays is null * @throws IllegalArgumentException if the arrays are different sizes */ public static Transformer switchTransformer(Predicate[] predicates, Transformer[] transformers, Transformer defaultTransformer) { return SwitchTransformer.getInstance(predicates, transformers, defaultTransformer); } /** * Create a new Transformer that calls one of the transformers depending * on the predicates. * <p> * The Map consists of Predicate keys and Transformer values. A transformer * is called if its matching predicate returns true. Each predicate is evaluated * until one returns true. If no predicates evaluate to true, the default * transformer is called. The default transformer is set in the map with a * null key. If no default transformer is set, null will be returned in a default * case. The ordering is that of the iterator() method on the entryset collection * of the map. * * @see org.apache.commons.collections.functors.SwitchTransformer * * @param predicatesAndTransformers a map of predicates to transformers * @return the transformer * @throws IllegalArgumentException if the map is null * @throws IllegalArgumentException if the map is empty * @throws IllegalArgumentException if any transformer in the map is null * @throws ClassCastException if the map elements are of the wrong type */ public static Transformer switchTransformer(Map predicatesAndTransformers) { return SwitchTransformer.getInstance(predicatesAndTransformers); } /** * Create a new Transformer that uses the input object as a key to find the * transformer to call. * <p> * The Map consists of object keys and Transformer values. A transformer * is called if the input object equals the key. If there is no match, the * default transformer is called. The default transformer is set in the map * using a null key. If no default is set, null will be returned in a default case. * * @see org.apache.commons.collections.functors.SwitchTransformer * * @param objectsAndTransformers a map of objects to transformers * @return the transformer * @throws IllegalArgumentException if the map is null * @throws IllegalArgumentException if the map is empty * @throws IllegalArgumentException if any transformer in the map is null */ public static Transformer switchMapTransformer(Map objectsAndTransformers) { Transformer[] trs = null; Predicate[] preds = null; if (objectsAndTransformers == null) { throw new IllegalArgumentException("The object and transformer map must not be null"); } Transformer def = (Transformer) objectsAndTransformers.remove(null); int size = objectsAndTransformers.size(); trs = new Transformer[size]; preds = new Predicate[size]; int i = 0; for (Iterator it = objectsAndTransformers.entrySet().iterator(); it.hasNext();) { Map.Entry entry = (Map.Entry) it.next(); preds[i] = EqualPredicate.getInstance(entry.getKey()); trs[i] = (Transformer) entry.getValue(); i++; } return switchTransformer(preds, trs, def); } /** * Gets a Transformer that expects an input Class object that it will instantiate. * * @see org.apache.commons.collections.functors.InstantiateTransformer * * @return the transformer */ public static Transformer instantiateTransformer() { return InstantiateTransformer.NO_ARG_INSTANCE; } /** * Creates a Transformer that expects an input Class object that it will * instantiate. The constructor used is determined by the arguments specified * to this method. * * @see org.apache.commons.collections.functors.InstantiateTransformer * * @param paramTypes parameter types for the constructor, can be null * @param args the arguments to pass to the constructor, can be null * @return the transformer * @throws IllegalArgumentException if the paramTypes and args don't match */ public static Transformer instantiateTransformer(Class[] paramTypes, Object[] args) { return InstantiateTransformer.getInstance(paramTypes, args); } /** * Creates a Transformer that uses the passed in Map to transform the input * object (as a simple lookup). * * @see org.apache.commons.collections.functors.MapTransformer * * @param map the map to use to transform the objects * @return the transformer * @throws IllegalArgumentException if the map is null */ public static Transformer mapTransformer(Map map) { return MapTransformer.getInstance(map); } /** * Gets a Transformer that invokes a method on the input object. * The method must have no parameters. If the input object is null, * null is returned. * <p> * For example, <code>TransformerUtils.invokerTransformer("getName");</code> * will call the <code>getName/code> method on the input object to * determine the transformer result. * * @see org.apache.commons.collections.functors.InvokerTransformer * * @param methodName the method name to call on the input object, may not be null * @return the transformer * @throws IllegalArgumentException if the methodName is null. */ public static Transformer invokerTransformer(String methodName) { return InvokerTransformer.getInstance(methodName, null, null); } /** * Gets a Transformer that invokes a method on the input object. * The method parameters are specified. If the input object is null, * null is returned. * * @see org.apache.commons.collections.functors.InvokerTransformer * * @param methodName the name of the method * @param paramTypes the parameter types * @param args the arguments * @return the transformer * @throws IllegalArgumentException if the method name is null * @throws IllegalArgumentException if the paramTypes and args don't match */ public static Transformer invokerTransformer(String methodName, Class[] paramTypes, Object[] args) { return InvokerTransformer.getInstance(methodName, paramTypes, args); } /** * Gets a transformer that returns a <code>java.lang.String</code> * representation of the input object. This is achieved via the * <code>toString</code> method, <code>null</code> returns 'null'. * * @see org.apache.commons.collections.functors.StringValueTransformer * * @return the transformer */ public static Transformer stringValueTransformer() { return StringValueTransformer.INSTANCE; } }