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Commons Collections example source code file (ListUtils.java)

This example Commons Collections source code file (ListUtils.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Commons Collections tags/keywords

arraylist, arraylist, collection, empty_list, factory, iterator, iterator, list, list, listutils, object, object, predicate, transformer, util

The Commons Collections ListUtils.java source code

/*
 *  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.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;

import org.apache.commons.collections.list.FixedSizeList;
import org.apache.commons.collections.list.LazyList;
import org.apache.commons.collections.list.PredicatedList;
import org.apache.commons.collections.list.SynchronizedList;
import org.apache.commons.collections.list.TransformedList;
import org.apache.commons.collections.list.TypedList;
import org.apache.commons.collections.list.UnmodifiableList;

/**
 * Provides utility methods and decorators for {@link List} instances.
 *
 * @since Commons Collections 1.0
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author Federico Barbieri
 * @author Peter Donald
 * @author Paul Jack
 * @author Stephen Colebourne
 * @author Neil O'Toole
 * @author Matthew Hawthorne
 */
public class ListUtils {

    /**
     * An empty unmodifiable list.
     * This uses the {@link Collections Collections} implementation 
     * and is provided for completeness.
     */
    public static final List EMPTY_LIST = Collections.EMPTY_LIST;
    
    /**
     * <code>ListUtils should not normally be instantiated.
     */
    public ListUtils() {
    }

    //-----------------------------------------------------------------------
    /**
     * Returns a new list containing all elements that are contained in
     * both given lists.
     *
     * @param list1  the first list
     * @param list2  the second list
     * @return  the intersection of those two lists
     * @throws NullPointerException if either list is null
     */
    public static List intersection(final List list1, final List list2) {
        final ArrayList result = new ArrayList();
        final Iterator iterator = list2.iterator();

        while (iterator.hasNext()) {
            final Object o = iterator.next();

            if (list1.contains(o)) {
                result.add(o);
            }
        }

        return result;
    }

    /**
     * Subtracts all elements in the second list from the first list,
     * placing the results in a new list.
     * <p>
     * This differs from {@link List#removeAll(Collection)} in that
     * cardinality is respected; if <Code>list1 contains two
     * occurrences of <Code>null and list2 only
     * contains one occurrence, then the returned list will still contain
     * one occurrence.
     *
     * @param list1  the list to subtract from
     * @param list2  the list to subtract
     * @return  a new list containing the results
     * @throws NullPointerException if either list is null
     */
    public static List subtract(final List list1, final List list2) {
        final ArrayList result = new ArrayList(list1);
        final Iterator iterator = list2.iterator();

        while (iterator.hasNext()) {
            result.remove(iterator.next());
        }

        return result;
    }

    /**
     * Returns the sum of the given lists.  This is their intersection
     * subtracted from their union.
     *
     * @param list1  the first list 
     * @param list2  the second list
     * @return  a new list containing the sum of those lists
     * @throws NullPointerException if either list is null
     */ 
    public static List sum(final List list1, final List list2) {
        return subtract(union(list1, list2), intersection(list1, list2));
    }

    /**
     * Returns a new list containing the second list appended to the
     * first list.  The {@link List#addAll(Collection)} operation is
     * used to append the two given lists into a new list.
     *
     * @param list1  the first list 
     * @param list2  the second list
     * @return  a new list containing the union of those lists
     * @throws NullPointerException if either list is null
     */
    public static List union(final List list1, final List list2) {
        final ArrayList result = new ArrayList(list1);
        result.addAll(list2);
        return result;
    }

    /**
     * Tests two lists for value-equality as per the equality contract in
     * {@link java.util.List#equals(java.lang.Object)}.
     * <p>
     * This method is useful for implementing <code>List when you cannot
     * extend AbstractList. The method takes Collection instances to enable other
     * collection types to use the List implementation algorithm.
     * <p>
     * The relevant text (slightly paraphrased as this is a static method) is:
     * <blockquote>
     * Compares the two list objects for equality.  Returns
     * <tt>true if and only if both
     * lists have the same size, and all corresponding pairs of elements in
     * the two lists are <i>equal.  (Two elements e1 and
     * <tt>e2 are equal if (e1==null ? e2==null :
     * e1.equals(e2))</tt>.)  In other words, two lists are defined to be
     * equal if they contain the same elements in the same order.  This
     * definition ensures that the equals method works properly across
     * different implementations of the <tt>List interface.
     * </blockquote>
     *
     * <b>Note: The behaviour of this method is undefined if the lists are
     * modified during the equals comparison.
     * 
     * @see java.util.List
     * @param list1  the first list, may be null
     * @param list2  the second list, may be null
     * @return whether the lists are equal by value comparison
     */
    public static boolean isEqualList(final Collection list1, final Collection list2) {
        if (list1 == list2) {
            return true;
        }
        if (list1 == null || list2 == null || list1.size() != list2.size()) {
            return false;
        }

        Iterator it1 = list1.iterator();
        Iterator it2 = list2.iterator();
        Object obj1 = null;
        Object obj2 = null;

        while (it1.hasNext() && it2.hasNext()) {
            obj1 = it1.next();
            obj2 = it2.next();

            if (!(obj1 == null ? obj2 == null : obj1.equals(obj2))) {
                return false;
            }
        }

        return !(it1.hasNext() || it2.hasNext());
    }
    
    /**
     * Generates a hash code using the algorithm specified in 
     * {@link java.util.List#hashCode()}.
     * <p>
     * This method is useful for implementing <code>List when you cannot
     * extend AbstractList. The method takes Collection instances to enable other
     * collection types to use the List implementation algorithm.
     * 
     * @see java.util.List#hashCode()
     * @param list  the list to generate the hashCode for, may be null
     * @return the hash code
     */
    public static int hashCodeForList(final Collection list) {
        if (list == null) {
            return 0;
        }
        int hashCode = 1;
        Iterator it = list.iterator();
        Object obj = null;
        
        while (it.hasNext()) {
            obj = it.next();
            hashCode = 31 * hashCode + (obj == null ? 0 : obj.hashCode());
        }
        return hashCode;
    }   

    //-----------------------------------------------------------------------
    /**
     * Returns a List containing all the elements in <code>collection
     * that are also in <code>retain. The cardinality of an element e
     * in the returned list is the same as the cardinality of <code>e
     * in <code>collection unless retain does not contain e, in which
     * case the cardinality is zero. This method is useful if you do not wish to modify
     * the collection <code>c and thus cannot call collection.retainAll(retain);.
     * 
     * @param collection  the collection whose contents are the target of the #retailAll operation
     * @param retain  the collection containing the elements to be retained in the returned collection
     * @return a <code>List containing all the elements of c
     * that occur at least once in <code>retain.
     * @throws NullPointerException if either parameter is null
     * @since Commons Collections 3.2
     */
    public static List retainAll(Collection collection, Collection retain) {
        List list = new ArrayList(Math.min(collection.size(), retain.size()));

        for (Iterator iter = collection.iterator(); iter.hasNext();) {
            Object obj = iter.next();
            if (retain.contains(obj)) {
                list.add(obj);
            }
        }
        return list;
    }

    /**
     * Removes the elements in <code>remove from collection. That is, this
     * method returns a list containing all the elements in <code>c
     * that are not in <code>remove. The cardinality of an element e
     * in the returned collection is the same as the cardinality of <code>e
     * in <code>collection unless remove contains e, in which
     * case the cardinality is zero. This method is useful if you do not wish to modify
     * <code>collection and thus cannot call collection.removeAll(remove);.
     * 
     * @param collection  the collection from which items are removed (in the returned collection)
     * @param remove  the items to be removed from the returned <code>collection
     * @return a <code>List containing all the elements of c except
     * any elements that also occur in <code>remove.
     * @throws NullPointerException if either parameter is null
     * @since Commons Collections 3.2
     */
    public static List removeAll(Collection collection, Collection remove) {
        List list = new ArrayList();
        for (Iterator iter = collection.iterator(); iter.hasNext();) {
            Object obj = iter.next();
            if (remove.contains(obj) == false) {
                list.add(obj);
            }
        }
        return list;
    }

    //-----------------------------------------------------------------------
    /**
     * Returns a synchronized list backed by the given list.
     * <p>
     * You must manually synchronize on the returned buffer's iterator to 
     * avoid non-deterministic behavior:
     *  
     * <pre>
     * List list = ListUtils.synchronizedList(myList);
     * synchronized (list) {
     *     Iterator i = list.iterator();
     *     while (i.hasNext()) {
     *         process (i.next());
     *     }
     * }
     * </pre>
     * 
     * This method uses the implementation in the decorators subpackage.
     * 
     * @param list  the list to synchronize, must not be null
     * @return a synchronized list backed by the given list
     * @throws IllegalArgumentException  if the list is null
     */
    public static List synchronizedList(List list) {
        return SynchronizedList.decorate(list);
    }

    /**
     * Returns an unmodifiable list backed by the given list.
     * <p>
     * This method uses the implementation in the decorators subpackage.
     *
     * @param list  the list to make unmodifiable, must not be null
     * @return an unmodifiable list backed by the given list
     * @throws IllegalArgumentException  if the list is null
     */
    public static List unmodifiableList(List list) {
        return UnmodifiableList.decorate(list);
    }

    /**
     * Returns a predicated (validating) list backed by the given list.
     * <p>
     * Only objects that pass the test in the given predicate can be added to the list.
     * Trying to add an invalid object results in an IllegalArgumentException.
     * It is important not to use the original list after invoking this method,
     * as it is a backdoor for adding invalid objects.
     *
     * @param list  the list to predicate, must not be null
     * @param predicate  the predicate for the list, must not be null
     * @return a predicated list backed by the given list
     * @throws IllegalArgumentException  if the List or Predicate is null
     */
    public static List predicatedList(List list, Predicate predicate) {
        return PredicatedList.decorate(list, predicate);
    }

    /**
     * Returns a typed list backed by the given list.
     * <p>
     * Only objects of the specified type can be added to the list.
     * 
     * @param list  the list to limit to a specific type, must not be null
     * @param type  the type of objects which may be added to the list
     * @return a typed list backed by the specified list
     */
    public static List typedList(List list, Class type) {
        return TypedList.decorate(list, type);
    }
    
    /**
     * Returns a transformed list backed by the given list.
     * <p>
     * Each object is passed through the transformer as it is added to the
     * List. It is important not to use the original list after invoking this 
     * method, as it is a backdoor for adding untransformed objects.
     *
     * @param list  the list to predicate, must not be null
     * @param transformer  the transformer for the list, must not be null
     * @return a transformed list backed by the given list
     * @throws IllegalArgumentException  if the List or Transformer is null
     */
    public static List transformedList(List list, Transformer transformer) {
        return TransformedList.decorate(list, transformer);
    }
    
    /**
     * Returns a "lazy" list whose elements will be created on demand.
     * <p>
     * When the index passed to the returned list's {@link List#get(int) get}
     * method is greater than the list's size, then the factory will be used
     * to create a new object and that object will be inserted at that index.
     * <p>
     * For instance:
     *
     * <pre>
     * Factory factory = new Factory() {
     *     public Object create() {
     *         return new Date();
     *     }
     * }
     * List lazy = ListUtils.lazyList(new ArrayList(), factory);
     * Object obj = lazy.get(3);
     * </pre>
     *
     * After the above code is executed, <code>obj will contain
     * a new <code>Date instance.  Furthermore, that Date
     * instance is the fourth element in the list.  The first, second, 
     * and third element are all set to <code>null.
     *
     * @param list  the list to make lazy, must not be null
     * @param factory  the factory for creating new objects, must not be null
     * @return a lazy list backed by the given list
     * @throws IllegalArgumentException  if the List or Factory is null
     */
    public static List lazyList(List list, Factory factory) {
        return LazyList.decorate(list, factory);
    }

    /**
     * Returns a fixed-sized list backed by the given list.
     * Elements may not be added or removed from the returned list, but 
     * existing elements can be changed (for instance, via the 
     * {@link List#set(int,Object)} method).
     *
     * @param list  the list whose size to fix, must not be null
     * @return a fixed-size list backed by that list
     * @throws IllegalArgumentException  if the List is null
     */
    public static List fixedSizeList(List list) {
        return FixedSizeList.decorate(list);
    }

}

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