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Commons Collections example source code file (Bag.java)

This example Commons Collections source code file (Bag.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

bag, bag, collection, collection, iterator, iterator, set, set, util

The Commons Collections Bag.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.Collection;
import java.util.Iterator;
import java.util.Set;

/**
 * Defines a collection that counts the number of times an object appears in
 * the collection.
 * <p>
 * Suppose you have a Bag that contains <code>{a, a, b, c}.
 * Calling {@link #getCount(Object)} on <code>a would return 2, while
 * calling {@link #uniqueSet()} would return <code>{a, b, c}.
 * <p>
 * <i>NOTE: This interface violates the {@link Collection} contract. 
 * The behavior specified in many of these methods is <i>not the same
 * as the behavior specified by <code>Collection.
 * The noncompliant methods are clearly marked with "(Violation)".
 * Exercise caution when using a bag as a <code>Collection.
 * <p>
 * This violation resulted from the original specification of this interface.
 * In an ideal world, the interface would be changed to fix the problems, however
 * it has been decided to maintain backwards compatibility instead.
 *
 * @since Commons Collections 2.0
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author Chuck Burdick
 * @author Stephen Colebourne
 */
public interface Bag extends Collection {

    /**
     * Returns the number of occurrences (cardinality) of the given
     * object currently in the bag. If the object does not exist in the
     * bag, return 0.
     * 
     * @param object  the object to search for
     * @return the number of occurrences of the object, zero if not found
     */
    int getCount(Object object);

    /**
     * <i>(Violation)
     * Adds one copy the specified object to the Bag.
     * <p>
     * If the object is already in the {@link #uniqueSet()} then increment its
     * count as reported by {@link #getCount(Object)}. Otherwise add it to the
     * {@link #uniqueSet()} and report its count as 1.
     * <p>
     * Since this method always increases the size of the bag,
     * according to the {@link Collection#add(Object)} contract, it 
     * should always return <code>true.  Since it sometimes returns
     * <code>false, this method violates the contract.
     *
     * @param object  the object to add
     * @return <code>true if the object was not already in the uniqueSet
     */
    boolean add(Object object);

    /**
     * Adds <code>nCopies copies of the specified object to the Bag.
     * <p>
     * If the object is already in the {@link #uniqueSet()} then increment its
     * count as reported by {@link #getCount(Object)}. Otherwise add it to the
     * {@link #uniqueSet()} and report its count as <code>nCopies.
     * 
     * @param object  the object to add
     * @param nCopies  the number of copies to add
     * @return <code>true if the object was not already in the uniqueSet
     */
    boolean add(Object object, int nCopies);

    /**
     * <i>(Violation)
     * Removes all occurrences of the given object from the bag.
     * <p>
     * This will also remove the object from the {@link #uniqueSet()}.
     * <p>
     * According to the {@link Collection#remove(Object)} method,
     * this method should only remove the <i>first occurrence of the
     * given object, not <i>all occurrences.
     *
     * @return <code>true if this call changed the collection
     */
    boolean remove(Object object);

    /**
     * Removes <code>nCopies copies of the specified object from the Bag.
     * <p>
     * If the number of copies to remove is greater than the actual number of
     * copies in the Bag, no error is thrown.
     * 
     * @param object  the object to remove
     * @param nCopies  the number of copies to remove
     * @return <code>true if this call changed the collection
     */
    boolean remove(Object object, int nCopies);

    /**
     * Returns a {@link Set} of unique elements in the Bag.
     * <p>
     * Uniqueness constraints are the same as those in {@link java.util.Set}.
     * 
     * @return the Set of unique Bag elements
     */
    Set uniqueSet();

    /**
     * Returns the total number of items in the bag across all types.
     * 
     * @return the total size of the Bag
     */
    int size();

    /**
     * <i>(Violation)
     * Returns <code>true if the bag contains all elements in
     * the given collection, respecting cardinality.  That is, if the
     * given collection <code>coll contains n copies
     * of a given object, calling {@link #getCount(Object)} on that object must
     * be <code>>= n for all n in coll.
     * <p>
     * The {@link Collection#containsAll(Collection)} method specifies
     * that cardinality should <i>not be respected; this method should
     * return true if the bag contains at least one of every object contained
     * in the given collection.
     * 
     * @param coll  the collection to check against
     * @return <code>true if the Bag contains all the collection
     */
    boolean containsAll(Collection coll);

    /**
     * <i>(Violation)
     * Remove all elements represented in the given collection,
     * respecting cardinality.  That is, if the given collection
     * <code>coll contains n copies of a given object,
     * the bag will have <code>n fewer copies, assuming the bag
     * had at least <code>n copies to begin with.
     *
     * <P>The {@link Collection#removeAll(Collection)} method specifies
     * that cardinality should <i>not be respected; this method should
     * remove <i>all occurrences of every object contained in the 
     * given collection.
     *
     * @param coll  the collection to remove
     * @return <code>true if this call changed the collection
     */
    boolean removeAll(Collection coll);

    /**
     * <i>(Violation)
     * Remove any members of the bag that are not in the given
     * collection, respecting cardinality.  That is, if the given
     * collection <code>coll contains n copies of a
     * given object and the bag has <code>m > n copies, then
     * delete <code>m - n copies from the bag.  In addition, if
     * <code>e is an object in the bag but
     * <code>!coll.contains(e), then remove e and any
     * of its copies.
     *
     * <P>The {@link Collection#retainAll(Collection)} method specifies
     * that cardinality should <i>not be respected; this method should
     * keep <i>all occurrences of every object contained in the 
     * given collection.
     *
     * @param coll  the collection to retain
     * @return <code>true if this call changed the collection
     */
    boolean retainAll(Collection coll);

    /**
     * Returns an {@link Iterator} over the entire set of members,
     * including copies due to cardinality. This iterator is fail-fast
     * and will not tolerate concurrent modifications.
     * 
     * @return iterator over all elements in the Bag
     */
    Iterator iterator();

    // The following is not part of the formal Bag interface, however where possible
    // Bag implementations should follow these comments.
//    /**
//     * Compares this Bag to another.
//     * This Bag equals another Bag if it contains the same number of occurrences of
//     * the same elements.
//     * This equals definition is compatible with the Set interface.
//     * 
//     * @param obj  the Bag to compare to
//     * @return true if equal
//     */
//    boolean equals(Object obj);
//
//    /**
//     * Gets a hash code for the Bag compatible with the definition of equals.
//     * The hash code is defined as the sum total of a hash code for each element.
//     * The per element hash code is defined as
//     * <code>(e==null ? 0 : e.hashCode()) ^ noOccurances).
//     * This hash code definition is compatible with the Set interface.
//     * 
//     * @return the hash code of the Bag
//     */
//    int hashCode();
    
}

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