This extension of HashMap support duplicate keys

    
/*
 * Copyright (c) 2006, Chuck Mortimore - xmldap.org
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *     * Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in the
 *       documentation and/or other materials provided with the distribution.
 *     * Neither the names xmldap, xmldap.org, xmldap.com nor the
 *       names of its contributors may be used to endorse or promote products
 *       derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE REGENTS AND CONTRIBUTORS BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */


//package org.xmldap.util;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.List;

/**
 * This extension of HashMap support duplicate keys
 */
public class Bag extends LinkedHashMap {
  public List getValues(Object key) {
    if (super.containsKey(key)) {
      return (List) super.get(key);
    } else {
      return new ArrayList();
    }
  }

  public Object get(Object key) {
    ArrayList values = (ArrayList) super.get(key);
    if (values != null && !values.isEmpty()) {
      return values.get(0);
    } else {
      return null;
    }
  }

  public boolean containsValue(Object value) {
    return values().contains(value);
  }

  public int size() {
    int size = 0;
    Iterator keyIterator = super.keySet().iterator();

    while (keyIterator.hasNext()) {
      ArrayList values = (ArrayList) super.get(keyIterator.next());
      size = size + values.size();
    }

    return size;
  }

  public Object put(Object key, Object value) {
    ArrayList values = new ArrayList();

    if (super.containsKey(key)) {
      values = (ArrayList) super.get(key);
      values.add(value);

    } else {
      values.add(value);
    }

    super.put(key, values);

    return null;
  }

  public void remove(Object key, Object value) {
    List values = getValues(key);
    if (values != null) {
      values.remove(value);
      if (values.isEmpty()) {
        remove(key);
      }
    }
  }

  public Collection values() {
    List values = new ArrayList();
    Iterator keyIterator = super.keySet().iterator();

    while (keyIterator.hasNext()) {
      List keyValues = (List) super.get(keyIterator.next());
      values.addAll(keyValues);
    }

    return values;
  }
}

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This extension of HashMap support duplicate keys : HashMap « Collections Data Structure « Java

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