Ordered Map : Customized Map « Collections « Java Tutorial

import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;

 * This is a simple Map implementation backed by a List of Map.Entry objects. 
 * It has the property that iterators return entries in the order in whick
 * they were inserted.
 * Operations involving searching, including get() and put(), have cost linear 
 * to the size of the map. In other words, avoid this implementation if your 
 * Map might get large.
 * If we could assume Java 1.4+, we'd just use java.util.LinkedHashMap 
 * instead of this class.  But we can't.
 * @author Mike Williams
public class OrderedMap extends AbstractMap {

    private Set _entrySet = new OrderedSet();

    public Set entrySet() {
        return _entrySet;

    public Object put(Object key, Object value) {
        Entry existingEntry = getEntryWithKey(key);
        if (existingEntry == null) {
            entrySet().add(new Entry(key, value));
            return null;
        Object previousValue = existingEntry.getValue();
        return previousValue;

    private Entry getEntryWithKey(Object key) {
        Iterator i = entrySet().iterator();
        while (i.hasNext()) {
            Entry e = (Entry) i.next();
            if (eq(e.getKey(), key)) {
                return e;
        return null;
    static class OrderedSet extends AbstractSet {

        private List _elementList = new LinkedList();

        public int size() {
            return _elementList.size();

        public Iterator iterator() {
            return _elementList.iterator();

        public boolean add(Object o) {
            return true;

    static class Entry implements Map.Entry {
        Object _key;
        Object _value;

        public Entry(Object key, Object value) {
            _key = key;
            _value = value;

        public Object getKey() {
            return _key;

        public Object getValue() {
            return _value;

        public Object setValue(Object value) {
            Object oldValue = _value;
            _value = value;
            return oldValue;

        public boolean equals(Object o) {
            if (!(o instanceof Map.Entry)) {
                return false;
            Map.Entry e = (Map.Entry) o;
            return eq(_key, e.getKey()) && eq(_value, e.getValue());

        public int hashCode() {
            return ((_key == null) ? 0 : _key.hashCode()) ^
                   ((_value == null) ? 0 : _value.hashCode());

        public String toString() {
            return _key + "=" + _value;


    private static boolean eq(Object o1, Object o2) {
        return (o1 == null ? o2 == null : o1.equals(o2));

9.33.Customized Map
9.33.1.Implementation of a bit map of any size, together with static methods to manipulate int, byte and byte[] values as bit maps
9.33.2.A fixed size map implementation.
9.33.3.A memory-efficient hash map.
9.33.4.CaseBlindHashMap - a HashMap extension, using Strings as key values.
9.33.5.Clones a map and prefixes the keys in the clone
9.33.6.Converts array into a java.util.Map.
9.33.7.Ordered Map
9.33.8.A hash map that uses primitive ints for the key rather than objects.
9.33.9.HashNMap stores multiple values by a single key value. Values can be retrieved using a direct query or by creating an enumeration over the stored elements.
9.33.10.A Map where keys are compared by object identity, rather than equals()
9.33.11.A java.util.Map implementation using reference values
9.33.12.A simple hashmap from keys to integers
9.33.13.An IdentityMap that uses reference-equality instead of object-equality
9.33.14.Creates a mutable map from two arrays with keys and values
9.33.15.Fixed size hash map using String values as keys mapped to primitive int values.
9.33.16.Hash map for counting references to Object keys.
9.33.17.Hash map using String values as keys mapped to primitive int values.
9.33.18.IntMap provides a simple hashmap from keys to integers
9.33.19.List ordered map
9.33.20.Lookup table that stores a list of strings
9.33.21.Map implementation Optimized for Strings keys
9.33.22.Map with keys iterated in insertion order