Hướng dẫn và ví dụ Java IdentityHashMap

public class IdentityHashMap<K,V> extends AbstractMap<K,V>
                     implements Map<K,V>, java.io.Serializable, Cloneable

package org.o7planning.identityhashmap.ex;

public class InternalArrayLength_test {

    private static final int MINIMUM_CAPACITY = 4;

    private static final int MAXIMUM_CAPACITY = 1 << 29;

    private static int capacity(int expectedMaxSize) {
        // assert expectedMaxSize >= 0;
        return (expectedMaxSize > MAXIMUM_CAPACITY / 3) ? MAXIMUM_CAPACITY
                : (expectedMaxSize <= 2 * MINIMUM_CAPACITY / 3) ? MINIMUM_CAPACITY
                        : Integer.highestOneBit(expectedMaxSize + (expectedMaxSize << 1));
    }

    public static void main(String[] args) {
        for (int i = 1; i < 15; i++) {
            int mapSize = i * 25;

            int arrayLength = 2 * capacity(mapSize);
            System.out.printf("Map size: %3d --> Internal Array length: %d%n",mapSize, arrayLength);
        }
    }
}

private static int hash(Object x, int length) {
    int h = System.identityHashCode(x);
    // Multiply by -127, and left-shift to use least bit as part of hash
    return ((h << 1) - (h << 8)) & (length - 1);
}
// Return default not null Object if key is null.
private static Object maskNull(Object key) {
    return (key == null ? NULL_KEY : key);
}

final Object notNullKey = maskNull(key);
int len = table.length; // Length of Object[] table.
int idx = hash(notNullKey, len);

private static int hash(Object x, int length) {
    int h = System.identityHashCode(x);
    // Multiply by -127, and left-shift to use least bit as part of hash
    return ((h << 1) - (h << 8)) & (length - 1);
}
// Return default not null Object if key is null.
private static Object maskNull(Object key) {
    return (key == null ? NULL_KEY : key);
}

final Object notNullKey = maskNull(key);
int len = table.length; // Length of Object[] table.
int idx = hash(notNullKey, len);

// true or false?
notNullKey == table[idx]

table[index] = null;
table[index+1] = null;

package org.o7planning.identityhashmap.ex;

import java.util.IdentityHashMap;

public class IdentityHashMapEx1 {

    public static void main(String[] args) {
        String key1 = "Tom";
        String key2 = new String("Tom");
        
        // key1 == key2 ? false
        System.out.println("key1 == key2 ? " + (key1== key2)); // false

        IdentityHashMap<String, String> map = new IdentityHashMap<String, String>();
        
        map.put(key1, "Value 1");
        map.put(key2, "Value 2");
        
        System.out.println("Map Size: " + map.size());
        
        System.out.println(map);
    }  
}