HashMap vs. IdentityHashMap

main.java

import java.util.*;

class Main {
	private static final Random random = new Random();
	
	static class A {
	} /// end of A
	
	static class B {
	    private final int hashCode = random.nextInt();
	    
	    @Override
	    public int hashCode() {
	        return hashCode;
	    }
	} /// end of B
	
	static final int keyCount = 100000;
	static final int iterations = 200;
	
	static void testIdentityHashCode() {
	    List<A> keys = new ArrayList<A>();
	    Map<A, Integer> map = new IdentityHashMap<A, Integer>();
	    for (int i = 0; i < keyCount; ++i) {
	        A key = new A();
	        keys.add(key);
	        map.put(key, i);
	    }
	    int sum = 0;
	    for (int j = 0; j < iterations; ++j) {
            for (A key : keys) {
                sum += map.get(key);
            }
	    }
	    System.err.println(sum);
	}
	
	static void testHashCode() {
	    List<B> keys = new ArrayList<B>();
	    Map<B, Integer> map = new HashMap<B, Integer>();
	    for (int i = 0; i < keyCount; ++i) {
	        B key = new B();
	        keys.add(key);
	        map.put(key, i);
	    }
	    int sum = 0;
	    for (int j = 0; j < iterations; ++j) {
            for (B key : keys) {
                sum += map.get(key);
            }
	    }
	    System.err.println(sum);
	}
	
	public static void main(String[] args) throws Exception {
	    for (;;) {
	        {
                long a = System.currentTimeMillis();
                testHashCode();
                System.err.println("hash code: " + (System.currentTimeMillis() - a));
	        }
	        {
                long a = System.currentTimeMillis();
                testIdentityHashCode();
                System.err.println("identity hash code: " + (System.currentTimeMillis() - a));
	        }
	        {
                long a = System.currentTimeMillis();
                testHashCode();
                System.err.println("hash code: " + (System.currentTimeMillis() - a));
	        }
	        {
                long a = System.currentTimeMillis();
                testIdentityHashCode();
                System.err.println("identity hash code: " + (System.currentTimeMillis() - a));
	        }
	    }
	}
} /// end of Main