IdentityHashMap

Java中的hashMap是不允许有重复的key存在的，那么什么才是重复的呢？其实就是说什么才是相等的？

Hashmap中用equal来判断相等。

IdentityHashMap中用==来判断相等。

例子：

public class IdentityHashMapTst {
	public static void main(String[] args) {
		Map<Integer,String> map1 = new HashMap<Integer,String>();
		Map<Integer,String> map2 = new IdentityHashMap<Integer,String>();
		
		Integer i1, i2;
		
		i1 = 1;
		i2 = 1;
		System.out.println("== \t"+ (i1==i2));
		System.out.println("equals \t"+ (i1.equals(i2)));
		
		map1.put(i1, "22");		map1.put(i2, "22");
		map2.put(i1, "22");		map2.put(i2, "22");
		System.out.println(map1.keySet().size());
		System.out.println(map2.keySet().size());

		i1 = 555;
		i2 = 555;
		System.out.println("== \t"+ (i1==i2));
		System.out.println("equals \t"+ (i1.equals(i2)));

		map1.put(i1, "22");		map1.put(i2, "22");
		map2.put(i1, "22");		map2.put(i2, "22");
		System.out.println(map1.keySet().size());
		System.out.println(map2.keySet().size());
	}
}

运行结果：

写道

== true
equals true
1
1
== false
equals true
2
3