Java: What is the difference between WeakHashMap and IdentityHashMap?

1// WeakHashMap — keys auto-removed when GC'd
2WeakHashMap<Object, String> metadata = new WeakHashMap<>();
3
4Object key1 = new Object();
5metadata.put(key1, "data for key1");
6key1 = null; // Key eligible for GC
7
8System.gc();
9Thread.sleep(100); // Give GC time
10
11// Entry may be removed — key was GC'd
12System.out.println(metadata.size()); // May be 0
13
14// Practical use: metadata cache
15WeakHashMap<Class<?>, String> classNameCache = new WeakHashMap<>();
16
17public String getClassName(Class<?> clazz) {
18    return classNameCache.computeIfAbsent(clazz,
19        c -> c.getSimpleName() + "@" + System.identityHashCode(c));
20}
21// Cache entries removed when Class objects are unloaded

1// IdentityHashMap — uses == for keys
2Map<Object, String> map = new IdentityHashMap<>();
3
4String key1 = new String("hello");
5String key2 = new String("hello");
6
7map.put(key1, "first");
8map.put(key2, "second");
9
10// key1 == key2 is FALSE (different objects)
11// Two entries! key1.equals(key2) is TRUE, but == is FALSE
12System.out.println(map.size()); // 2
13
14// Object graph traversal
15public void traverseGraph(Object root) {
16    Map<Object, String> visited = new IdentityHashMap<>();
17    Deque<Object> stack = new ArrayDeque<>();
18    stack.push(root);
19
20    while (!stack.isEmpty()) {
21        Object obj = stack.pop();
22        if (visited.containsKey(obj)) continue;
23        visited.put(obj, "visited");
24        // Process object...
25    }
26}
27// IdentityHashMap prevents revisiting the same object instance