Collections: IdentityHashMap compares keys with ==, not equals

IdentityHashMap uses reference identity for both keys and values, so two equal-but-distinct String objects occupy separate entries where a normal HashMap would merge them into one.

Code

Map<String, Integer> identity = new IdentityHashMap<>();
String a = new String("key");
String b = new String("key");
identity.put(a, 1);
identity.put(b, 2);
System.out.println("IdentityHashMap size: " + identity.size());
System.out.println("a.equals(b): " + a.equals(b));
Map<String, Integer> normal = new HashMap<>();
normal.put(a, 1);
normal.put(b, 2);
System.out.println("HashMap size: " + normal.size());

Output

IdentityHashMap size: 2
a.equals(b): true
HashMap size: 1