Java.Collections.What is the difference between HashMap and IdentityHashMap? What is IdentityHashMap used for?

Let’s get into the difference between HashMap and IdentityHashMap, and when to use this strange beast of a map.

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🎯 TL;DR:

✅ HashMap compares keys using .equals()
✅ IdentityHashMap compares keys using == (object reference equality)

That’s the core difference — but it leads to some very different behaviors.

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🧪 HashMap: Normal, Everyday Map

• Uses .equals() and .hashCode() to compare keys.
• Two different objects with the same content are treated as the same key.

Map<String, String> map = new HashMap<>();
map.put(new String("key"), "value1");
map.put(new String("key"), "value2");

System.out.println(map.size()); // 1 — keys are equal by content

🧪 IdentityHashMap: Compares by ==

• Uses reference equality — two keys are equal only if they are the same object.
• Ignores .equals() and .hashCode().

Map<String, String> map = new IdentityHashMap<>();
map.put(new String("key"), "value1");
map.put(new String("key"), "value2");

System.out.println(map.size()); // 2 — keys are different objects!

🔍 Use Cases for IdentityHashMap

You don’t use IdentityHashMap in everyday apps. It’s used in very specific cases, like:

✅ 1. Object identity tracking

• You want to associate data with exact object instances, not logically equal objects.
• Useful in graph algorithms or serialization frameworks.

✅ 2. Performance optimization

• Avoids calling equals() and hashCode(), which might be expensive for large objects.
• But only when you care about references, not content.

✅ 3. Java internals

• Used internally by the JVM, e.g., in ClassLoaders or tracking references during garbage collection, etc.

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⚠️ Gotchas

• IdentityHashMap violates Map contract in some cases (e.g., it doesn’t behave like a Set of keys in the usual way).
• Iteration order is also not guaranteed or predictable.
• Dangerous to use unless you really know what you’re doing.

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🧠 Summary Table

Feature	HashMap	IdentityHashMap
Key equality	.equals()	== (reference equality)
Hashing	.hashCode()	System.identityHashCode()
Allows duplicates	❌ No (based on content)	✅ Yes (if different instances)
Use cases	General-purpose	Object tracking, performance
Null keys	✅ 1 allowed	✅ Multiple allowed