HashMap vs WeakHashMap vs IdentityHashMap

HashMap uses equals() for key comparison and keeps entries strongly
WeakHashMap allows entries to be removed by GC when keys are no longer referenced
IdentityHashMap uses == instead of equals() for key comparison

Core Difference

Point	HashMap	WeakHashMap	IdentityHashMap
Key comparison	equals()	equals()	== (reference)
Reference type	Strong	Weak (keys)	Strong
GC behavior	Entries not GCed	Keys can be GCed	Entries not GCed
Allows null key	✅ One	✅ One	✅ Multiple
Allows null values	✅ Yes	✅ Yes	✅ Yes
Typical use	General-purpose map	Cache / memory-sensitive	Reference-based keys
Commonly used	✅ Very common	⚠️ Less common	⚠️ Rare

HashMap

A HashMap is the most commonly used Map implementation.

Characteristics

Uses hashCode() and equals() for key comparison
Holds strong references to keys and values
Entries remain until explicitly removed

When to use

General-purpose key–value storage
When you control lifecycle of keys

WeakHashMap

A WeakHashMap stores keys using weak references.

Characteristics

If a key is no longer strongly referenced, GC can remove the entry
Helps avoid memory leaks
Values are removed automatically when key is GCed

Typical use cases

Caches
Metadata storage
Listener registries

“WeakHashMap entries can be garbage collected when keys are no longer referenced.”

IdentityHashMap

An IdentityHashMap compares keys using reference equality (==).

Characteristics

Ignores equals() method
Two different objects with same content are treated as different keys
Allows multiple null keys (because comparison is reference-based)

Typical use cases

Framework internals
Object graph processing
When identity matters more than equality

HashMap → Normal map
WeakHashMap → GC-aware map
IdentityHashMap → Reference-based map

HashMap uses equals() for key comparison, WeakHashMap allows entries to be garbage collected when keys are weakly referenced, and IdentityHashMap uses reference equality (==) for keys.