IdentityHashMap

Overview

• IdentityHashMap is a Map implementation that uses reference equality (==) instead of equals()
• Part of java.util
• Used when object identity matters more than logical equality

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Key Difference

👉 Normal HashMap:

key1.equals(key2)

👉 IdentityHashMap:

key1 == key2

What does this mean?

Two objects:

String a = new String("A");
String b = new String("A");

a.equals(b) → ✅ true
a == b → ❌ false

Behavior

public static void main(String[] args) {
    Map<String, String> map = new IdentityHashMap<>();

    map.put(a, "First");
    map.put(b, "Second");

    System.out.println(map);
}

👉 Output:

{A=First, A=Second}

✔ Both entries exist (treated as different keys)

Key Characteristics

Feature	Description
Equality Check	Uses == (reference equality)
Duplicates	Allowed (if different objects)
Order	Not guaranteed
Thread Safety	Not synchronized
Null Keys	Allowed

Internal Working

• Uses array-based structure, not standard buckets like HashMap
• Stores keys and values in alternating slots:

  [ key1, value1, key2, value2, … ]

• Uses System.identityHashCode() instead of hashCode()

Important Methods Behavior

Method Signature	Description	Behavior / Edge Case
V put(K key, V value)	Adds entry	Allows logically equal but different objects
V get(Object key)	Retrieves value	Works only if same reference used
boolean containsKey(Object key)	Checks key	Uses ==, not equals()
V remove(Object key)	Removes entry	Removes only exact reference

Code Examples

1. Identity vs Equality

public static void main(String[] args) {
    Map<String, String> map = new IdentityHashMap<>();

    String a = new String("A");
    String b = new String("A");

    map.put(a, "First");

    System.out.println(map.get(b)); // null ❌
    
}

👉 Because a != b (different references)

2. Normal HashMap Comparison

public static void main(String[] args) {
    Map<String, String> map = new HashMap<>();

    String a = new String("A");
    String b = new String("A");

    map.put(a, "First");

    System.out.println(map.get(b)); // First ✅
    
}

When to Use

When object identity matters

Examples:

• Object graph processing
• Serialization frameworks
• Tracking object references (not values)
• Proxy/debugging tools

When NOT to Use

• General-purpose mapping ❌
• When logical equality matters ❌
• Most business applications ❌

IdentityHashMap vs HashMap

Feature	HashMap	IdentityHashMap
Equality	equals()	==
Hashing	hashCode()	identityHashCode()
Duplicate keys	Not allowed	Allowed (if different refs)
Use case	General	Special cases

Important Gotchas ⚠️

1. Retrieval issue

map.put(new String(“A”), “Value”);

map.get(new String(“A”)); // null ❌

1. Breaks Map contract (subtle point)

• Map contract expects equals() usage
• IdentityHashMap violates this expectation intentionally

Developer Notes

• Rarely used in normal applications
• Useful in framework-level programming
• Do NOT replace HashMap with this casually