What is the difference between HashMap and IdentityHashMap?

The main difference lies in how they compare keys.

• HashMap uses the equals() method and the hash code of the key to determine equality, which corresponds to the standard semantics of object equality.
• IdentityHashMap compares keys strictly by reference (using ==), ignoring the equals() method. This means that two different objects, even if they are "equal" according to their equals() method, will be considered different keys in IdentityHashMap if they have different memory addresses.

Example:

// HashMap
Object key1 = new String(" key");
Object key2 = new String(" key");
Map<Object, String> hashMap = new HashMap<>();
hashMap.put(key1, "value 1");
hashMap.put(key2, "value 2"); // overwrites the value for " key"

// IdentityHashMap
Map<Object, String> identityHashMap = new IdentityHashMap<>();
identityHashMap.put(key1, "value 1");
identityHashMap.put(key2, "value 2"); // adds a second entry, since key1 != key2 by reference

IdentityHashMap is used less frequently, for example:

• During serialization/deserialization, when object identity within a single object graph is important.
• In some specific algorithms where objects need to be distinguished by their identity rather than their logical equivalence.
• For detecting cyclic references.