equals() vs hashCode() in Java

equals() and hashCode() are fundamental methods defined in the java.lang.Object class.

They play a critical role in:

Object comparison
Collections (HashMap, HashSet)
Correct behavior of domain objects

equals()

equals() is used to check logical equality between two objects.

Default implementation in Object:

public boolean equals(Object obj) {
    return this == obj;
}

This means:
Default equals() checks reference equality, not content.

hashCode()

hashCode() returns an integer hash value representing the object.

Used by:

HashMap
HashSet
Hashtable

Default implementation:

Usually derived from memory address
Different objects → different hash codes

Java defines a strict contract between equals() and hashCode().

The Contract

1️⃣ If two objects are equal according to equals()

👉 they must have the same hashCode()

2️⃣ If two objects have the same hashCode()

👉 they may or may not be equal

Why This Contract Exists

Hash-based collections work in two steps:

Use hashCode() to find the bucket
Use equals() to find the exact object

If this contract is broken:

Objects become unreachable
Data loss happens
Bugs are very hard to detect

Why It Is Recommended to Override BOTH

Overriding only one breaks the contract.

Case 1️⃣ Override equals() ONLY ❌

@Override
public boolean equals(Object o) {
return this.id == ((Customer)o).id;
}

But hashCode() is still default.

Result:

equals() → true
hashCode() → different

HashSet treats them as different objects ❌

Case 2️⃣ Override hashCode() ONLY ❌

@Override
public int hashCode() {
return id;
}

But equals() still uses reference comparison.

Result:

Same bucket
equals() fails

Duplicate objects ❌

Case 3️⃣ Override BOTH (Correct) ✅

@Override
public boolean equals(Object o) {

if (this == o) return true;

if (!(o instanceof Customer)) return false;

Customer c = (Customer) o;
return id == c.id;
}

@Override
public int hashCode() {
return Objects.hash(id);
}

✔ Correct behavior
✔ Works in all collections

Where This Matters the Most

HashMap
HashSet
ConcurrentHashMap
Hibernate entities
Cache keys

In enterprise apps:

Incorrect equals/hashCode = production bugs

Where equals() DOES content comparison by default

1️⃣ String
2️⃣ Wrapper Classes
3️⃣ Enum
4️⃣ Record (Java 16+)
5️⃣ Collection Implementations