Understanding equals() and hashCode() in Java

The Contract Between equals() and hashCode()

In Java, the equals() and hashCode() methods, both defined in the Object class, are fundamental for determining object equality. When you create custom classes, you often need to override equals() to define logical equality (based on object state) rather than reference equality (based on memory address).

However, there’s a critical rule you must follow: If you override equals(), you MUST override hashCode().

This is because of the contract between them, which is essential for the correct functioning of hash-based collections like HashMap, HashSet, and Hashtable.

The contract states:

1. If two objects are equal according to the equals(Object) method, then calling the hashCode() method on each of the two objects must produce the same integer result.
2. It is not required that if two objects are unequal according to the equals(Object) method, then calling the hashCode() method on each of the two objects must produce distinct integer results. However, producing distinct results for unequal objects may improve the performance of hash tables.

Why is this Contract Important?

Hash-based collections use the hashCode() method to determine where to store an object in memory (which “bucket” to put it in). When you try to retrieve an object (e.g., with map.get(key) or set.contains(object)), the collection does the following:

1. It calculates the hash code of the object you’re looking for.
2. It uses this hash code to quickly find the bucket where the object should be.
3. It then iterates through the (usually small) number of objects in that bucket, using the equals() method to find the exact match.

What Happens if You Break the Contract?

Let’s say you have a User class and you override equals() but not hashCode().

class User {
    private int id;
    private String email;

    // Constructor, getters...

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        User user = (User) o;
        return id == user.id && email.equals(user.email);
    }

    // Missing hashCode() override!
}

Now, let’s use this class in a HashSet:

User user1 = new User(1, "test@example.com");
User user2 = new User(1, "test@example.com");

System.out.println("user1.equals(user2): " + user1.equals(user2)); // true

Set<User> userSet = new HashSet<>();
userSet.add(user1);

System.out.println("userSet.contains(user2): " + userSet.contains(user2)); // false!

Why does contains() return false?

1. When userSet.add(user1) is called, HashSet calculates user1.hashCode() (using the default implementation from Object, which is based on memory address) and stores user1 in a bucket corresponding to that hash code.
2. When userSet.contains(user2) is called, HashSet calculates user2.hashCode(). Since user1 and user2 are different objects in memory, their default hash codes are different.
3. HashSet looks in the bucket for user2’s hash code, which is a different bucket from where user1 is stored. It doesn’t find anything, so it immediately returns false without ever calling equals().

The collection fails to find an object that is logically equal because the broken contract led it to look in the wrong place.

How to Correctly Override hashCode()

To fix this, you must implement hashCode() so that it produces the same hash for objects that are considered equal. A good hashCode() implementation should use the same fields that are used in the equals() method.

Correct Implementation:

import java.util.Objects;

class User {
    private int id;
    private String email;

    // Constructor, getters...

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        User user = (User) o;
        return id == user.id && Objects.equals(email, user.email);
    }

    @Override
    public int hashCode() {
        // Use Objects.hash() to easily generate a hash code from the fields.
        return Objects.hash(id, email);
    }
}

With this corrected hashCode(), user1.hashCode() and user2.hashCode() will be the same. The HashSet will now correctly find the bucket and then use equals() to confirm the match, returning true for contains().

Key Takeaway

Always override hashCode() when you override equals(). The easiest and safest way to do this is by using the java.util.Objects.hash() utility method, passing it the same fields you used in your equals() implementation.