Why is it important to override the equals() and hashCode() methods together in Java?

In Java, the methods equals() and hashCode() are closely related and are used for object comparison and working with hash-based collections (e.g., HashMap, HashSet).

If you override equals() but not hashCode(), objects considered equal by equals() may have different hash codes. This violates the contract that equal objects must have the same hashCode(). As a result, hash-based collections may not work correctly: for example, they may fail to find an object or may store duplicates.

Therefore, it is important to override both methods together to ensure correct behavior of objects in collections and during comparison.

Example of overriding:

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    MyClass myClass = (MyClass) o;
    return field1.equals(myClass.field1) && field2 == myClass.field2;
}

@Override
public int hashCode() {
    return Objects.hash(field1, field2);
}