Can Two Java Objects with Identical Hashcodes Be Unequal?

Do two Java objects with the same hash codes mean they are equal?

In Java, the relationship between hash codes and object equality is defined by the `hashCode()` and `equals()` methods. However, it is important to understand that two objects can have the same hash code but still be unequal, which is a consequence of how these methods are implemented and the nature of hash functions.

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

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

Causes

• Hash collisions: Different objects can produce the same hash code due to the properties of hash functions, leading to what is known as a hash collision.
• Incorrect implementation of the `equals()` method: If two objects do not correctly override the `equals()` method, they might superficially appear equal based on hash codes, but logically they are not equivalent.
• Different states of objects: If the objects maintain state that has changed over time, they might end up with the same hash code while being logically different.

Solutions

• Always override both `hashCode()` and `equals()` methods together following the contracts defined in the Object class.
• Ensure that the `equals()` method is based on the fields that are used in the `hashCode()` method to maintain consistency.
• Use IDE-generated methods or libraries to minimize mistakes when overriding these methods.

Mistake: Not overriding hashCode() when overriding equals().

Solution: Always implement `hashCode()` if you override `equals()` to ensure consistent behavior.

Mistake: Using mutable fields in hashCode() and equals().

Solution: Use immutable fields for hash code and equality comparisons to avoid inconsistent results.