Code Smell 281 - When Equals and HashCodes Misbehave

by Maximiliano ContieriNovember 25th, 2024

Too Long; Didn't Read

Misaligned equals() and hashCode() methods can break hash collections, causing bugs, poor performance, and duplicate entries. Avoid mutable keys, use effective hashing, and stick to best practices for reliable collections.

featured image - Code Smell 281 - When Equals and HashCodes Misbehave

Misaligned equals() and hashCode() break collections.

Problems

The least surprise principle violation
Contract violations
Mutable key issues
Duplicate hash codes
Debugging becomes hard
Poor hash distribution

Solutions

Avoid mutable keys
Use effective hashes
Test behavior carefully
Avoid redefining equal and hash
Honor the Bijection

Context

When you work with hashed collections like HashMap or HashSet, you should pay special attention to equals() and hashCode().

A mismatch or poor implementation can lead to unpredictable bugs.

equals() method defines logical equality, while hashCode() determines an object’s bucket for faster access.

When these two methods fail to align, collections lose their reliability, leading to poor performance or issues like duplicate entries caused by hash collections.

The best solution is never to override the hash and equals and rely on object identity.

This is what happens in the real world using the MAPPER).

Whenever you get an external object you need to map it to your bijection correspondence and not create a brand new one.

Once within your controlled system, rely on identity and forget equality issues.

Sample Code

Wrong

class BrokenObject {
    private int value;

    public BrokenObject(int value) {
        this.value = value;
    }

    @Override
    public boolean equals(Object obj) {
        return true; // Always equal
    }

    @Override
    public int hashCode() {
        return super.hashCode(); // Uses default implementation
    }
}

Right

class FixedObject {
    private final int value;

    public FixedObject(int value) {
        this.value = value;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        FixedObject that = (FixedObject) obj;
        return value == that.value;
    }

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

// This is the best solution

class CleanObject {
    private final int value;

    public FixedObject(int value) {
        this.value = value;
    }

    // - @Override
    // - public boolean equals(Object obj) {}

    // - @Override
    // - public int hashCode() { 
    }
}