How to implement equals and hashCode

Short answer

If two objects are equal by equals(), they must have the same hashCode(). That is why these methods are always overridden together:

class Point {
    final int x;
    final int y;
 
    Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
 
    @Override
    public boolean equals(Object o) {
        return o instanceof Point other && x == other.x && y == other.y;
    }
 
    @Override
    public int hashCode() {
        return Objects.hash(x, y);
    }
}
 
void main() {
    IO.println(new Point(1, 2).equals(new Point(1, 2)));
    IO.println(new HashSet<>(List.of(new Point(1, 2))).contains(new Point(1, 2)));
}

Output:

true
true

Simpler — a record

A record generates both methods from all its fields:

record Point(int x, int y) {}
 
void main() {
    IO.println(new Point(1, 2).equals(new Point(1, 2)));
    IO.println(new Point(1, 2).hashCode() == new Point(1, 2).hashCode());
}

Output:

true
true

Comparing by some fields

Sometimes only an identifier defines equality. Use the same fields in both methods:

class User {
    final long id;
    String name;
 
    User(long id, String name) {
        this.id = id;
        this.name = name;
    }
 
    @Override
    public boolean equals(Object o) {
        return o instanceof User other && id == other.id;
    }
 
    @Override
    public int hashCode() {
        return Long.hashCode(id);
    }
}
 
void main() {
    IO.println(new User(7, "Anna").equals(new User(7, "Anna Smith")));
}

Output:

true

Strings and null

For object fields that may be null, use Objects.equals():

class Contact {
    final String email;
 
    Contact(String email) {
        this.email = email;
    }
 
    @Override
    public boolean equals(Object o) {
        return o instanceof Contact other && Objects.equals(email, other.email);
    }
 
    @Override
    public int hashCode() {
        return Objects.hashCode(email);
    }
}
 
void main() {
    IO.println(new Contact(null).equals(new Contact(null)));
}

Output:

true

Common mistake

Overriding only equals(). The objects are equal, but a HashSet cannot find them, because it looks them up by different hash codes:

class Point {
    final int x;
 
    Point(int x) {
        this.x = x;
    }
 
    @Override
    public boolean equals(Object o) {
        return o instanceof Point other && x == other.x;
    }
}
 
void main() {
    var set = new HashSet<Point>();
    set.add(new Point(1));
 
    IO.println(new Point(1).equals(new Point(1)));
    IO.println(set.contains(new Point(1)));
}

Output:

true
false

Thanks for reading!

Short answer

Simpler — a record

Comparing by some fields

Strings and null

Common mistake

Functions used in this recipe

Thanks for reading!