Overriding equals without hashCode

Definition

If you override equals but not hashCode, two equal objects usually get different hash codes. A HashSet or HashMap then looks in the wrong bucket, so it usually cannot find an object that is equal to one it already holds, and duplicates can appear.

Example code

Equal objects with different hash codes, and the fix

import java.util.HashSet;
import java.util.Set;

class BadId {
    final int value;
    BadId(int value) { this.value = value; }
    @Override public boolean equals(Object o) { return o instanceof BadId b && b.value == value; }
    // no hashCode(): it inherits Object.hashCode(), which is different for every object
}

class GoodId {
    final int value;
    GoodId(int value) { this.value = value; }
    @Override public boolean equals(Object o) { return o instanceof GoodId g && g.value == value; }
    @Override public int hashCode() { return Integer.hashCode(value); }
}

class Demo {
    public static void main(String[] args) {
        BadId a = new BadId(7), b = new BadId(7);
        System.out.println("equal? " + a.equals(b));
        System.out.println("same hash? " + (a.hashCode() == b.hashCode()));      // false: hash collections get confused

        Set<GoodId> good = new HashSet<>();
        good.add(new GoodId(7));
        System.out.println("found? " + good.contains(new GoodId(7)));            // true: equal objects share a hash
    }
}

// Output:
// equal? true
// same hash? false
// found? true