The equals contract

Definition

equals says when two objects count as the same value. A correct equals is reflexive (x equals x), symmetric (if x equals y then y equals x), transitive, consistent and never true for null. Always take an Object parameter so you really override it.

Example code

A correct equals method

class Email {
    private final String address;

    Email(String address) { this.address = address.toLowerCase(); }

    @Override
    public boolean equals(Object other) {
        if (this == other) return true;                      // reflexive
        if (!(other instanceof Email that)) return false;    // handles null and other types
        return address.equals(that.address);
    }

    @Override
    public int hashCode() { return address.hashCode(); }     // always override both together
}

class Demo {
    public static void main(String[] args) {
        Email a = new Email("Asha@Example.com");
        Email b = new Email("asha@example.com");
        System.out.println(a.equals(b));
        System.out.println(b.equals(a));          // symmetric
        System.out.println(a.equals(null));       // never true for null
        System.out.println(a == b);               // == compares identity, not value
    }
}

// Output:
// true
// true
// false
// false