The <Object>.equals function (by definition) must be equivalent in the following ways:

  • reflexive - x.equals(x)
  • symmetric - x.equals(y) && y.equals(x)
  • transitive - x.equals(y) && y.equals(z) && x.equals(z)
  • consistent - while (true) x.equals(y)
  • x.equals(null) == false

Implementing <Object>.equals

In writing an equals function, we perform the following routine:

  • Add the @Override annotation before the method signature
  • Check that the supplied obj is not null
  • Check that the classes match
  • Cast the obj to the correct type
  • Perform necessary equality checking
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class MyObject() {
  int i;
  String b;

  MyObject(int i, String s) {
    this.i = i;
    this.s = s;
  }

  @Override
  boolean equals(Object obj) {
    if (obj == null) return false;
    if (this.getClass() != obj.getClass()) return false;
    MyObject o = (MyObject) obj;

    return ( this.i == o.i && this.s.equals(o.s) );
  }
}

Note: Reminder, that to compare strings we need the <String>.equals function, rather than the == operator

Inherited classes

For inherited classes, it is beneficial to test the equality of their superclasses.

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class MyOtherNewObject extends MyNewObject {
  boolean b;

  // ...

  @Override
  boolean equals(Object obj) {
    if (obj == null) return false;
    if (this.getClass() != obj.getClass()) return false;

    MyObject n = (MyObject) obj;
    if (!super.equals(n)) return false;

    MyOtherNewObject o = (MyOtherNewObject) obj;
    return this.b == o.b;
  }

// ...