Enforce “equals” in an interface

Question

I have an interface and I want everyone who implements this interface to implements an overrridden \"equals\" method.

Is there a way to make sure that happens?

Answer 1

No, you only can create an abstract class instead of an interface like this:

public abstract class MyApi {

  public final boolean equals(Object other) {
    if (other == this) {
      return true;
    }
    if (other instanceof MyApi) {
      return equals((MyApi)other);
    }
    return false;
  }

  protected abstract boolean equals(MyApi other);

}

or a more simple version:

public abstract class MyApi {

  public boolean equals(Object other) {
    throw new UnsupportedOperationException("equals() not overridden: " + getClass());
  }

}

EDIT (gave it a try after the comment from @CodeConfident, thanks! Never assumed it would work):

You can also simply declare equals() in an abstract class (not in an interface!) and therefore hide the Object implementation and enforce a new implementation in any subclass:

public abstract class MyApi {

  public abstract boolean equals(Object obj);

  public abstract int hashCode();

}

Anyway you should always implement equals() and hashCode() together to fulfil the contract.