A.java

public interface A {
  public int compareTo(A other);
}

B.java

public class B implements A {
  private String superSecretName;

  // other stuff
  // NO GETTER for superSecretName

  @Override
  public int compareTo(A other) {
    ...
    if (!superSecretName.equals(other.superSecretName)) { return 1; }
    ...
  }
}

C.java

public class C implements A {
  private long superSecretNumber ;

  // other stuff
  // NO GETTER for superSecretName

  @Override
  public int compareTo(A other) {
    ...
    if (superSecretNumber != other.superSecretNumber) { return superSecretNumber - other.superSecretNumber; }
    ...
  }
}

D.java

public class D extends B, C {


  @Override
  public int compareTo(A other) {
    // Now what?
  }
}