override int... param with int[] breaks polymorphism

[Richard Hayward]

I've been trying a few experiments to explore K&B p74 q3.

Suppose I have a class Voop3 with method
void doStuff(int... doArgs){...}

Surprisingly, I can override that method in the descendant class Voop3Son with
void doStuff(int[] doArgs){...}

class Voop3Test {

  public static void main(String[] args) {
    Voop3Son v3s = new Voop3Son();
    // v3s.doStuff(1,2,3);  //compile failure!
    v3s.doStuff( new int[] {1,2,3} );

    Voop3 v3 = new Voop3();
    v3.doStuff(1,2,3,4);
    v3.doStuff( new int[] {1,2,3,4,5} );

    v3 = new Voop3Son();
    v3.doStuff(1,2,3,4,5,6);
    v3.doStuff(new int[] {1,2,3,4,5,6,7});
  }

}

class Voop3Son extends Voop3{
  void doStuff(int[] doArgs){
    System.out.println("Voop3Son doArgs.length=" + doArgs.length);
  }
}


class Voop3{
  void doStuff(int... doArgs){
    System.out.println("Voop3 doArgs.length=" + doArgs.length);
  }
}

The reason I find this surprising is that it seems to break polymorphism.

An instance of Voop3Son called v3s is-a Voop3.
I would expect it to be able to do anything a Voop3 instance could do.
In particular, Voop3 has a method
void doStuff(int... doArgs){...}
I could call that method on an instance of Voop3, as I do at line 09.
However, if I try and make that same method call on v3s, as at line 05, it won't even compile.

In order to call that method, with those parameters, on a Voop3Son instance I have to refer to it via Voop3 reference variable as at line 13.

Isn't this breaking polymorphism?

Regards
Richard

[Himai Minh]

Var-args is equivalent to an array of args.
For example:
doStuff(int... a) is the same as doStuff(int[] a)

In your example, Voop3Son overrides the doStuff method in Voop3.

I hope it makes sense.

[Richard Hayward]

Hi Himai,

Himai Minh wrote:
doStuff(int... a) is the same as doStuff(int[] a)

The two versions of doStuff are not the same.

class EquivalenceTest {

  public void doStuff(int... a) {} 

  public static void main(String[] args){
    EquivalenceTest et = new EquivalenceTest();
    et.doStuff(1,2,3);
  }
 
}

will compile

class EquivalenceTest {

  public void doStuff(int[] a) {} 

  public static void main(String[] args){
    EquivalenceTest et = new EquivalenceTest();
    et.doStuff(1,2,3);  //compile failure!
  }
 
}

Will not compile.

Regards
Richard

[Himai Minh]

Yes. They are not the same.
But var-args is treated the same as an array of args.

For the doStuff(int...a) method , you can pass in new int [] {1,2,3} as the argument. I am sure it will compile.