ArrayList of abstract subclasses (must have different hashcode)

Question

I am creating a game for a school project, and I have a 2 classes called Pieces and Powers, with subclasses Piece_Yellow, Piece_Blue ... and Power_Explode, Power_ChangeColor etc...

I was doing it with enum and someone from this website (mikera to be more precise) help me changing that and creating a better interaction with the pieces and the powers.

But now I have to change the old code, and I have problems with that because I was doing like this :

int x = 10, y = 5;
for (int i=0; i<10; i++) {
    if (pecas[x][y] == null)
        pecas[x][y] = new Piece(arrayPecas.get(rand.nextInt(arrayPecas.size())));
}

Like this my array was partially filled with 10 new pieces Objects with different colors (but no powers, that was the problem), and every single one had a different hashcode (for finding , comparing and deleting the pieces later)...

But Since we can't initiliaze an abstract class, the only solution that I've found to this problem was to do this :

int x = 10, y = 5;
for (int i=0; i<10; i++) {
    if (pecas[x][y] == null)
        pecas[x][y] = arrayPecas.get(rand.nextInt(arrayPecas.size()));
}

Like this the object is still added, but now I have 1 problems : 1 - All the hashcode from the pieces with the same color are the same... I don't really know how to solve this. I've read that I could override the hashcode method but there is no information to make the difference between them (and I can't store the position because I had to change it every time the piece change position).

Answer 1

It doesn't seem that really the hashCodes that are your problem. It's that the objects are simply equal. Code that relies on two unequal objects having different hashCodes isn't strictly correct. return 1; is a perfectly legal implementation of Object#hashCode().

The simplest thing seems to be to put a clone() method on Piece that all the subclasses can implement in order to return copies in places where you want different, distinct, objects.

public abstract class Piece {

  public abstract Piece clonePiece();

}

public class YellowPiece extends Piece {

  @Override
  public Piece clonePiece() {
    return new YellowPiece(this.relevantThing1, this.relevantThing2 // etc etc)
  }
}

int x = 10, y = 5;
for (int i=0; i<10; i++) {
    if (pecas[x][y] == null)
        pecas[x][y] = arrayPecas.get(rand.nextInt(arrayPecas.size())).clonePiece();
}