Accessing Subclass Field from Array of Superclass?

Question

I just recently started learning java, so I'm having a little trouble understanding what I think is a polymorphism issue. I'm trying to program a chess game, so I have a an array of superclass called GamePiece for which the subclass PawnPiece extends. I have a method in both classes called MovePiece() which changes the position of the piece. When I use MovePiece(), changes the position values of the PawnPiece, but when I try to call the positions in my main code, it gives me the unchanged position of 'GamePiece'. Here's some of my code:

public class GamePiece {
    int position;

    //Constructor
    public GamePiece(int x){
       position=x;
    }
    public void MovePiece(int positionOfMove){};
}

public class PawnPiece extends GamePiece{
    int positionPawn;

    //Subclass Constructor
    public PawnPiece(int x){
        super(x);
    }

    public void MovePiece(int positionOfMovePawn){
       positionPawn=x;
}

public classChessMain{
    public static void main(String[] arg){
        GamePiece[] allPieces = new GamePiece[32];
        int startPositionPawn = 9;     //Arbitrary#
        allPieces[8]=new PawnPiece(int startPositionPawn); //index is arbitrary
        int playerMove = 5;     //Arbitrary#
        allPieces[8].MovePiece(playerMove);
    }
}

The last line gives me the initial position (9 in this case), but I know if I could access the position of PawnPiece, it would give me 5. Any help all you coding wizards out there? I'd really appreciate it.

Answer 1

A couple issues:

1. The class GamePiece and the method MovePiece should be abstract. This way, you will force any subclass that chooses to be a GamePiece to implement its own MovePiece method.

2. You are storing the position in two places:

   • You have int position in GamePiece
   • You have int positionPawn in PawnPiece.

   You probably want only one of these: get rid of the positionPawn in PawnPiece and use position in GamePiece.

3. (Optional) Java convention is to have method names begin with a lower case letter: rename MovePiece to movePiece

Answer 2

From the code given I can guess that in the PawnPiece you are updating variable positionPawn, and then check variable position. So you are writing to wrong place.