Java重写继承的方法

Question

So I naively wrote an abstract class (A game board) containing a field with no access modifier for an array used to store the game state (a Square[]). This class has various methods implemented using this Square[]. I also have two subclasses both inheriting from this class, using these same methods from their parent class.

Now, for one reason or another, I have to make my Square[] private. I thought a simple fix would be to make a getter method as usual,

Class Board {
...
    /**An array contain all the Squares for my board. */
    private Square[] squares;

    public Square[] getSquares() {
        return squares;
    }
}

class MutableBoard extends Board {

    /** An N x N board in initial configuration. */
    MutableBoard(int N) {
        _N = N;
        squares = new Square[_N * _N];
        for (int i = 0; i < squares.length; i++) {
            squares[i] = new Square();
        }
    }

    @Override
    void copy(Board board) {
        _N = board.size();
        _moves = board.numMoves();
        squares = new Square[board.getSquares().length]; //Errors here when used
        for (int i = 0; i < squares.length; i++) {
            squares[i] = board.squares[i].clone();
        }
    }
}

class ConstantBoard extends Board {

    /** A new ConstantBoard that allows a read-only view of BOARD. */
    ConstantBoard(Board board) {
        _board = board;
        squares = board.getSquares();
    }
}

but I keep getting null pointer exceptions every time I try to access or modify my array in my subclasses using inherited methods. I think because my Square[] is defined in my parent class, its trying to access Board's copy of the array, not the subclasses copy. Does anyone know an easy fix to this that doesn't involve just putting all my methods in both subclasses (and defeating the whole point of inheritance)?

For example, in my copy method, I want to be able to pass in both a MutableBoard, and Constant Board, but I think its trying to access a copy of Board's squares somehow, which isn't even initialized.

Answer 1

You should return Square[]

public Square[] getSquares() {
    return squares;
}

squares[] is of type Square , therefore the method should return a Square[] not a Board

You also need to make sure you have squares[] initialized

private Square[] squares = new Square[someSize];

for (int i = 0; i < suqares.length; i++){
    squares[i] = new Square();
}

Also, you haven't delcared _N before trying to initialize it in you MutableBoard

 _N = N;

public class MutableBoard extends Board {
    int _N;

    public MutablerBoard(int N){
        _N = N;
    }
}

-or- 

public abstract class Board {
    int _N;
}

same for _board

Also, you should make square[] in your Board class public static . You can't modify it from MutableBoard if its private and you can't access the "current state" of square from ConstantBoard if it is not static .

Answer 2

Didn't getSquares() should return squares?

public Square[] getSquares() {
        return squares;
 }

You declare that you want to return instance of Board class and you return array of Squares.

Answer 3

Assuming you mean:

public Square[] getSquares() {
 return squares;
}

You have not initialised the array squares . You'll need:

Square[] squares = new Square[SIZE];

Even then, although the array will be initialised, each Square in the array will still be null. You'll need to initialsie them too:

for (int i=0; i<SIZE; i++) {
  squares[i] = new Square();  //or some other way of getting a `Square`
}

Above, SIZE is some static final int indicating the number of squares.

EDIT:

Your MutableBoard cannot see the squares array in Board because its private, and subclasses do not inherit private fields or methods. The code you have should not compile. The squares variable in the MutableBoard constructor is not the squares field in Board .

I suggest you change squares to be protected so subclasses can inherit it.