java - Android - 构造函数变量中的空指针异常

Question

我正在为 Android 应用程序中的 GridView 开发自定义适配器。其定义如下:

public class BoardAdapter  extends BaseAdapter {

public static final int EMPTY = 0;
public static final int RED = 1;
public static final int BLACK = 2;
public static final int RED_PROMOTED = 3;
public static final int BLACK_PROMOTED = 4;


Context context;
int[][] board = null;


public int[] pieces = new int [64];
{
        for(int i=0;i<8;i++)
        {
            for(int j=0;j<8;j++)
            {
    //null pointer exception here           
                if(board[i][j] == RED)
                {
                    pieces[8*i+j] = R.drawable.red_piece;
                }
                else if(board[i][j] == BLACK)
                {
                    pieces[8*i+j] = R.drawable.black_piece;
                }
                else pieces[8*i+j] = 0;
            }
        }
};


public BoardAdapter (Context ctx, int[][] board)
{
    this.context = ctx;
    this.board = board;
}

@Override
public int getCount() {
    // TODO Auto-generated method stub
    return pieces.length;
}

@Override
public Object getItem(int pos) {
    // TODO Auto-generated method stub
    return 0;
}

@Override
public long getItemId(int pos) {
    // TODO Auto-generated method stub
    return pieces[pos];
}

@Override
public View getView(int pos, View convertView, ViewGroup parent) {
    // TODO Auto-generated method stub
    ImageView imageView = new ImageView(context);
    imageView.setImageResource(pieces[pos]);
    imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
    imageView.setLayoutParams(new GridView.LayoutParams(100, 100));
    return imageView;       
}

}

我在 Activity 的 onCreate 方法中创建对象:

    protected void onCreate(Bundle savedInstanceState) 
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.game);  
    board = (GridView) findViewById (R.id.board);
    game.printBoard();
    //null pointer exception here
    board.setAdapter(new BoardAdapter(this, game.getBoard()));

}

当我打印电路板时，日志显示正确的值。所以我确信我将初始化的板传递给 BoardAdapter 构造函数。我不知道为什么在对象创建时引用该数组的元素时会抛出空指针异常...

Answer 1

由于您编写了一个初始化程序 block 并使用了空的board。

public int[] pieces = new int [64];

//您在这里启动了一个 block 。请注意board尚未初始化。

{
        for(int i=0;i<8;i++)
        {
            for(int j=0;j<8;j++)
            {
    //null pointer exception here           
                if(board[i][j] == RED)
                {
                    pieces[8*i+j] = R.drawable.red_piece;
                }
                else if(board[i][j] == BLACK)
                {
                    pieces[8*i+j] = R.drawable.black_piece;
                }
                else pieces[8*i+j] = 0;
            }
        }
};

您要做的就是创建一个名为 intializer 的方法并在那里执行

public int[] pieces = new int [64];
private void intialize(){

{
        for(int i=0;i<8;i++)
        {
            for(int j=0;j<8;j++)
            {
    //null pointer exception here           
                if(board[i][j] == RED)
                {
                    pieces[8*i+j] = R.drawable.red_piece;
                }
                else if(board[i][j] == BLACK)
                {
                    pieces[8*i+j] = R.drawable.black_piece;
                }
                else pieces[8*i+j] = 0;
            }
        }
}

}

现在在构造函数中调用该方法。

public BoardAdapter (Context ctx, int[][] board)
{
    this.context = ctx;
    this.board = board;
    intialize();
}