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我正在为 android 应用程序中的网格视图开发自定义适配器。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;       
}

}

我在活动的 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()));

}

当我打印板时，日志显示正确的值。所以我确信，我将一个初始化的板传递给 BoardAdapter 构造函数。我不知道，为什么在创建对象时引用该数组的元素时会引发空指针异常...

4

2 回答 2

3

由于您编写了一个initializer 块并使用了空板。

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

//你在这里开始了。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;
            }
        }
};

您所做的是创建一个名为 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();
}
于 2013-09-25T14:24:35.570 回答
1

按处理顺序

...
int[][] board = null; // this.board value assigned as null

public int[] pieces = new int [64]; //defining value, doesn't matter now

//WARNING this is an instance initializer block! Gets to run before the code of the constructor...
{ 
    for(int i=0;i<8;i++)
    {
        for(int j=0;j<8;j++)
        {
//null pointer exception here           
            if(board[i][j] == RED) //access stuff that does not exist...
            {
                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;
        }
    }
};

...

在未来的某个地方，构造函数将被称为

  ...
  this.board = board; //board is assigned a value
  ...

编辑

I want to assign the values of a 2 dimensional array board into a one-dimensional array pieces, so that I can get the appropriate images for the grid elements

然后你应该创建一个方法来完成它，而不是静态初始化块（假设 voard 总是 8 x 8，并且不为空）：

public int[] getPieces()
{ 
    int[] pieces = new int[64];
    for(int i=0;i<8;i++)
    {
        for(int j=0;j<8;j++)
        {
            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;
        }
    }
    return pieces;
};

并随时在 BoardAdapter 实例上调用它。

于 2013-09-25T14:25:22.750 回答