GUI类在监视器上不显示任何内容

Question

我是新的GUI，我想知道为什么这段代码不显示任何东西。请记住，代码是不完整的，请原谅我的家伙，代码有点长。

//imports
public class SudokuGame extends Frame{
private SudokuBoard sb = new SudokuBoard(this);
/**
 * a button to restart game
 */
private Button b = new Button("Restart");
/**
 * 9 x 9 buttons to store the numbers used in the game
 */
private Button[][] grid= new Button[9][9];
/**
 * TextField used to store the remaining empty blocks
 */
private TextField t;
/**
 * Constructor
 * @param ssm
 */
public SudokuGame(SomeSudokuMatrix ssm) {
    /**
     * sets window's name
     */
    super( "Sudoku" );
    /**
     * sets background color
     */
    setBackground( Color.WHITE );


    /**
     * adds SudokuBoard to the SudokuGame which extends frame
     */
    add( sb, BorderLayout.CENTER );
    /**
     * initialize the remaining blocks to TextField
     */
    t = new TextField("Remain:" + sb.getEmpty()); 
    /**
     * new panel
     */
    Panel p = new Panel();
    /**
     * sets panel size to 100 x 10
     */
     p.setSize(100,10);
    /**
     * sets restart button size to 10 x 5 and location to 48, 5
     */
    b.setBounds(48,5,10,5);  
    /**
     * adds restart button to the Panel
     */
    p.add( b );
    /**
     * adds panel to south 
     */
    add( p, BorderLayout.SOUTH );
    /**
     * adds ActionListner to restart button
     */
    b.addActionListener( sb );

    /**
     * adds TextField to the Panel
     */
    p.add(t); 
    /**
     * Sets the TextField size to 10 x 5 and location to 53, 5
     */
    t.setBounds(53,5,10,5);
    /**
     * adds the buttons that hold the values
     */
    for(int i = 0; i < 9; i++){
        for(int j = 0; j < 9; j++){
            grid[i][j]=new Button(null);
            add(grid[i][j]);
        }
    }
    /**
     * Automatically resizing
     */
    pack();
    /**
     * Shows SudokuGame 
     */
    setVisible(true);
    /**
     * Shows the Panel
     */
    p.setVisible(true);       
 }
 /**
 * sets the label of the button
 * 
 * @param s   String to add to the button
 * @param i   the position
 * @param j   the position
 */
public void setText(String s, int i, int j){
    /**
     * if the button is not empty sets the label of the button
     */
    if(!s.equals("-1")){
    grid[i][j].setLabel(s); 
    }
}  //to do: change TextField when filling empty block
public void setText(String s){
    t.setText(s);
}
}

这是GUI类(框架)，没有画布或动作监听器或任何其他运行游戏的类。

Answer 1

你已经写了

private Button[][] grid= new Button[9][9];

应该是

 private Button[][] grid= new Button[9][];

接下来你直接开始添加Button from grid array作为:

   for(int i = 0; i < 9; i++){
        for(int j = 0; j < 9; j++){
            add(grid[i][j]);
        }
    }

这是错误的，因此它抛出NullPointerException。请参阅IDE的控制台。解决方案是首先实例化grid数组，如下所示:

 for(int i = 0; i < 9; i++){
    grid[i]=new Button[9];
  }
for(int i = 0; i < 9; i++){
    for(int j = 0; j < 9; j++){
      grid[i][j]=new Button("SomeLabel");
    }
}

初始化网格数组后，按如下方式向框架中添加按钮:

for(int i = 0; i < 9; i++){
    for(int j = 0; j < 9; j++){
        add(grid[i][j]);
    }
}