添加ActionListener時數組中的空指針異常

Question

對於一個學校項目，我一直在開展掃雷遊戲。它本質上將成爲遊戲的一個克隆，但現在，當我嘗試爲JButton添加動作偵聽器時，我得到了一個nullpointerexception。任何幫助？這是我的代碼：

import javax.swing.JFrame; 
    import javax.swing.JButton; 
    import javax.swing.JOptionPane; 

    import java.awt.event.ActionListener; 
    import java.awt.event.ActionEvent; 
    import java.awt.event.MouseAdapter; 
    import java.awt.event.MouseEvent; 
    import java.awt.GridLayout; 
    import java.awt.Dimension; 
    import java.util.Random; 

    public class Grid { 

    public static int c, d; //Necessary for the allowance of usage within the listeners from lines 47 - 68. 

    JFrame frame = new JFrame(); 
    public static boolean[][] isBomb; 
    public static int bombProbability; 

    public static JButton[][] grid; 

    Random Bomb = new Random(); 

    public Grid(int width, int length){ 

     bombProbability = (int) Double.parseDouble(JOptionPane.showInputDialog("Input the likeliness of a bomb:")); 

     frame.setLayout(new GridLayout(width, length)); 
     grid = new JButton[width][length]; 

     isBomb = new boolean[width + 2][length + 2]; 

     for(int a = 1; a <= length; a++){ 
      for(int b = 1; b <= width; b++){ 

       grid[a][b] = new JButton();    
       frame.add(grid[a][b]); 

       if((Bomb.nextInt(99) + 1) <= bombProbability){ 
        isBomb[a][b] = true; 
        grid[a][b].setText(String.valueOf(isBomb[a][b])); //Delete this before final product 
       } 
      } 
     } 
     frame.setTitle("Minesweeper!"); 
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     frame.setMinimumSize(new Dimension(500, 500)); 
     frame.pack(); 
     frame.setVisible(true); 

     for(c = 0; c < length; c++){ 
      for(d = 0; d < width; d++){ 
       grid[c][d].addActionListener(new ActionListener() { 
        public void actionPerformed(ActionEvent arg0) { 
         if(isBomb[c][d] == true){ 
          JOptionPane.showMessageDialog(null, "BOOM! You're DEAD!"); 
          System.exit(0); 
         } 
         else{ 
          indexCells(c,d); 
         } 
        } 
       }); 
      } 
     } 
    } 

    public static void indexCells(int c,int d){ 
     int[][] nearbyBombs = new int[c+2][d+2]; 

     for (int i = 1; i <= c; i++){ 
      for (int j = 1; j <= d; j++){ 
       // (ii, jj) indexes neighboring cells 
       for (int ii = i - 1; ii <= i + 1; ii++){ 
        for (int jj = j - 1; jj <= j + 1; jj++){ 
         if (isBomb[ii][jj]){ 
          nearbyBombs[i][j]++; 
         } 
        } 
       } 
       grid[i][j].setText(String.valueOf(nearbyBombs[i][j])); 
       if(nearbyBombs[i][j] == 0){ 
        for(int iii = i - 1; iii <= i + 1; iii ++){ 
         for(int jjj = j - 1; jjj <= j + 1; jjj ++){ 
          indexCells(iii,jjj); 
         } 
        }     
       } 
      } 
     } 
    } 

    public static void main(String []args){ 
     //int columns = (int) Double.parseDouble(JOptionPane.showInputDialog(null, "Input the number of columns:")); 
     //int rows = (int) Double.parseDouble(JOptionPane.showInputDialog(null, "Input the number of rows:")); 

     new Grid(10, 10); 
    } 
} 

問題是與行54（grid[c][d].addActionListener(new ActionListener() {）。任何幫助將不勝感激。

Answer 1

首先，你這樣做......

for(int a = 1; a <= length; a++){ 
    for(int b = 1; b <= width; b++){ 
     grid[a][b] = new JButton(); 

然後你這樣做......

for(c = 0; c < length; c++){ 
    for(d = 0; d < width; d++){ 
     grid[c][d].addActionListener(new ActionListener() { 

但grid[0][0]從來沒有被初始化，是null ...

你應該做更像...

for(int a = 0; a < length; a++){ 
    for(int b = 0; b < width; b++){ 
     grid[a][b] = new JButton();