將ArrayList中的按鈕添加到JPanel中的NullPointerException

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我收到一個NullPointerException異常，我不知道爲什麼。如果有人可以查看我的代碼並告訴我它有什麼問題，那就太好了。將ArrayList中的按鈕添加到JPanel中的NullPointerException

提示：構建按鈕的金字塔的 1）塊寬度 2）塊高度 3）行號

import java.awt.*; 
import java.util.*; 
import javax.swing.*; 


public class BlockPyramidBuilder extends JPanel{ 

public static int height; 
public static int width; 
public static int rowNum; 
public static JFrame frame = new JFrame(); 
public static Graphics g; 
public static BlockPyramidBuilder test; 

public static void main(String[]Args){ 
    Scanner inputReader = new Scanner(System.in); 

    System.out.print("Block Height ------->"); 
    height = inputReader.nextInt(); 
    System.out.print("Block Width ------->");  
    width = inputReader.nextInt(); 
    System.out.print("Row Num ------->"); 
    rowNum = inputReader.nextInt(); 

    test = new BlockPyramidBuilder(height, width, rowNum); 

    frame.add(test); 
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    frame.pack(); 
    frame.setVisible(true); 

} 
public BlockPyramidBuilder(int height, int width, int rowNum){ 
    ArrayList<MyBox> array = new ArrayList<MyBox>(); 
    int center = frame.getWidth(); 
    int x = 0; 
    int y = 0;   


    for(int i = 1; i <= rowNum; i++){ 
     y = height * (i-1); 
     for(int j = 1; j <= i; j++){ 
      x = center + (width/2) * j; 
      System.out.println("x: " + x + "  y: " + y); 

      MyBox r = new MyBox(x, y, width, height); 
      array.add(r); 
     } 
    } 
    for(int i = 0; i < array.size(); i++){ 
     test.add(array.get(i)); 
    } 

} 

} 
class MyBox extends JButton{ 
public MyBox(int x, int y, int height, int width){ 
    super(); 
    setBackground(new java.awt.Color(37, 80, 130)); 
    setFont(new java.awt.Font("Trebuchet MS", 0, 24)); 
    setForeground(new java.awt.Color(38,38,38)); 
    setSize(height, width); 
    setLocation(x, y); 

} 
}

代碼得到正確的座標輸入到繪製的框，但是有將MyBox添加到JPanel時出錯。另外，我想知道

來源

2013-12-19 user3117395

'還有，我想知道嗎？想知道什麼？想知道如何在沒有堆棧的情況下幫助你？ – nhgrif