java - 为什么尝试在表格上绘制图形时会出现 NullPointerException？

Question

当我在构造函数中调用我创建的方法 draw() 时，我不断收到 NullPointerException。这尤其令人沮丧，因为我找到了解决方法，但这不是我想要的。这是有效的代码。 公共(public)类 TutorialGrid 扩展 JFrame{

private JPanel contentPane;
private Graphics g;
private int currentlength;
private int currentwidth;
private Integer[][] maze = new Integer[20][30];
private static TutorialGrid frame;
public static JTable table;
private JTextField title;
/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                frame = new TutorialGrid();
                frame.setVisible(true);

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

/**
 * Create the frame.
 */
public TutorialGrid(){
    setResizable(false);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(0, 0, 1366, 768);
    contentPane = new JPanel();
    contentPane.setBorder(new BevelBorder(BevelBorder.LOWERED, null, null, null, null));
    setContentPane(contentPane);
    contentPane.setLayout(null);

    ImageIcon icon = new ImageIcon("C:\\Users\\Brendan\\Desktop\\GAME\\Images\\BG7.jpg");
    input();


    table = new JTable();
    table.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent e) {
            draw(maze);
        }
    });
    table.setAutoResizeMode(JTable.AUTO_RESIZE_OFF);
    table.setAutoCreateRowSorter(true);
    table.setCellSelectionEnabled(true);
    table.setColumnSelectionAllowed(true);
    table.setDoubleBuffered(true);
    table.setDragEnabled(true);
    table.setFillsViewportHeight(true);
    table.setFocusCycleRoot(true);
    table.setFocusTraversalPolicyProvider(true);
    table.setIgnoreRepaint(true);
    table.setInheritsPopupMenu(true);
    table.setSurrendersFocusOnKeystroke(true);
    table.setBackground(new Color(0, 0, 0));
    table.setForeground(new Color(255, 255, 255));
    table.setBorder(new LineBorder(new Color(139, 0, 0)));
    table.setBounds(180, 40, 1000, 600);
    contentPane.add(table);

    title = new JTextField();
    title.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent e) {
            draw(maze);
        }
    });
    title.setHorizontalAlignment(SwingConstants.CENTER);
    title.setText("Click to Start");
    title.setBorder(new LineBorder(new Color(0, 128, 0)));
    title.setEditable(false);
    title.setForeground(Color.WHITE);
    title.setBackground(new Color(0, 0, 0));
    title.setBounds(606, 11, 151, 20);
    contentPane.add(title);
    title.setColumns(10);
    JLabel lblBgpanel = new JLabel("", icon,JLabel.CENTER);
    lblBgpanel.setBounds(0, 0, 1360, 740);
    contentPane.add(lblBgpanel);
}

正如你所看到的，我在表格和标题上都有 mouseListeners，它们成功地调用了绘制方法，没有任何问题。它在 table 上绘制了我想要的网格，但是为了绘制它，我必须单击这些容器之一。我希望它在 JFrame 初始化时绘制网格。但如果我简单地把draw(maze);在构造函数中它给了我一个空指针异常。这是用于绘制网格的绘制和输入方法的代码。

        public void draw(Integer[][] maze){
    int x= 125;
    int y =50;
    int width1 =25;
    int length1 =25;
    g=table.getGraphics();
    for(int i=0; i<20; i++)
       {

         for(int j=0; j<30; j++)
         {

             if(maze[i][j] == maze[currentlength][currentwidth])
             {
                g.setColor(Color.YELLOW);
                g.fillRect(x,y,width1,length1);
                g.setColor(Color.RED);
                g.drawRect(x,y,width1,length1);
                x = x+25;
             }
             else if(maze[i][j] == 1)
             {
                    g.setColor(Color.BLACK);
                    g.fillRect(x,y,width1,length1);
                    g.setColor(Color.RED);
                    g.drawRect(x,y,width1,length1);
                    x = x+25;
             }

                else if(maze[i][j] == 0)
                {
                    g.setColor(Color.BLUE);
                    g.fillRect(x,y,width1,length1);
                    g.setColor(Color.RED);
                    g.drawRect(x,y,width1,length1);
                    x = x+25;
                }
                else if(maze[i][j] == -2)
                {
                    g.setColor(Color.GREEN);
                    g.fillRect(x,y,width1,length1);
                    g.setColor(Color.RED);
                    g.drawRect(x,y,width1,length1);
                    x = x+25;
                }
                    else if(maze[i][j] == -10)
                    {
                        g.setColor(Color.WHITE);
                        g.fillRect(x,y,width1,length1);
                        g.setColor(Color.RED);
                        g.drawRect(x,y,width1,length1);
                        x = x+25;
                    }
         }
         y=y+25;
         x=125;
       }
}

        public void input(){
    //Imports and reads grid file
       Scanner scan = null;

       try
       {

           FileReader grid = new FileReader("C:\\Users\\Brendan\\Desktop\\tutorialgrid.txt");
           scan = new Scanner(grid);


       }
       catch(FileNotFoundException e)
       {
           System.out.println(e.getMessage() + "Could not find that file");
           System.exit(0);
       }

       for(int i=0; i<20; i++)
       {
         for(int j=0; j<30; j++)
         {
            maze[i][j]=scan.nextInt();
            if(maze[i][j] == -1)
            {
                currentlength = i;
                currentwidth = j;
            }
            if(maze[i][j] == -10)
            {
            }
         }
       }
}

}

所有这些都在同一个类中。这就是我想要做的，但给了我一个错误。我在构造函数的底部添加了绘制(迷宫)，当我尝试运行它时，它就会在我身上爆炸。

    public TutorialGrid(){
    setResizable(false);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(0, 0, 1366, 768);
    contentPane = new JPanel();
    contentPane.setBorder(new BevelBorder(BevelBorder.LOWERED, null, null, null, null));
    setContentPane(contentPane);
    contentPane.setLayout(null);

    ImageIcon icon = new ImageIcon("C:\\Users\\Brendan\\Desktop\\GAME\\Images\\BG7.jpg");
    input();


    table = new JTable();
    table.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent e) {
            draw(maze);
        }
    });
    table.setAutoResizeMode(JTable.AUTO_RESIZE_OFF);
    table.setAutoCreateRowSorter(true);
    table.setCellSelectionEnabled(true);
    table.setColumnSelectionAllowed(true);
    table.setDoubleBuffered(true);
    table.setDragEnabled(true);
    table.setFillsViewportHeight(true);
    table.setFocusCycleRoot(true);
    table.setFocusTraversalPolicyProvider(true);
    table.setIgnoreRepaint(true);
    table.setInheritsPopupMenu(true);
    table.setSurrendersFocusOnKeystroke(true);
    table.setBackground(new Color(0, 0, 0));
    table.setForeground(new Color(255, 255, 255));
    table.setBorder(new LineBorder(new Color(139, 0, 0)));
    table.setBounds(180, 40, 1000, 600);
    contentPane.add(table);

    title = new JTextField();
    title.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent e) {
            draw(maze);
        }
    });
    title.setHorizontalAlignment(SwingConstants.CENTER);
    title.setText("Click to Start");
    title.setBorder(new LineBorder(new Color(0, 128, 0)));
    title.setEditable(false);
    title.setForeground(Color.WHITE);
    title.setBackground(new Color(0, 0, 0));
    title.setBounds(606, 11, 151, 20);
    contentPane.add(title);
    title.setColumns(10);
    JLabel lblBgpanel = new JLabel("", icon,JLabel.CENTER);
    lblBgpanel.setBounds(0, 0, 1360, 740);
    contentPane.add(lblBgpanel);
    draw(maze);
}

这是错误。

    java.lang.NullPointerException
at game.TutorialGrid.draw(TutorialGrid.java:136)
at game.TutorialGrid.<init>(TutorialGrid.java:111)
at game.TutorialGrid$1.run(TutorialGrid.java:41)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

game.TutorialGrid.draw第136行是draw方法开始的行。游戏教程网格。第 111 行是构造函数中的最后一行，我在其中放置了绘制(迷宫)。game.TutorialGrid$1.run第41行是线frame = new TutorialGrid();

感谢帮助。

Answer 1

getGraphics() 返回 null，直到事物可见为止。