空指针异常

Question

我对StackOverFlow相当陌生。我在Java方面有多年的经验，但在这段代码中似乎找不到触发Null指针异常的行。所有进口都准备好了。

public class BallShooter extends JFrame{

    private JFrame ballshooter;
    private BallShooter bs;
    private MenuPanel mp;
    private MainPanel mep;
    private GamePanel gp;
    private Balls balls;

    private CardLayout card;
    private int[] leaderboard; 
    private boolean ballHitWall;
    public BallShooter()
    {   

        mep = new MainPanel();
        mp = new MenuPanel();
        gp = new GamePanel();

        ballshooter = new JFrame();
        ballshooter.setLocation(0, 0);
        ballshooter.setSize(800, 700);
        ballshooter.setDefaultCloseOperation(EXIT_ON_CLOSE);
        ballshooter.setBackground(Color.GRAY);
        ballshooter.setResizable(false);

        ballshooter.getContentPane().add(mep);
        ballshooter.setVisible(true);
        card = (CardLayout)(mep.getLayout());

    }
    public static void main(String [] args)
    {
        BallShooter balls = new BallShooter();
    }
    class MainPanel extends JPanel 
    {
        public MainPanel()
        {
            setSize(800,700);
            setVisible(true);
            setBackground(Color.GRAY);
            setLayout(new CardLayout());
            add(mp); <-- line 52
            add(gp);

        }
    }
    class MenuPanel extends JPanel implements ActionListener
    {
        private JButton startGame;
        private JButton leaderboard;
        private JButton instructions;
        public MenuPanel()
        {
            setLayout(null);
            setSize(800,700);
            setBackground(Color.GRAY);

            startGame = new JButton("Start the GAME.");
            leaderboard = new JButton("Go to LEADERBOARD.");
            instructions = new JButton("Instructions.");

            startGame.addActionListener(this);
            leaderboard.addActionListener(this);
            instructions.addActionListener(this);

            startGame.setBounds(300,100,200,150);
            leaderboard.setBounds(300,250,200,150);
            instructions.setBounds(300,400,200,150);


            add(startGame);
            add(leaderboard);
            add(instructions);
        }
        public void actionPerformed(ActionEvent e) {
            String in = e.getActionCommand();
            if(in.equals("Start the GAME."))
            {
                card.next(mep);
            }

        }

    }
    class GamePanel extends JPanel implements ActionListener
    {

        private JButton stats;

        private int pos1,pos2,pos3,pos4,pos5,pos6,pos7,pos8,pos9,pos10,pos11,pos12,pos13,pos14,pos15,pos16,pos17,pos18,pos19,pos20 ;
        private boolean onePos,twoPos,threePos,fourPos,fivePos,sixPos,sevenPos,eightPos,ninePos,tenPos,elevenPos,twelvePos,thirteenPos,fourteenPos,fifteenPos,sixteenPos,seventeenPos,eighteenPos,ninteenPos,twentyPos = true;
        private int x,y;
        public GamePanel()
        {
            balls = new Balls();
            onePos = true;
            setSize(800,700);
            setBackground(Color.GRAY);
            setLayout(null);
            if(onePos)
            {
                pos1 = 1;
                x = 100;
                y = 100;

                balls.setBounds(x,y, 50,50);
                gp.add(balls);

            }


        }

        public void actionPerformed(ActionEvent e) {}
        public void paintComponent(Graphics g)
        {
            super.paintComponent(g);
            g.setColor(Color.WHITE);
            g.drawRect(100, 100, 600, 500);

        }

    }
    class Balls extends JPanel {
        private int color;
        private int vX, vY;
        private int ballW = 50, ballH = 50;
        private int x,y;
        public Balls()
        {
            setSize(50,50);
        }
        public void paintComponent(Graphics g)
        {

            super.paintComponent(g);
            color = (int)(Math.random()*4+1);
            if(color == 1)
            {
                g.setColor(Color.YELLOW);
                g.drawOval(0, 0, ballW, ballH);
            }
            else if(color == 2)
            {
                g.setColor(Color.GREEN);
                g.drawOval(0, 0, ballW, ballH);
            }
            else if(color == 3)
            {
                g.setColor(Color.RED);
                g.drawOval(0, 0, ballW, ballH);
            }
            else if(color == 4)
            {
                g.setColor(Color.BLUE);
                g.drawOval(0, 0, ballW, ballH);
            }

        }

    }
}

我不知道怎么回事。它只是飞过我的头顶。错误：

Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:1097)
at java.awt.Container.add(Container.java:417)
at BallShooter$MainPanel.<init>(BallShooter.java:52)
at BallShooter.<init>(BallShooter.java:24)
at BallShooter.main(BallShooter.java:42)

如果有人能在这方面帮助我，那将是非常感谢的。编辑第一次投递，所以格式化有一个错误。类头没有进入代码部分，也没有进入最后一个括号。

Answer 1

您正在初始化非静态嵌套类MainPanel，它在初始化mp和gp字段之前使用BallShooter类中的字段(参见MainPanel构造函数中调用#add的行)。在创建了所有菜单之后，尝试调用mep#add，或者考虑一些类似于#init方法的内容(甚至是不同的层次结构/设计)。

Answer 2

General rule --当您读取这样的堆栈跟踪时，请查找第一行，它是您的类之一，而不是JDK或库类。在这种情况下，问题可以在BallShooter.java的第52行中找到。

当该行运行时，mp是null，因此您试图向容器中添加一个null，因此出现了异常。

要解决这个问题，首先创建另外两个面板，最后创建MainPanel。