java - 编号和循环问题

Question

我是编程新手，很难解决这个问题。我正在尝试根据在 JTextField 中输入的数字 build 一堵墙，但该数字不能超过 20。我无法显示错误消息，也无法 build 砖墙。有人可以帮我弄清楚我做错了什么吗？

public class Wall extends JApplet implements ActionListener {

JTextField enter;
Boolean submit;
JLabel bricks;
JButton build;
JPanel top;
Image zombie;
int value;

public void init() {
    setLayout(new BorderLayout());

    top = new JPanel();
    build = new JButton("AHHHHHH...ZOMBIES!");                          //button for building wall
    bricks = new JLabel("Enter between 1 & 20 rows to contruct:");
    enter = new JTextField(2);

    top.add(build);         //add zombie button
    top.add(bricks);        //add intructions
    top.add(enter);         //add text field
    add(top, BorderLayout.NORTH);


    build.addActionListener(this);


}

public void actionPerformed(ActionEvent ae) {
    if (ae.getSource() == build) {
        int value = Integer.parseInt(build.getText());
        if (value > 0 && value < 21) {
            submit = true;
            repaint();
        } else {
            submit = false;
            repaint();
        }

    }
}

public void paint(Graphics g) {
    super.paint(g);

    //add zombie image
    Image zombie = getImage(getCodeBase(), "Zombie.jpg");
    g.drawImage(zombie, 0, 45, this);

    if (submit = false) //add error message
    {
        g.setColor(Color.WHITE);
        g.setFont(new Font("TimesRoman", Font.BOLD, 40));
        g.drawString("You must enter a number between 1 & 20!", 400, 100);  //add message
    }

    int brick_width = 50;
    int brick_height = 20;
    int spacing = 1;
    int x = 0;
    while (x < 21) {
        drawBrick(g, nextInt(brick_width + spacing), nextInt(brick_height + spacing));
        x = x + getWidth() + 50;
        x = x - 25 + getWidth() + 50;
        x++;

    }

}

public void drawBrick(Graphics g, int x, int y) {
    g.setColor(new Color(150, 0, 0));
        g.fillRect(0, 635, 50, 20);
    }
}

Answer 1

而不是在 top-level containers 上绘画比如直接用JApplet，就得用JPanel在上面画画。只需覆盖 JPanel() 的 paintComponent() 方法。永远不要忘记在覆盖的方法中调用 super.paintComponent(g)。

示例代码:

top = new JPanel() {
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        ...
        // your custom painting code goes here
    }
};

注意:

submit == false 和 !submit 是检查它的正确方法，但它仍然会导致 NullPointerException 因为您从未初始化实例变量提交。

使用原始 boolean 而不是 Boolean 来避免此类异常或正确初始化它。

Boolean submit = false;