paintComponent中的初始化不起作用 [英] Initialization in paintComponent doesn't work

问题描述

我正在做一些练习来理解Java和Swing API。为什么在Disegno构造函数中有nullPointerException？我想打印两个矩形的坐标，但它们似乎没有初始化。

I'm doing some exercise to understand Java and Swing API. Why do I have a nullPointerException in the Disegno constructor? I want to print the coordinates of the two rectangles, but they seem not to be initialitied.

import java.awt.*;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Disegno extends JFrame{

    Disegno(){
        this.setSize(500, 500);
        this.setDefaultCloseOperation(this.EXIT_ON_CLOSE);
        MyPanel aba = new MyPanel();
        this.setContentPane(aba);
        this.setVisible(true);

        System.out.println(aba.rect.blue.x + "-" + aba.rect.blue.y);
        System.out.println(aba.rect.yellow.x + "-" + aba.rect.yellow.y);
    }

    public static void main(String[] args){
        new Disegno();
    }
}

class MyPanel extends JPanel{

    JPanel up, down;
    RectArea rect;

    MyPanel(){
        this.setLayout(new BorderLayout());

        up = new JPanel();
        this.add(up, BorderLayout.NORTH);
        up.setBackground(Color.red);
        up.setVisible(true);

        down = new JPanel();
        down.setBackground(Color.green);
        this.add(down, BorderLayout.SOUTH);
        down.setVisible(true);

        rect = new RectArea();
        this.add(rect, BorderLayout.CENTER);

        this.setVisible(true);
    }
}

class RectArea extends JPanel{

    Rectangle blue, yellow;
    boolean check = false;

    RectArea(){
        super();
        this.setVisible(true);
    }

    public void initRect(){
        blue = new Rectangle(0, 0, 100, 100);
        yellow = new Rectangle(this.getWidth(), this.getHeight(), 100, 100);
        System.out.println("ok");
    }

    @Override
    protected void paintComponent(Graphics g){
        super.paintComponent(g);
        if(check == false){
            this.initRect();
            check = true;
        }

        System.out.println(this.getWidth() + "-" + this.getHeight());
        g.setColor(Color.blue);
        g.fillRect(blue.x, blue.y, blue.width, blue.height);
        g.setColor(Color.yellow);
        g.fillRect(yellow.x - yellow.width, yellow.y - yellow.height, yellow.width, yellow.height);
    }
}

推荐答案

其他有用地建议了检测和避免 NullPointerException 的方法。不幸的是，当 你的 paintComponent（）的实现被调用时，你不能依赖。相反，

Others have helpfully suggested ways to detect and avoid the NullPointerException. Unfortunately, you can't rely on when your implementation of paintComponent() will be called. Instead,

• 根据当前寡妇的大小确定所需的几何图形;在下面的示例中调整窗口大小以查看黄色似乎粘贴到右下角。

因为 MyPanel 不包含自己的组件，所以应该覆盖 getPreferredSize（），如@nIcE cOw显示此处。

Because MyPanel contains no components of its own, you should override getPreferredSize(), as @nIcE cOw shows here.

使用 pack（）调整封闭的窗口。

构建于事件派遣线程。

附录：我无法理解为什么要重写方法 getPreferredSize（）。

JComponent的子类覆盖 getPreferredSize（） 以便 pack（） 可以调整窗口的大小以适合其子组件的首选大小和布局。这样，您就不必担心用户是否有不同的字体，例如。 MyPanel 只是绘制几何形状，所以你是首选大小的老板。正如此处所述，演示可以使用 setPreferredSize（）以方便，但你应该理解这样做的局限性。

Subclasses of JComponent override getPreferredSize() so that pack() can size the Window "to fit the preferred size and layouts of its subcomponents." That way you don't have to worry if the user has a different font, for example. MyPanel just draws geometric shapes, so you're the boss on preferred size. As discussed here, a demo may use setPreferredSize() for convenience, but you should understand the limitations of doing so.

import java.awt.*;
import javax.swing.JFrame;
import javax.swing.JPanel;

/**
* @see https://stackoverflow.com/q/11376272/230513
*/
public class Disegno extends JFrame {

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new Disegno();
            }
        });
    }

    Disegno() {
        this.setSize(500, 500);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        MyPanel aba = new MyPanel();
        this.add(aba);
        this.pack();
        this.setLocationRelativeTo(null);
        this.setVisible(true);
    }

    class MyPanel extends JPanel {

        private JPanel up, down;
        private RectArea rect;

        MyPanel() {
            super(new BorderLayout());

            up = new JPanel();
            up.setBackground(Color.red);
            this.add(up, BorderLayout.NORTH);

            rect = new RectArea();
            this.add(rect, BorderLayout.CENTER);

            down = new JPanel();
            down.setBackground(Color.green);
            this.add(down, BorderLayout.SOUTH);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(320, 240);
        }
    }

    class RectArea extends JPanel {

        private Rectangle blue = new Rectangle(0, 0, 100, 100);
        private Rectangle yellow = new Rectangle(0, 0, 100, 100);

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            System.out.println(this.getWidth() + " x " + this.getHeight());
            g.setColor(Color.blue);
            g.fillRect(blue.x, blue.y, blue.width, blue.height);

            g.setColor(Color.yellow);
            int dx = getWidth() - yellow.width;
            int dy = getHeight() - yellow.height;
            g.fillRect(dx, dy, yellow.width, yellow.height);
        }
    }
}

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