java - paintComponent中的初始化不起作用

我正在做一些练习，以了解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，但是它们似乎没有初始化。

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);
    }
}

最佳答案

其他人则有益地建议了检测和避免NullPointerException的方法。不幸的是，您不能依赖何时调用paintComponent()的实现。代替，

根据当前寡妇的大小确定所需的几何形状；在下面的示例中调整窗口的大小，以查看yellow似乎如何粘在右下角。

因为MyPanel不包含其自身的组件，所以您应该覆盖getPreferredSize()，因为@nIcE cOw显示here。

使用pack()调整封闭的Window的大小。

建立在event dispatch thread上。

附录:我不明白为什么您要覆盖getPreferredSize()方法。
JComponent的子类覆盖 getPreferredSize() ，以便 pack() 可以调整Window的大小以“适合其子组件的首选大小和布局”。这样，您不必担心用户是否使用其他字体。 MyPanel仅绘制几何形状，因此您是首选大小的老板。如here所讨论的，为了方便起见，演示可能会使用setPreferredSize()，但是您应该了解这样做的局限性。

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);
        }
    }
}