paintComponent中的初始化不起作用

Question

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.

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);
    }
}

Answer 1

其他人已经有人提出了检测和避免的方法NullPointerException.不幸的是,您不能依赖于何时paintComponent()调用您的实现.代替,

• 根据当前寡妇的大小确定所需的几何形状; 调整窗口的大小在下面的例子来看看如何yellow似乎坚持到右下角.

• 因为不MyPanel包含自己的组件,所以应该覆盖getPreferredSize(),如@nIcE cOw 在此处所示.

• 用于pack()确定封闭的大小Window.

• 构建在事件派发线程上.

附录:我无法理解为什么要覆盖该方法 getPreferredSize().

JComponent覆盖的子类,getPreferredSize()以便pack()可以调整Window"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只绘制几何形状,所以你是首选大小的老板.正如这里所讨论的,演示可能是setPreferredSize()为了方便起见,但你应该理解这样做的局限性.

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);
        }
    }
}