Java不会在JPanel上绘图

Quadrition 发表于 Dev

正交

这是我的主要课程：

public class Sad {

    private JFrame frame;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Sad window = new Sad();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the application.
     */
    public Sad() {
        initialize();
    }

    /**
     * Initialize the contents of the frame.
     */
    private void initialize() {
        frame = new JFrame();
        frame.setBounds(100, 100, 512, 399);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(new CardLayout(0, 0));

        JPanel panel = new JPanel();
        frame.getContentPane().add(panel, "name_12361565901507");
        panel.setLayout(null);

        JButton btnNes = new JButton("Nes");
        btnNes.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {

                Grafik grafik = new Grafik(20, 20, 100);
                panel.add(grafik);
            }
        });
        btnNes.setBounds(90, 146, 89, 23);
        panel.add(btnNes);
    }


}

这是绘画课

public class Grafik extends JPanel{

    private int x;
    private int y;
    private int r;

    public Grafik(int x, int y, int r){
        this.x = x;
        this.y = y;
        this.r = r;
    }

    public void paintComponent(Graphics g) {
        Graphics2D g2 =(Graphics2D) g;

        Ellipse2D circ = new Ellipse2D.Float(x, y, r, r);
        g2.setColor(Color.RED);
        g2.draw(circ);


        }

}

它们在同一包装中。当我单击按钮时，它本应以红色绘制椭圆，但它什么也没显示。有人能帮我吗？顺便说一句抱歉英语不好

卢卡斯·罗特（Lukas Rotter）

这是因为你不叫panel.setBounds()，revalidate()和repaint()。

但是无论如何您都不应该使用空布局：使用布局管理器。
您应该super.paintComponent(g)在paintComponent方法的开头进行调用。
与其在每次按下按钮后向面板添加新组件，不如只是在Grafik实例内部切换一个布尔值，该值确定椭圆是否可见。
如果希望椭圆“平滑”，可以致电g2.setRenderingHint(hintKey, hintValue)。

修改后的代码包括我的建议：

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Ellipse2D;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Sad {

    private JFrame frame;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Sad window = new Sad();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the application.
     */
    public Sad() {
        initialize();
    }

    /**
     * Initialize the contents of the frame.
     */
    private void initialize() {
        frame = new JFrame();
        frame.setBounds(100, 100, 512, 399);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        Grafik grafik = new Grafik(20, 20, 100);

        JPanel panel = new JPanel();
        panel.setLayout(new BorderLayout());
        panel.add(grafik);

        JButton btnNes = new JButton("Nes");
        btnNes.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {
                grafik.setEllipseVisible(true);
                panel.repaint();
            }
        });

        JPanel btnPanel = new JPanel();
        btnPanel.add(btnNes);
        panel.add(btnPanel, BorderLayout.SOUTH);

        frame.setContentPane(panel);
    }

}

class Grafik extends JPanel {

    private int x;
    private int y;
    private int r;
    private boolean ellipseVisible;

    public Grafik(int x, int y, int r) {
        this.x = x;
        this.y = y;
        this.r = r;
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (isEllipseVisible()) {
            Graphics2D g2 = (Graphics2D) g;
            g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

            Ellipse2D circ = new Ellipse2D.Float(x, y, r, r);
            g2.setColor(Color.RED);
            g2.draw(circ);
        }
    }

    public boolean isEllipseVisible() {
        return ellipseVisible;
    }

    public void setEllipseVisible(boolean ellipseVisible) {
        this.ellipseVisible = ellipseVisible;
    }

}

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