Question

我在理解JPanel，JFrame和图形类的整个结构以及扩展和覆盖等方面遇到了一些麻烦。我似乎已经完成了所有工作，直到我添加了图形类，然后我的按钮等我的JPanel / JFrame上的按钮才显示出来了。我发现它与覆盖或超级有关？但我真的需要一些澄清。非常感谢！

我已经缩小了代码以便于查看。

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.TitledBorder;

public class windowBuild extends JFrame {

    private static final long serialVersionUID = 1L;
    private JPanel contentPane;
    private int energy = 4;
    private JButton btnClaw = new JButton("Claw");
    private Image bg;
    private boolean loaded = false;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                windowBuild frame = new windowBuild();
                frame.setVisible(true);

            }
        });
    }

    private class ButtonHandler implements ActionListener {

        public void actionPerformed(ActionEvent e) {
            String which = e.getActionCommand();
            if (which.equals("Claw")) {
                energy = energy - 1;
                System.out
                        .println("Player one's dragon clawed the opponent. Dragon's energy is now at: "
                                + energy);
            }

        }

    }

    public void loadImage() {
        bg = new ImageIcon("C:\\res\\dragonDuelBackground.jpeg").getImage();
        loaded = true;
        repaint();
    }

    public windowBuild() {
        ButtonHandler bh;
        System.out.println("Starting frame...");
        bh = new ButtonHandler();
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 800, 600);
        contentPane = new JPanel();
        contentPane.setBorder(new TitledBorder(null, "Dragon Duel",
                TitledBorder.CENTER, TitledBorder.TOP, null, Color.CYAN));
        setContentPane(contentPane);
        contentPane.setLayout(null);

        btnClaw.setBounds(273, 511, 109, 39);
        contentPane.add(btnClaw);
        btnClaw.addActionListener(bh);

    }
//********************************************************************
//  public void paint(Graphics g) {
//      if (loaded) {
//          g.drawImage(bg, 400, 400, null);
//      }
//  }
//***************Uncomment this and the code won't work anymore**********
}

Answer 1

取消注释，代码将不再起作用

不要覆盖顶级容器的paint（）（JFrame，JDialog ...）。通过覆盖JPanel（或JComponent）的paintComponent()方法来完成自定义绘制。然后将面板添加到框架中。不要忘记调用super.paintComponent（...）。

阅读Custom Painting教程中的Swing教程，了解更多信息和示例。

Answer 2

避免直接从JFrame延伸。您没有向框架添加任何新功能，并且如果您这样做，则限制了UI的可重用性（因为您无法向其他任何内容添加框架）
不要覆盖顶级容器的paint。有很多很好的理由，但主要的问题是框架容器有许多不同的层（JLayeredPane，contentPane）可以涂抹任何你做的事情
您必须致电super.paintXxx以确保油漆链保持完整。

相反，创建一个单独的类（例如从JPanel扩展）并覆盖它的paintComponent方法。或者更好。简单地使用JLabel来显示图像

请查看Performing Custom Painting和Painting in AWT and Swing了解详情。

另外，我没有看到你在哪里调用loadImage，所以永远不会加载图像

我还建议您使用ImageIO读取Reading/Loading an Image，当它无法读取图像文件时会抛出Exception，这比使用它更有用只是无声地失败的ImageIcon

更新了简单示例

enter image description here

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.TitledBorder;

public class TestGraphics {

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

    public TestGraphics() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private int energy = 4;
        private JButton btnClaw = new JButton("Claw");
        private BufferedImage bg;

        public TestPane() {
            setBackground(Color.WHITE);
            try {
                bg = ImageIO.read(new File("dragon.gif"));
            } catch (IOException ex) {
                ex.printStackTrace();
            }

            ButtonHandler bh;
            bh = new ButtonHandler();
            setBorder(new TitledBorder(null, "Dragon Duel",
                    TitledBorder.CENTER, TitledBorder.TOP, null, Color.CYAN));
            setLayout(new BorderLayout());

            JPanel buttonPane = new JPanel();
            buttonPane.setOpaque(false);
            buttonPane.add(btnClaw);

            add(buttonPane, BorderLayout.SOUTH);
            btnClaw.addActionListener(bh);
        }

        @Override
        public Dimension getPreferredSize() {
            return bg == null ? new Dimension(200, 200) : new Dimension(bg.getWidth(), bg.getHeight());
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            if (bg != null) {
                Graphics2D g2d = (Graphics2D) g.create();
                int x = (getWidth() - bg.getWidth()) / 2;
                int y = (getHeight() - bg.getHeight()) / 2;
                g2d.drawImage(bg, x, y, this);
                g2d.dispose();
            }
        }

        private class ButtonHandler implements ActionListener {

            public void actionPerformed(ActionEvent e) {
                String which = e.getActionCommand();
                if (which.equals("Claw")) {
                    energy = energy - 1;
                    System.out
                            .println("Player one's dragon clawed the opponent. Dragon's energy is now at: "
                            + energy);
                }

            }
        }
    }
}

Ps-你最好有充分的理由不使用布局管理器

Graphics类似乎重写了JFrame / JPanel

2 个答案: