java - 为什么矩形不显示？

Question

我检查了这段代码几个小时，但矩形没有显示，谁能告诉我为什么它没有显示？:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class Main {

    public static void main(String[] args) {

        JFrame frame = new JFrame();
        int resx = 700,resy = 500;
        frame.setSize(resx,resy);
        frame.setLocationRelativeTo(null);
        frame.setTitle("Game");
        frame.setResizable(false);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new FlowLayout());
        try {
            frame.setContentPane(new JLabel(new ImageIcon(ImageIO.read(new File("C:\\Users\\ivans\\Pictures\\Cookies.png")))));
        } catch (IOException e) {
        }
        frame.repaint();
        frame.setLayout(new FlowLayout());
        frame.add(new JPanel(){
            @Override
            public void paintComponent(Graphics g){
                super.paintComponent(g);
                g.setColor(Color.WHITE);
                g.fillRect(50,50,450,650);
            }
        }, BorderLayout.CENTER);
        frame.repaint();
        frame.setVisible(true);
    }

}

每次我尝试激活背景时，都不显示矩形，每次激活矩形时，都不显示背景。请帮忙!

Answer 1

您将 JFrame 的 contentPane 设置为 JLabel(一个不使用布局的容器)，因此向其中添加组件将不允许显示该组件，除非您完全指定该组件的大小和位置(即其边界) 。这是我避免对 contentPanes 使用 JLabels 的原因之一(而且它不会根据它所持有的组件设置其首选大小)，而是通常更喜欢在后台 JPanel 的 PaintComponent 方法中进行绘图。

侧面建议:

• 主方法中发生了太多事情 - 除非该程序除了演示目的之外没有任何其他用途
• 您将 JFrame 的原始 contentPane(JPanel)设置为 FlowLayout，但要明白，一旦更改 contentPane，这就没有意义了。
• 尽管您假设 contentPane 使用 FlowLayout，但您试图将绘图 JPanel 添加到 BorderLayout 位置，这是没有意义的。
• 你有一个空的 catch block ，这是几乎不应该做的事情。
• 将图像作为资源而不是文件获取。
• 避免使用绝对文件路径，而优先使用资源的相对路径。
• 如果可以避免，请不要设置事物的大小。
• 避免所谓的“魔数(Magic Number)”，例如 g.fillRect(50,50,450,650);，因为这会使您的代码难以调试和增强。

例如，类似:

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.*;

@SuppressWarnings("serial")
public class ShowRectangle extends JPanel {
    private static final int RECT_X = 50;
    private static final int RECT_Y = RECT_X;
    private static final int RECT_W = 200;
    private static final int RECT_H = 200;
    private static final String URL_SPEC = "https://duke.kenai.com/guitar/DukeAsKeith-daylightSmall.png";
    private BufferedImage img;

    public ShowRectangle(BufferedImage img) {
        this.img = img;
    }

    // have same JPanel draw image and graphic element
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (img != null) {
            g.drawImage(img, 0, 0, this);
        }

        // avoid magic numbers
        // g.fillRect(50,50,450,650);
        g.fillRect(RECT_X, RECT_Y, RECT_W, RECT_H);
    }

    // Size the JPanel to the image size
    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet() || img == null) {
            return super.getPreferredSize();
        }
        return new Dimension(img.getWidth(), img.getHeight());
    }

    private static void createAndShowGui(BufferedImage image) {
        ShowRectangle mainPanel = new ShowRectangle(image);

        JFrame frame = new JFrame("ShowRectangle");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String[] args) {

        try {
            URL imageUrl = new URL(URL_SPEC);
            BufferedImage img = ImageIO.read(imageUrl);
            SwingUtilities.invokeLater(() -> createAndShowGui(img));
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(-1);
        }
    }
}

例如，如果您避免使用“魔术”数字，则很容易使黑色矩形可拖动，因为它现在是由变量值绘制的，您可以在 MouseAdapter(MouseListener 和 MouseMotionListener 组合)内部更改这些值。例如:

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;

import javax.imageio.ImageIO;
import javax.swing.*;

@SuppressWarnings("serial")
public class ShowRectangle extends JPanel {
    private static final int RECT_X = 50;
    private static final int RECT_Y = RECT_X;
    private static final int RECT_W = 200;
    private static final int RECT_H = 200;
    private static final String URL_SPEC = "https://duke.kenai.com/guitar/DukeAsKeith-daylightSmall.png";
    private int rectX = RECT_X;
    private int rectY = RECT_Y;
    private BufferedImage img;

    public ShowRectangle(BufferedImage img) {
        this.img = img;
        MouseAdapter myMouse = new MyMouse();
        addMouseListener(myMouse);
        addMouseMotionListener(myMouse);
    }

    // have same JPanel draw image and graphic element
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (img != null) {
            g.drawImage(img, 0, 0, this);
        }

        // avoid magic numbers
        // g.fillRect(50,50,450,650);
        g.fillRect(rectX, rectY, RECT_W, RECT_H);
    }

    // Size the JPanel to the image size
    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet() || img == null) {
            return super.getPreferredSize();
        }
        return new Dimension(img.getWidth(), img.getHeight());
    }

    private class MyMouse extends MouseAdapter {
        private Point p1;
        private Point rectP = null;

        @Override
        public void mousePressed(MouseEvent e) {
            p1 = e.getPoint();
            if (new Rectangle(rectX, rectY, RECT_W, RECT_H).contains(p1)) {
                rectP = new Point(rectX, rectY);
            }
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            moveRect(e.getPoint());
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            moveRect(e.getPoint());
            rectP = null;
        }

        private void moveRect(Point p2) {
            if (rectP == null) {
                return;
            }
            rectX = rectP.x + p2.x - p1.x;
            rectY = rectP.y + p2.y - p1.y;
            repaint();
        }

    }

    private static void createAndShowGui(BufferedImage image) {
        ShowRectangle mainPanel = new ShowRectangle(image);

        JFrame frame = new JFrame("ShowRectangle");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String[] args) {

        try {
            URL imageUrl = new URL(URL_SPEC);
            BufferedImage img = ImageIO.read(imageUrl);
            SwingUtilities.invokeLater(() -> createAndShowGui(img));
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(-1);
        }
    }
}