JFrame 上对象消失的问题

Question

我的程序有问题。矩形在框架的某个区域消失了，我不知道为什么。我最近写了一个类似的程序并且运行得很好。不同之处在于我画的是圆形，而不是矩形。

你能帮我找出问题所在吗？

public class Runner {
    
    public static void main(String[] args) {
        AppFrame app = new AppFrame();
        app.setVisible(true);
    }
}

---

public class Drawing extends JPanel{
    
    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }

    
    private int x;
    private int y;

    public void paint(Graphics g) {
        super.paint(g);
        
        Graphics2D g2d = (Graphics2D) g;
        g2d.setColor(Color.RED);
        g2d.fill(new Rectangle2D.Double(x, y, 40, 40));
    }

    public void repaintObject() {
        repaint();
    }}

---

public class AppFrame extends JFrame implements ActionListener {
    
    private int height =600;
    private int width = 600;
    
    private JButton upButton;
    private JButton downButton;
    private JButton rightButton;
    private JButton leftButton;
    
    private JPanel drawingPanel;
    private JPanel buttonsPanel;
    
    private Drawing object;
    private int x;
    private int y;
    
    public AppFrame() {
        
        this.setLayout(new GridLayout(2,1));
        
        drawingPanel= new JPanel();
        buttonsPanel= new JPanel();
        
        drawingPanel.setLayout(new GridLayout(1,1));
        buttonsPanel.setLayout(new BorderLayout());
        
        upButton = new JButton("Up");
        downButton = new JButton("Down");
        rightButton = new JButton("Right");
        leftButton = new JButton("Left");
        
        buttonsPanel.add(upButton, BorderLayout.CENTER);
        buttonsPanel.add(downButton, BorderLayout.PAGE_END);
        buttonsPanel.add(rightButton, BorderLayout.LINE_END);
        buttonsPanel.add(leftButton, BorderLayout.LINE_START);
        
        x=100;
        y=100;
        
        object = new Drawing();
        object.setX(x);
        object.setY(y);
        drawingPanel.add(object);
        
        this.add(drawingPanel);
        this.add(buttonsPanel);
        
        upButton.addActionListener(this);
        downButton.addActionListener(this);
        rightButton.addActionListener(this);
        leftButton.addActionListener(this);
    
        this.setSize(height, width);
        
        setLocationRelativeTo(null);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        
    }
    
    @Override
    public void actionPerformed(ActionEvent e) {
        Object source = e.getSource();
        
        System.out.println(x+"    "+ y);
        if(source == upButton) {
        
            if(y>=50) {
                y = y - 50;
                object.setY(y);
                object.repaintObject();
            }
            
        } else if(source == downButton) {
            if(y<350) {
                y = y+50;
                object.setY(y);
                object.repaintObject();
            }
        } else if(source == rightButton) {
            if(x<550) {
                x = x+50;
                object.setX(x);
                object.repaintObject();
            }
            
        } else if(source == leftButton) {
            if(x>=50) {
                x=x-50;
                object.setX(x);
                object.repaintObject();
            }
        }   }}

Answer 1

您可以使用

paint()

/

JLabel

来绘制矩形（或任何您想要的形状，只要您覆盖

JPanel

），而不是通过覆盖

paintComponent()

在不同位置绘制矩形），然后使用该组件的

setLocation()

来更改其位置（根据您希望矩形移动的速度 和）。为了避免矩形超出主面板的边界，您可以使用矩形的当前位置、矩形的尺寸以及主面板的尺寸执行一些检查。因此，无论您最初为矩形或面板/框架指定什么大小，或者在程序运行时调整框架窗口的大小，矩形仍然能够在其父组件的边界内移动。下面是一个工作示例。如果您想使用鼠标移动矩形，请查看此处。

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class App extends JFrame implements ActionListener {
    JLabel label;
    private JButton upButton;
    private JButton downButton;
    private JButton rightButton;
    private JButton leftButton;
    JPanel p; // main panel
    int rectHeight = 40;
    int rectWidth = 40;
    int speed = 50;

    public App() {
        label = new JLabel();
        label.setBounds(10, 10, rectWidth, rectHeight);
        label.setBackground(Color.RED);
        label.setOpaque(true);
        p = new JPanel();
        p.setLayout(null);
        p.add(label);

        upButton = new JButton("Up");
        downButton = new JButton("Down");
        rightButton = new JButton("Right");
        leftButton = new JButton("Left");
        JPanel buttonsPanel = new JPanel();
        buttonsPanel.setLayout(new BorderLayout());
        buttonsPanel.add(upButton, BorderLayout.CENTER);
        buttonsPanel.add(downButton, BorderLayout.SOUTH);
        buttonsPanel.add(rightButton, BorderLayout.EAST);
        buttonsPanel.add(leftButton, BorderLayout.WEST);
        upButton.addActionListener(this);
        downButton.addActionListener(this);
        rightButton.addActionListener(this);
        leftButton.addActionListener(this);

        this.getContentPane().add(p, BorderLayout.CENTER);
        this.getContentPane().add(buttonsPanel, BorderLayout.SOUTH);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.setSize(640, 480);
        this.setLocationRelativeTo(null);
        this.setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        Object source = e.getSource();
        int xPos = (int) label.getLocation().getX();
        int yPos = (int) label.getLocation().getY();
        int pHeight = (int) p.getSize().getHeight();
        int pWidth = (int) p.getSize().getWidth();

        if (source == upButton) {
            if (yPos - speed > 0)
                label.setLocation(xPos, yPos - speed);
            else
                label.setLocation(xPos, 0);

        } else if (source == downButton) {
            if (yPos + speed < pHeight - rectHeight)
                label.setLocation(xPos, yPos + speed);
            else
                label.setLocation(xPos, pHeight - rectHeight);

        } else if (source == leftButton) {
            if (xPos - speed > 0)
                label.setLocation(xPos - speed, yPos);
            else
                label.setLocation(0, yPos);

        } else if (source == rightButton) {
            if (xPos + speed < pWidth - rectWidth)
                label.setLocation(xPos + speed, yPos);
            else
                label.setLocation(pWidth - rectWidth, yPos);
        }

    }

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