在Java中，为什么frame.repaint在这段代码中不起作用？

Question

我不确定为什么fr.repaint(); 方法在这里不起作用。 如果我使用 set visible false ，然后在进行更改后将其设置为true ，它会使屏幕 go 变黑，这不是我想要做的。

有什么帮助吗？

此代码用于学校项目，因此如果可能，请提供一些详细的解释以供理解。

import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.*;

class Main {
    static int startingposx;
    static int startingposy;

    static int endingposx;
    static int endingposy;

    public static void main(String[] args) {
        JFrame fr = new JFrame();
        fr.setBounds(10,10,512,512);
        
        fr.addMouseMotionListener(new MouseMotionListener() {
            @Override
            public void mouseDragged(MouseEvent e) {
            }
            @Override
            public void mouseMoved(MouseEvent e) {
            }
        });

        fr.addMouseListener(new MouseListener() {
            @Override
            public void mouseClicked(MouseEvent e) {
                
                
            }

            @Override
            public void mousePressed(MouseEvent e) {

                startingposx = e.getX();
                startingposy = e.getY();
            }

            @Override
            public void mouseReleased(MouseEvent e) {

                endingposx = e.getX();
                endingposy = e.getY();
                //fr.setVisible(false);// i tried to use this to refresh it but it makes the frame black for a second
                
                JPanel x = new JPanel(){
                    @Override
                    public void paint(Graphics g){
                            g.setColor(Color.RED);
                            g.fillRect(startingposx-(32-15),startingposy-32,30,30);
                            g.setColor(Color.BLUE);
                            g.fillRect(endingposx-(32-15),endingposy-32,30,30); 
                    }
                };
                
                fr.add(x);
                fr.repaint();// why does this not work??
                //fr.setVisible(true);
        
                
                System.out.print("\033[H\033[2J");
                System.out.println("starting pos: " + startingposx+ ","+ startingposy+"\n" + "ending pos: " + endingposx+ "," + endingposy);
            }

            @Override
            public void mouseEntered(MouseEvent e) {
            }
            
            @Override
            public void mouseExited(MouseEvent e) {
            }
        });
        
        fr.setVisible(true);        
        fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  }
}

Answer 1

_{框架的高度变短纯粹是为了节省空间}

请参阅代码中的更多注释。 请注意，蓝色矩形完全覆盖了红色矩形。

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

public class TestCustomPaint {
    // Don't use static! Make them instances of the x class and give it a constructor
    static int startingposx;
    static int startingposy;

    static int endingposx;
    static int endingposy;

    public static void main(String[] args) {
        JFrame fr = new JFrame();
        // this is the wrong size if the content should be square
        fr.setBounds(10,10,512,512);

        // unused MouseMotionListener removed
        fr.addMouseListener(new MouseListener() {
            @Override
            public void mouseClicked(MouseEvent e) { }

            @Override
            public void mousePressed(MouseEvent e) {
                startingposx = e.getX();
                startingposy = e.getY();
            }

            @Override
            public void mouseReleased(MouseEvent e) {
                endingposx = e.getX();
                endingposy = e.getY();

                fr.repaint();

                System.out.print("\033[H\033[2J");
                System.out.println("starting pos: " + startingposx+ ","+ startingposy+"\n" +
                        "ending pos: " + endingposx+ "," + endingposy);
            }

            @Override
            public void mouseEntered(MouseEvent e) { }

            @Override
            public void mouseExited(MouseEvent e) { }
        });

        // This is when the panel should be created and added.
        JPanel x = new JPanel(){
            @Override
            public void paintComponent(Graphics g){
                super.paintComponent(g);
                g.setColor(Color.RED);
                g.fillRect(startingposx-(32-15),startingposy-32,30,30);
                g.setColor(Color.BLUE);
                g.fillRect(endingposx-(32-15),endingposy-32,30,30);
            }
        };

        fr.add(x);
        fr.setVisible(true);
        fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}