java - Swing无法画圆

Question

所以我编写的这个程序应该在每次单击面板时画一个圆圈。由于某种原因，我最初在启动时右上角有一个半圆，但我无法让它画一个圆。谁能看出它出了什么问题吗？圆圈的直径应为 20 px，以单击的点为中心绘制。

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



public class QuizActionsInitial extends JFrame {
     public JButton redButton = new JButton("red");
     public JButton clearButton = new JButton("clear");
     boolean isRed = false;
     int x1,y1;
     boolean clear = false;
     CirclePanel myPanel;


 public QuizActionsInitial() {

     myPanel = new CirclePanel();

     add(myPanel, BorderLayout.SOUTH);
     JPanel southPanel = new JPanel(new FlowLayout());

     clearButton.addActionListener(new ActionListener(){
         public void actionPerformed(ActionEvent e){
             clear = true;
            }
        });

     myPanel.addMouseListener(new CircleListener());

     southPanel.add(redButton);
     southPanel.add(clearButton);
     add(southPanel, BorderLayout.NORTH);
     pack();
     setVisible(true);
 } // end constructor
 public class CirclePanel extends JPanel {
     public CirclePanel() {
         setPreferredSize(new Dimension(400,300));
         setBorder(BorderFactory.createLineBorder(Color.BLUE, 2));
        }

     public void paintComponent(Graphics gc){
         super.paintComponent(gc);
         gc.fillOval(x1-10,y1-10,20,20);
     }
        } // end class CirclePanel
 // end class CirclePanel

 public class CircleListener extends MouseAdapter{
     public void mouseClicked(MouseEvent e){
         if (clear = false){
             x1 = e.getX();
             y1 = e.getY();  
         }
         repaint();
         clear = false;
        }
    }

 public static void main(String args[]) {
     new QuizActionsInitial();
    } // end main
} // end class QuizActionsInitial

Answer 1

int x1,y1;将值初始化为0，这样你总是会在-10x-10处绘制一个初始圆

尝试使用 java.awt.Point 类，当它为 null 时，不要绘制任何内容...

//int x1,y1;
private Point point; 
//...
public void paintComponent(Graphics gc){
     super.paintComponent(gc);
     if (point != null) {
         gc.fillOval(point.x-10,point.y-10,20,20);
     }
}
//...
public void mouseClicked(MouseEvent e){
    if (!clear){
        point = evt.getPoint();
    }
    clear = false;
    repaint();
}

哦，if (clear = false){ 是一个赋值(使 clear 等于 false，这样 if 语句总是会失败)