Java–重新绘制JPanel会出错

我是Java的初学者,我正在尝试创建一个绘制矩形的应用程序,无论光标位于何处.我已经完成了所有工作,但是我无法使用mouseMoved(MouseEvent)方法重新绘制JPanel.没有重绘,矩形只绘制一次,就是这样.使用重绘,它编译得很好,但是当我运行它时,每次移动鼠标时,我都会得到这个大的“线程异常”AWT-EventQueue-0“java.lang.NullPointerException”错误.

那么,任何人都可以帮我解决这个问题吗？

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import javax.swing.event.*;
public class Game extends JPanel implements MouseMotionListener
{
public static void main(String[] args) {
new Game().game();
}
JPanel panel;
JButton button2;
JButton button;
public void game() {
JPanel panel = new Game();
button = new JButton("Ok");
panel.setLayout(new FlowLayout());
panel.add(button);
button2 = new JButton("Cancel");
JFrame frame = new JFrame("Game");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(500,500);
frame.setResizable(false);
frame.add(panel);
frame.setVisible(true);
panel.addMouseMotionListener(this);
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
PointerInfo a = MouseInfo.getPointerInfo();
Point b = a.getLocation();
int x = (int) b.getX();
int y = (int) b.getY();
g.fillRect(x,y,100,100);
}
public void mouseMoved(MouseEvent evt) {
panel.repaint; //This is the line of code that I need help with. Thanks!
}
public void mouseDragged(MouseEvent evt) {}
}


解决方法:

希望代码示例中的注释能够告诉您代码中的错误:-),否则总有理由提出您的疑问……

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class Game extends JPanel {
/*
* Now x and y are instance variables,
* whose values you can change at each
* MouseMove Event, and call repaint()
* to see the effects
*/
private int x;
private int y;
private MouseAdapter mouseActiOns=
new MouseAdapter() {
@Override
public void mouseMoved(MouseEvent me) {
/*
* Now as the Mouse moves, we simply
* updating the instance variables,
* i.e. x and y to the new values
* of the Mouse Location and calling
* repaint() to draw the rectangle.
* Since this class (Game) extends JPanel,
* hence all the functions of the JPanel
* belongs to this class, hence like
* as we call any other method of this
* class, without using the object,
* we can call repaint, likewise.
*/
x = me.getX();
y = me.getY();
repaint();
}
};
/*
* This JPanel panel is unnecessary in
* this case, since the class itself
* extends JPanel, hence you can use
* this (keyword) to access the instance
*/
//JPanel panel;
// Not needed for this case.
//JButton button2;
//JButton button;
public void game() {
JFrame frame = new JFrame("Game");
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.setResizable(false);
addMouseMotionListener(mouseActions);
/*
* Here this means the instance
* of the current class
*/
frame.add(this);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
/*
* While overriding methods of the
* super class, try to keep the
* ACCESS SPECIFIER, as close to
* the original thingy as possible
* In this case, it's protected
* and not public
*/
@Override
protected void paintComponent(Graphics g) {
/*
* Do not perform calculation in this method
* atleast.
*/
super.paintComponent(g);
g.fillRect(x, y, 100, 100);
}
public static void main(String[] args) {
Runnable runnable = new Runnable() {
@Override
public void run() {new Game().game();
}
};
EventQueue.invokeLater(runnable);
}
@Override
public Dimension getPreferredSize() {
return new Dimension(500, 500);
}
}