这个问题在这里已有答案：
我希望在一个面板中创建4个名为FBButton的JButton，可以单击该面板来更改预设图像并在面板上拖动。截至目前，按钮将从mouseListener点击，但拖动时会出现错误。我已经尝试将布局设置为null，但是当我这样做时，即使我初始化它们的大小和位置，按钮也不会出现在屏幕上。
为了实现这一点，我将代码分为三类：FBPlayerFrame，FBButton和FBMouseListener。我会尽可能地让每个人都成为最小的屁股，但如果你认为我添加了太多，我会道歉。
这是FBPlayerFrame
package swing;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class FBPlayerFrame extends JFrame {
JPanel p = new JPanel();
FBButton buttons[] = new FBButton[22];
Dimension dim = new Dimension(52, 52);
public static void main(String[] args) {
new FBPlayerFrame();
}
public FBPlayerFrame() {
super("Football Start");
setSize(400, 400);
setResizable(false);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
p.setLayout(new DragLayout());
for (int i = 0; i < 4; i++) {
buttons[i] = new FBButton();
buttons[i].setPreferredSize(new Dimension(52, 52));
buttons[i].setLocation(50, 40 + 60 * i);
p.add(buttons[i]);
buttons[i].addMouseMotionListener(new FBMouseListener());
buttons[i].addMouseListener(new FBMouseListener());
}
add(p);
setVisible(true);
}
}
这是FBButton
package swing;
import javax.swing.*;
import java.awt.event.*;
@SuppressWarnings("serial")
public class FBButton extends JButton implements ActionListener {
ImageIcon SN, SL,IN;
byte value = 0;
FBMouseListener listener;
public FBButton() {
listener = new FBMouseListener();
SN = new ImageIcon(this.getClass().getResource("square_null.png"));
SL = new ImageIcon(this.getClass().getResource("square_left.png"));
IN = new ImageIcon(this.getClass().getResource("invisible.png"));
addActionListener(this);
}
public void actionPerformed(ActionEvent e) {
value++;
value %= 3;
if (value == 1) {
setIcon(SN);
} else if (value == 2) {
setIcon(SL);
} else {
setIcon(IN);
}
}
}
最后一堂课是FBMouseListener
package swing;
import java.awt.Component;
import java.awt.Point;
import java.awt.event.*;
import javax.swing.event.MouseInputAdapter;
public class FBMouseListener extends MouseInputAdapter {
Point location;
MouseEvent pressed;
public void mousePressed(MouseEvent me) {
pressed = me;
System.out.println("Found mousePressed() in FBMouseListener()");
}
public void mouseDragged(MouseEvent me) {
System.out.println("Found mouseDragged() in FBMouseListener()");
Component component = me.getComponent();
location = component.getLocation(location);
int x = location.x - pressed.getX() + me.getX(); //ERROR HERE
int y = location.y - pressed.getY() + me.getY();
component.setLocation(x, y);
}
}
当我点击并拖动相应的println()确实出现。拖动带有一个我无法识别的错误。下面是最后一个println()和完整的错误。当我点击哪一行时，说它是FBMouseListener中的这一行（在代码中也标有注释）int x = location.x - pressed.getX() + me.getX(); //ERROR HERE。
Found mouseDragged() in FBMouseListener()Exception in thread "AWT-EventQueue-0"
java.lang.NullPointerException
at swing.FBMouseListener.mouseDragged(FBMouseListener.java:22)
at java.awt.AWTEventMulticaster.mouseDragged(AWTEventMulticaster.java:320)
at java.awt.Component.processMouseMotionEvent(Component.java:6581)
at javax.swing.JComponent.processMouseMotionEvent(JComponent.java:3342)
at java.awt.Component.processEvent(Component.java:6302)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4889)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4542)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2746)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
先感谢您。我一直试图解决这个问题，我很感激能得到的所有帮助。
您不应该保持对“已按下”的MouseEvent的引用，而是存储该位置本身。