package com.elyong.noway;
/**
* Created by ely ong on 2017/11/27.
*/
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class SimpleGui3C implements ActionListener{
JFrame frame;
public static void main(String[] args){
SimpleGui3C gui=new SimpleGui3C();
gui.go();
}
public void go(){
JFrame frame=new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JButton button=new JButton("Change colors");
button.addActionListener(this);
MyDrawPanel drawPanel=new MyDrawPanel();
frame.getContentPane().add(BorderLayout.SOUTH,button);
frame.getContentPane().add(BorderLayout.CENTER,drawPanel);
frame.setSize(300,300);
frame.setVisible(true);
}
public void actionPerformed(ActionEvent event){
frame.repaint();
}
}
class MyDrawPanel extends JPanel{
public void paintComponent(Graphics g){
Graphics2D g2d=(Graphics2D) g;
int red=(int)(Math.random()*255);
int green=(int)(Math.random()*255);
int blue=(int)(Math.random()*255);
Color startColor=new Color(red,green,blue);
red=(int)(Math.random()*255);
green=(int)(Math.random()*255);
blue=(int)(Math.random()*255);
Color endColor=new Color(red,green,blue);
GradientPaint gradient=new GradientPaint(70,70,startColor,150,150,endColor);
g2d.setPaint(gradient);
g2d.fillOval(70,70,100,100);
}
}
目的是写一个按下按钮圆圈就会改变颜色的程序。错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at com.elyong.noway.SimpleGui3C.actionPerformed(SimpleGui3C.java:27)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6533)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6298)
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:4525)
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)
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