Java:NullPointerException是一个简单的GUI程序

大家好，我是Java初学者。我试图实现一个简单的GUI程序，通过点击按钮来改变面板的颜色

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

public class Button_lable implements ActionListener {
public JFrame frame;
//JPanel panel;
//JLabel label;

public static void main(String[] args) {

    Button_lable gui = new Button_lable();
    gui.go();
}//end of main
public void go(){
    //System.out.println("Entered Go()");
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    JButton b_frame = new JButton("Click to change the color");
    b_frame.addActionListener(this);
    MyDrawpanel d_panel = new MyDrawpanel();

    frame.getContentPane().add(BorderLayout.SOUTH,b_frame);
    frame.getContentPane().add(BorderLayout.CENTER ,d_panel);

    frame.setSize(300, 300);
    frame.setVisible(true);


}//end of go
public void actionPerformed(ActionEvent e) {
    frame.repaint();
}


}//end of Button_lable


class MyDrawpanel extends JPanel {
    public void paintComponent(Graphics g){
Graphics2D grph = (Graphics2D) g;
int red = (int)(Math.random()* 255);
int green = (int)(Math.random()* 255);
int blue = (int)(Math.random()* 255);
Color strt_clr = new Color(red,green,blue);

red = (int)(Math.random()* 255);
green = (int)(Math.random()* 255);
blue = (int)(Math.random()* 255);
Color end_clr = new Color(red,green,blue);

GradientPaint gradient = new GradientPaint(70,70,strt_clr,150,150,end_clr);  
grph.setPaint(gradient);
grph.fillOval(50,25, 150, 150);
  }
}

---

我得到输出窗口。但是当我点击按钮时，我得到了以下异常

 Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Button_lable.actionPerformed(Button_lable.java:34)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

---

请给我一些建议

---

亲切问候。

您从未初始化此.frame，因此它是

null

在

go（）

中，创建并初始化另一个名为

frame

的变量：

JFrame frame = new JFrame();

frame = new JFrame();

frame = new JFrame();

您可能需要删除第一个

JFrame

：

JFrame frame = new JFrame();

frame = new JFrame();

frame = new JFrame();

---

您已经声明了两个名为

frame

的变量-一个类变量和一个内部变量

go

。 删除

go

中的声明&只需初始化

go

public void go(){
    //System.out.println("Entered Go()");
    JFrame frame = new JFrame();

将上述内容更改为

public void go(){
    //System.out.println("Entered Go()");
    frame = new JFrame();

---

由于您正在

go

中创建一个名为

frame

的新变量，因此类变量

frame

永远不会初始化。它保持为

null

，因此

NullPointerException

帧为null。只需使用一个框架设置器。比如：

void setFrame(JFrame theFrame) {
    this.frame = theFrame;
}

您还声明了两次框架。将其从go（）中移除

更改此行：

JFrame frame = new JFrame();

据此：

您已经将类变量定义为frame。 您正在实例化局部变量frame。因此，如果不指定，很明显它将抛出NPE。因为局部变量的作用域只存在于特定的方法中

你只需要做以下事情

public class Button_lable implements ActionListener {
public JFrame frame;
//JPanel panel;
//JLabel label;

public static void main(String[] args) {

    Button_lable gui = new Button_lable();
    gui.go();
}//end of main
public void go(){
    //System.out.println("Entered Go()");
    frame = new JFrame(); // here, the class variable is instantiated. So it wont give NPE.
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    JButton b_frame = new JButton("Click to change the color");
    b_frame.addActionListener(this);
    MyDrawpanel d_panel = new MyDrawpanel();

    frame.getContentPane().add(BorderLayout.SOUTH,b_frame);
    frame.getContentPane().add(BorderLayout.CENTER ,d_panel);

    frame.setSize(300, 300);
    frame.setVisible(true);


}

---

让我解释一下例外的原因。

---

第一个

公共JFrame具有全局范围。第二个
JFrame=newjframe（）初始化将创建它的新实例。在go（）方法中声明的框架具有局部作用域，它初始化它而不是全局作用域。这就是为什么框架的全局作用域实例未初始化的原因它导致了
NullPointerException
该类不应该被命名为Button\u label吗？你是这些异常的拥护者，不是NPE吗？；）因为这里没有PM，你在编程领域有多久了？你的个人资料上写的是两年，但不是多久。