Java：NullPointerException一个简单的GUI程序

嗨，我是新手Java学习者。我正在努力实现一个简单的GUI程序，通过点击按钮来改变面板的颜色。Java：NullPointerException一个简单的GUI程序

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); 
    } 
}

我得到的输出窗口。但是当我点击按钮时，出现以下异常。

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)

请指点。

亲切的问候。

来源

2013-03-31 user2228741

不应该将该类命名为Button_label吗？ – stevenelberger

你永远不会初始化this.frame，所以它是null。

在go()，您创建和初始化不同的变量，名为frame：

JFrame frame = new JFrame();

你可能想要删除的第一JFrame：

frame = new JFrame();

来源

2013-03-31 07:15:43 NPE

你是这些例外的冠军，你不是NPE吗？ ;） – karmanaut

@karmanaut：我的面包和黄油:) – NPE

既然没有PM在这里，你在编程领域有多久了？你的个人资料说2年，但在这个领域没有多久。 – karmanaut

您已经声明2个变量称为frame - 一类变量和内部go。取出声明中go &只是初始化内部go

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

更改上面

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

，因为你在go创建一个名为frame新的变量，类类变量变量frame永远不会初始化。它仍然是null因此NullPointerException

来源

2013-03-31 07:16:58 user93353

帧为空。只需使用一个setter的框架。例如：

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

此外，您已声明帧两次。从go（）中移除它;

来源

2013-03-31 07:17:16 lokoko

改变这一行：

JFrame frame = new JFrame();

通过这样的：

frame = new JFrame();

来源

2013-03-31 07:17:32

public class Button_lable implements ActionListener { 
public JFrame frame; // This is a class/global variable 
//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(); // This is local variable. 
    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); 


}

您已定义的类变量作为帧。您正在实例化局部变量帧。所以不分配它是非常明显的，它会抛出NPE。因为局部变量的范围仅在特定的方法中。

您只需要做以下操作。

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); 


}

来源

2013-03-31 07:26:58 PVR

让我来解释Exception的原因。

您在第一个括号下声明的第一个public JFrame frame;具有全局范围。你初始化的第二个JFrame frame = new JFrame();创建它的一个新实例。您在go（）方法中声明的框架具有局部范围，并且它初始化它不是全局范围。这就是为什么global scope instance of frame is uninitialized。 这是造成NullPointerException。

来源

2015-07-23 07:21:34 Nepster

Java：NullPointerException一个简单的GUI程序

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