Java：简单GUI程序的NullPointerException

Question

嗨，我是Java新手。 我正试图实现一个简单的GUI程序，以在单击按钮时单击以更改面板的颜色。

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。 但是当我单击按钮时，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)

---

请指教。

亲切的问候。

Answer 1

您永远不会初始化this.frame ，因此它为null 。

在go() ，创建并初始化一个不同的变量，称为frame ：

JFrame frame = new JFrame();

您可能要删除第一个JFrame ：

frame = new JFrame();

Answer 2

您已经声明了2个称为frame变量-一个是类变量，另一个是在go 。 删除go内的声明并仅初始化go内的类变量

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

将以上更改为

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

因为要在go中创建一个称为frame的新变量，所以永远不会初始化类变量frame 。 它仍然为null ，因此为NullPointerException

Answer 3

框架为空。 只需使用二传手为框架。 就像是 ：

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

另外，您已两次声明帧。 从go（）中删除它；

Answer 4

更改此行：

JFrame frame = new JFrame();

这样：

frame = new JFrame();

Answer 5

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);


}

您已将类变量定义为框架。 您正在实例化局部变量框架。 因此，如果不进行分配，很明显它将抛出NPE。 因为局部变量的范围仅驻留在特定方法中。

您只需要执行以下操作。

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);


}

Answer 6

让我解释一下异常的原因。

第一个public JFrame frame; 您声明第一个花括号具有全局范围。 第二个JFrame frame = new JFrame(); 您初始化会为其创建一个新实例。 您在go（）方法中声明的框架具有局部范围，并且不会将其初始化为全局范围。 这就是为什么global scope instance of frame is uninitialized 。 它导致NullPointerException 。