NullPointerException異常的擺幅的actionPerformed

Question

This is a login form for a slot machine game which will evoke a "NextPage" object. Idon't think this LoginDemo.java code has to do anything with the error but I prefer topost the entire code. Thanks in advance..

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

class Login extends JFrame implements ActionListener 
{ 
    JButton SUBMIT; 
    JPanel panel; 
    JLabel label1,label2; 
    final JTextField text1,text2; 
    Login() 
    { 
     label1 = new JLabel(); 
     label1.setText("Username:"); 
     label1.setBounds(50,50,100,30); 
     text1 = new JTextField(15); 
     text1.setBounds(250,50,100,30); 
     label2 = new JLabel(); 
     label2.setText("Password:"); 
     label2.setBounds(50,250,100,30); 
     text2 = new JPasswordField(15); 
     text2.setBounds(250,250,100,30); 
     //this.setLayout(new BorderLayout()); 

     SUBMIT=new JButton("SUBMIT"); 
     SUBMIT.setBounds(350,350,100,30); 

     //  panel=new JPanel(new GridLayout(3,1)); 
      panel=new JPanel(null); 
     panel.setBounds(50,50,1000,1000); 
     // panel.setLayout(new FlowLayout()); 
     panel.add(label1); 
     panel.add(text1); 
     panel.add(label2); 
     panel.add(text2); 
     panel.add(SUBMIT); 
      add(panel,BorderLayout.CENTER); 
       SUBMIT.addActionListener(this); 
       setTitle("LOGIN FORM"); 
    } 
    public void actionPerformed(ActionEvent ae) 
    { 
     String value1=text1.getText(); 
     String value2=text2.getText(); 
     if (value1.equals("a") && value2.equals("a")) 
     { 
      NextPage page=new NextPage(); 
      page.setVisible(true); 
      //JLabel label = new JLabel("Welcome:"+value1); 
       // page.getContentPane().add(label); 
      setVisible(false); 
     } 
     else 
     { 
      //System.out.println("enter the valid username and password"); 
      JOptionPane.showMessageDialog(this,"Incorrect login or password","Error",JOptionPane.ERROR_MESSAGE); 
     } 
    } 
} 
class LoginDemo 
{ 
    public static void main(String arg[]) 
{ 
    try 
    { 
     Login frame=new Login(); 
     frame.setSize(1000,1000); 
     frame.setVisible(true); 
    } 
    catch(Exception e) 
    { 
     JOptionPane.showMessageDialog(null, e.getMessage()); 
    } 

    } 
} 



    //NextPage.java 

import javax.swing.*; 
import java.awt.*; 
import java.awt.event.*; 
import java.applet.*; 
import java.util.Random; 
import java.sql.*; 
import java.lang.*; 
import java.awt.image.*; 
import java.awt.image.BufferedImage; 
import javax.imageio.ImageIO; 
import java.io.*; 
import java.awt.event.ActionEvent; 
import java.awt.event.ActionListener; 

public class NextPage extends JFrame implements Runnable, ActionListener 
{ 
     BufferedImage img1,i0,i1,i2,i3,i4,i5,i6,i7,i8,i9,i10,i11,i12,star1; 
    JFrame f1; 
    Thread t1,t2,t3; 
    int x1,y1,i,j,k,l,m,n,r1,r2,r3; 
    JButton b1; 
    Random r=new Random(); 
    TextField tf1; 
     JPanel p1; 

    NextPage() 
    { 
     setBounds(50,50,1000,1000); 
     p1=(JPanel)(getContentPane()); 
     p1.setBounds(50,50,1000, 1000); 
     p1.setLayout(null); 
     //tf1=new TextField(20); 
     //tf1.setBounds(50,50,100,30); 
     setDefaultCloseOperation(javax.swing.WindowConstants.DISPOSE_ON_CLOSE); 
     setTitle("Welcome"); 

     b1=new JButton("START"); 

     //p1.add(tf1); 

     b1.setBounds(150,400,100,30); 
     //try{ 
     b1.addActionListener(this); 
     //} 
     /*catch(Exception e) 
     { 
      System.out.println(""+e); 
     }*/ 
     add(b1); 
     setVisible(true); 
     try 
     {  
      i0=ImageIO.read(new File("tea.jpg")); 
       i1=ImageIO.read(new File("coffee.jpg")); 
      i2=ImageIO.read(new File("bhel.jpg")); 
      i3=ImageIO.read(new File("pizza.jpg")); 
      i4=ImageIO.read(new File("icecream.jpg")); 
      i5=ImageIO.read(new File("dosa.jpg")); 
      i6=ImageIO.read(new File("idli.jpg")); 
      i7=ImageIO.read(new File("panipuri.jpg")); 
      i8=ImageIO.read(new File("orange.jpg")); 
      i9=ImageIO.read(new File("watermelon.jpg")); 
      i10=ImageIO.read(new File("pavbhaji.jpg"));  
      i11=ImageIO.read(new File("noodles.jpg")); 
      i12=ImageIO.read(new File("sandwich.jpg"));  
     } 
     catch (IOException ie) 
     { 
      System.out.println("Error:"+ie.getMessage()); 
     } 

     j=0; m=0; n=0; 

     r1= r.nextInt(13); 
     r2= r.nextInt(13); 
     r3= r.nextInt(13); 

    } 
    public void init() 
    { 
     t1=new Thread(this); 
     t2=new Thread(this); 
     t3=new Thread(this); 
    } 

    public void start() 
    { 
     t2.start(); 
     t3.start(); 
    } 
    public void run() 
    { 
     try 
     { 
      if(Thread.currentThread()==t1) 
      for(j=0; j<150; j++) 
      //for(i=0; i<=150; i+=20) 
      { 
       r1= r.nextInt(13); 
       repaint(); 
       t1.sleep(10); 
      } 

      if(Thread.currentThread()==t2) 
      for(m=0; m<160; m++) 
      //for(k=0; k<=150; k+=20) 
      { 
       r2= r.nextInt(13); 
       repaint(); 
       t2.sleep(10); 
      } 
      if(Thread.currentThread()==t3) 
      for(n=0; n<170; n++) 
      //for(l=0; l<=150; l+=20) 
      { 
       r3= r.nextInt(13); 
       repaint(); 
       t3.sleep(10); 
      } 
     } 
     catch(Exception e) 
     { 
      System.out.println(""+e); 
     } 

    } 

    public void paint(Graphics g) 
    { 
     g.setColor(Color.blue); 

     g.drawRect(50,300,150,80); 
     g.drawRect(200,300,150,80); 
     g.drawRect(350,300,150,80); 

      switch(r1) 
      { 
       case 0: g.drawImage(i0,65,300,this); 
       break; 
       case 1: g.drawImage(i1,65,300,this); 
       break; 
       case 2: g.drawImage(i2,65,300,this); 
       break; 
       case 3: g.drawImage(i3,65,300,this); 
       break; 
       case 4: g.drawImage(i4,65,300,this); 
       break; 
       case 5: g.drawImage(i5,65,300,this); 
       break; 
       case 6: g.drawImage(i6,65,300,this); 
       break; 
       case 7: g.drawImage(i7,65,300,this); 
       break; 
       case 8: g.drawImage(i8,65,300,this); 
       break; 
       case 9: g.drawImage(i9,65,300,this); 
       break; 
       case 10: g.drawImage(i10,65,300,this); 
       break; 
       case 11: g.drawImage(i11,65,300,this); 
       break; 
       case 12: g.drawImage(i12,65,300,this); 
       break; 
      } 

      switch(r2) 
      { 
       case 0: g.drawImage(i0,215,300,this); 
       break; 
       case 1: g.drawImage(i1,215,300,this); 
       break; 
       case 2: g.drawImage(i2,215,300,this); 
       break; 
       case 3: g.drawImage(i3,215,300,this); 
       break; 
       case 4: g.drawImage(i4,215,300,this); 
       break; 
       case 5: g.drawImage(i5,215,300,this); 
       break; 
       case 6: g.drawImage(i6,215,300,this); 
       break; 
       case 7: g.drawImage(i7,215,300,this); 
       break; 
       case 8: g.drawImage(i8,215,300,this); 
       break; 
       case 9: g.drawImage(i9,215,300,this); 
       break; 
       case 10: g.drawImage(i10,215,300,this); 
       break; 
       case 11: g.drawImage(i11,215,300,this); 
       break; 
       case 12: g.drawImage(i12,215,300,this); 
       break; 
      } 

      switch(r3) 
      { 
       case 0: g.drawImage(i0,365,300,this); 
       break; 
       case 1: g.drawImage(i1,365,300,this); 
       break; 
       case 2: g.drawImage(i2,365,300,this); 
       break; 
       case 3: g.drawImage(i3,365,300,this); 
       break; 
       case 4: g.drawImage(i4,365,300,this); 
       break; 
       case 5: g.drawImage(i5,365,300,this); 
       break; 
       case 6: g.drawImage(i6,365,300,this); 
       break; 
       case 7: g.drawImage(i7,365,300,this); 
       break; 
       case 8: g.drawImage(i8,365,300,this); 
       break; 
       case 9: g.drawImage(i9,365,300,this); 
       break; 
       case 10: g.drawImage(i10,365,300,this); 
       break; 
       case 11: g.drawImage(i11,365,300,this); 
       break; 
       case 12: g.drawImage(i12,365,300,this); 
       break; 
      } 
     } 
     public void actionPerformed(ActionEvent ae) 
     { 
      if(ae.getSource()==b1) 
      t1.start(); 
     } 

     /*public static void main(String args[]) 
     { 
      NextPage o=new NextPage(); 
     }*/ 
    } 
} 

Here are the exceptions that I get-

F:\java\jdk\bin>java LoginDemo Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at NextPage.actionPerformed(NextPage.java:235) at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995) at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318) atjavax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387) at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242) atjavax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236) at java.awt.Component.processMouseEvent(Component.java:6289) at javax.swing.JComponent.processMouseEvent(JComponent.java:3267) at java.awt.Component.processEvent(Component.java:6054) at java.awt.Container.processEvent(Container.java:2041) at java.awt.Component.dispatchEventImpl(Component.java:4652) at java.awt.Container.dispatchEventImpl(Container.java:2099) at java.awt.Component.dispatchEvent(Component.java:4482)at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577) at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238) at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168) at java.awt.Container.dispatchEventImpl(Container.java:2085) at java.awt.Window.dispatchEventImpl(Window.java:2478) at java.awt.Component.dispatchEvent(Component.java:4482) at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:644) at java.awt.EventQueue.access$000(EventQueue.java:85)

at java.awt.EventQueue$1.run(EventQueue.java:603)

at java.awt.EventQueue$1.run(EventQueue.java:601)

at java.security.AccessController.doPrivileged(Native Method)

atjava.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87

at

java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98

at java.awt.EventQueue$2.run(EventQueue.java:617)

at java.awt.EventQueue$2.run(EventQueue.java:615)

at java.security.AccessController.doPrivileged(Native Method)

atjava.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87

at java.awt.EventQueue.dispatchEvent(EventQueue.java:614)

at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)

at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)

at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161) at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Can't get any solution.. I am a learner..please help..

Answer 1

您使用變量t1來啓動應該分配給此變量的線程。永遠不會調用創建分配給此變量的線程實例的函數init()。有t1是null並調用null對象上的任何功能導致NullPointerException。

只需撥打init()函數即可。我認爲最初的想法是在構造函數中調用它，你忘記插入它。

對於將來的問題，請考慮只發布接近StackTrace報告錯誤的點的行。要找到感興趣的地方，請查看StackTrace並查找您編寫的類中的條目。在這種情況下，NextPage.actionPerformed(NextPage.java:235)是一個有趣的點。在你的問題最好標記這些點快速獲得一個很好的決議。

Answer 2

你不叫init方法，因此你t1變量仍然是null當你進入actionPerformed方法。

這在堆棧跟蹤中非常明顯（該行只有一件東西可以是null），並且如果從堆棧跟蹤中不明顯，調試器會立即顯示出來。用數百行不相關的代碼創建帖子的方法更快