如何解決「在線程中的異常」主要「java.lang.NullPointerException」

我正在寫一個Java程序，試圖使一個新的面板搞砸了，並以某種方式搞砸了，不會讓我運行程序。我回到了工作的最後一點，但仍然不好。我不知道我錯在哪裏。從錯誤我只能假設我迷失在主要方法和makeGUI方法之間的某處，但我看不到任何錯誤。如何解決「在線程中的異常」主要「java.lang.NullPointerException」

我得到的錯誤是;

Exception in thread "main" java.lang.NullPointerException 
at java.awt.Container.addImpl<Container.java:1090> 
at java.awt.Container.add<Container.java:966> 
at assignment2g2.buildGUI<assignment2g2.java:47> 
at assignment2g2.<init><assignment2g2.java:31> 
at assignment2g2.main<assignment2g2.java:124>

和我的代碼...

import javax.swing.*; 
import javax.swing.border.LineBorder; 
import javax.swing.event.MouseInputListener; 

import java.awt.*; 
import java.awt.event.*; 

public class assignment2g2 extends JFrame implements MouseInputListener { 

JPanel mousePanel, actionsPanel; 
JLabel mouseLabel; 
JLabel topLeft,topMid,topRightMid,topRight,midLeft,midMid,midRightMid,midRight,btmLeft,btmMid,btmRightMid,btmRight; 
ImageIcon circle = new ImageIcon("circle.jpg"); 
ImageIcon square = new ImageIcon("square.jpg"); 
ImageIcon triangle = new ImageIcon("triangle.jpg"); 

//JButton button1=new JButton("?"); 
//JButton button2=new JButton("?"); 

public assignment2g2() { 


    buildGUI(); 
    buildActionsPanel(); 
    mousePanel.add(actionsPanel); 
    getContentPane().add(mousePanel); 

    setSize(800,400); 
    setVisible(true); 

} 

public void buildGUI() { 



    mousePanel = new JPanel(); 
    mousePanel.setLayout(new BorderLayout()); 
    mousePanel.add(mouseLabel,BorderLayout.SOUTH); 
    mousePanel.addMouseListener(this); 
    mousePanel.addMouseMotionListener(this); 

    mouseLabel = new JLabel(); 
    mouseLabel.setBorder(new LineBorder(Color.BLACK)); 
    mouseLabel.setForeground(Color.RED); 
    mouseLabel.setText("Please select an answer"); 

    actionsPanel = new JPanel(); 
    actionsPanel.setLayout(new GridLayout(3,3)); 


} 

public void buildActionsPanel() { 

    topLeft = new JLabel(); 

    topMid = new JLabel(); 
    //topMid.setIcon(circle); 
    topMid.setText("circle"); 


    topRightMid = new JLabel(); 

    topRight = new JLabel(); 

    midLeft = new JLabel(); 
    midLeft.setBorder(new LineBorder(Color.BLACK)); 
    midLeft.addMouseListener(this); 
    midLeft.setText("Square"); 

    midMid = new JLabel(); 
    midMid.setBorder(new LineBorder(Color.BLACK)); 
    midMid.addMouseListener(this); 
    midMid.setText("Triangle"); 

    /*midRightMid = new JLabel(); 
    midRightMid.setBorder(new LineBorder(Color.BLACK)); 
    midRightMid.addMouseListener(this); 
    midRightMid.setText("Circle");*/ 

    midRight = new JLabel(); 
    midRight.setBorder(new LineBorder(Color.BLACK)); 
    midRight.addMouseListener(this); 
    midRight.setText("Circle"); 

    btmLeft = new JLabel(); 

    btmMid = new JLabel(); 

    btmRightMid = new JLabel(); 

    btmRight= new JLabel(); 




    actionsPanel.add(topLeft); 
    actionsPanel.add(topMid); 
    //actionsPanel.add(topRightMid); 
    actionsPanel.add(topRight); 
    actionsPanel.add(midLeft); 
    actionsPanel.add(midMid); 
    //actionsPanel.add(midRightMid); 
    actionsPanel.add(midRight); 
    actionsPanel.add(btmLeft); 
    actionsPanel.add(btmMid); 
    //actionsPanel.add(btmRightMid); 
    actionsPanel.add(btmRight); 


} 


public static void main(String[] args) { 
    new assignment2g2(); 
} 

public void mouseClicked(MouseEvent e) { 
} 

public void mouseEntered(MouseEvent e) { 

} 

public void mouseExited(MouseEvent e) { 

} 






public void mousePressed(MouseEvent e) { 
    mouseLabel.setText("Mouse Pressed Event"); 
    //if(e.getSource() instanceof JLabel) { 
    // JOptionPane.showMessageDialog(null, "Incorrect", "Incorrect", JOptionPane.INFORMATION_MESSAGE); 
    //}  


    if(e.getSource()==midRight) { 
     if((topMid.getText()).matches("circle")){ 
      JOptionPane.showMessageDialog(null, "Correct", "Correct", JOptionPane.INFORMATION_MESSAGE); 
      //topMid.setIcon(square); 
      topMid.setText("square"); 
     } 
     else{ 
      JOptionPane.showMessageDialog(null, "Incorrect", "Incorrect", JOptionPane.INFORMATION_MESSAGE); 
     } 
    } 
    if(e.getSource()==midMid) { 
     if((topMid.getText()).matches("triangle")){ 
      JOptionPane.showMessageDialog(null, "Correct", "Correct", JOptionPane.INFORMATION_MESSAGE); 
      //topMid.setIcon(square); 
      topMid.setText("circle"); 
     } 
     else{ 
      JOptionPane.showMessageDialog(null, "Incorrect", "Incorrect",  JOptionPane.INFORMATION_MESSAGE); 
     } 
    } 
    if(e.getSource()==midLeft) { 
     if((topMid.getText()).matches("square")){ 
      JOptionPane.showMessageDialog(null, "Correct", "Correct", JOptionPane.INFORMATION_MESSAGE); 
      //topMid.setIcon(square); 
      topMid.setText("triangle"); 
     } 
     else{ 
      JOptionPane.showMessageDialog(null, "Incorrect", "Incorrect", JOptionPane.INFORMATION_MESSAGE); 
     } 
    } 

} 

public void mouseReleased(MouseEvent e) { 
} 

public void mouseDragged(MouseEvent e) {  
} 

public void mouseMoved(MouseEvent e) { 
} 

}

如果有人可以幫助我，這將是巨大的。對不起事先，我知道代碼是有點亂的：L

來源

2014-12-07 Ryan Fitzmaurice

不，他們是不同的東西。我之前有過它，它運行正常。 – 2014-12-07 00:17:08

您在'buildGUI（）'方法中向您的容器（'mousePanel'）添加'null'組件（'mouseLabel'） – jhamon 2014-12-07 00:17:43

仔細查看堆棧跟蹤。它告訴你在'assign2g2.java'的第47行代碼中發生了異常。轉到那行代碼，找出那裏不應該是'null'的'null'。 – Jesper 2014-12-07 00:17:48

是因爲你在初始化之前mouseLabel分配

mousePanel.add(mouseLabel,BorderLayout.SOUTH);

。

mouseLabel = new JLabel();

來源

2014-12-07 00:19:13

你加入mouseLabel到mousePanel時mouseLabel還沒有被初始化。這就是爲什麼你有一個nullPointerException。

相反，你應該這樣做

public void buildGUI() { 



mousePanel = new JPanel(); 
mousePanel.setLayout(new BorderLayout()); 
mousePanel.addMouseListener(this); 
mousePanel.addMouseMotionListener(this); 

mouseLabel = new JLabel(); 
mouseLabel.setBorder(new LineBorder(Color.BLACK)); 
mouseLabel.setForeground(Color.RED); 
mouseLabel.setText("Please select an answer"); 

mousePanel.add(mouseLabel,BorderLayout.SOUTH);/*Add The mousePanel Object after it has being initialized*/ 

actionsPanel = new JPanel(); 
actionsPanel.setLayout(new GridLayout(3,3)); 
}

我希望這可以幫助兄弟。

來源

2014-12-07 00:25:40

如何解決「在線程中的異常」主要「java.lang.NullPointerException」

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