Question

我正在写一个java程序，搞砸了它试图制作一个新面板并以某种方式搞砸了它并且不让我运行该程序。我回到了它工作的最后一点，但仍然没有好处。我不知道我哪里出错了。从错误中我只能假设我在主方法和makeGUI方法之间的某个地方搞砸了，但我看不出有什么不对。

我得到的错误是;

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>

我的代码是......

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) {  
}

}

如果有人可以帮助我，那就太好了。提前抱歉，我知道代码有点混乱：L

Answer 1

是因为你指定了

mousePanel.add(mouseLabel,BorderLayout.SOUTH);

在初始化mouseLabel之前

。

mouseLabel = new JLabel();

Answer 2

当mouseLabel尚未初始化时，您正在将mouseLabel添加到mousePanel。这就是你有一个nullPointerException。

相反你应该这样做

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));
}

我希望这有助于兄弟。

如何修复＆＃34;线程中的异常＆＃34; main＆＃34;显示java.lang.NullPointerException＆＃34;

2 个答案: