程序抛出NullPointerException异常不明白！

问题描述

在myFrame2中的actionPerformed方法下面加上Form1 = Application.Form1;就没问题，但是在windowOpened方法下面已经有了为什么还出现这种问题？ import java.awt.*;public class j02110501_Project1  {  private static j02110501_Project1 Application;    myFrame1 Form1;  myFrame2 Form2;  myFrame3 Form3;    public static void main(String arg[])  {    Application = new j02110501_Project1();    Application.Form1 = new myFrame1( Application );    Application.Form2 = new myFrame2( Application );        Application.Form1.setVisible(true);  }}import java.awt.*;import java.awt.event.*;public class myFrame1 extends Frame  {  private j02110501_Project1 Application;  private myFrame1 Form1;  private myFrame2 Form2;  private myFrame3 Form3;    Button B1, B2;  Label L1, L2;  public myFrame1(j02110501_Project1 app)  {    Application = app;    Form1 = this;        this.setLayout(null);    this.setBounds(100,300,265,200);    this.setTitle("Form1:主窗体");    this.addWindowListener(new myWindowAdapter() );    B1 = new Button("Show Form2");    B1.setBounds(60,30,140,30);    B1.addActionListener(new myActionListener() );    this.add(B1);    L1 = new Label("0",Label.RIGHT);    L1.setBounds(60,70,140,30);    L1.setBackground(Color.PINK);    this.add(L1);    B2 = new Button("Show Form3");    B2.setBounds(60,120,140,30);    B2.addActionListener(new myActionListener());    this.add(B2);    L2 = new Label("0",Label.RIGHT);    L2.setBounds(60,160,140,30);    L2.setBackground(Color.PINK);    this.add(L2);  }    class myWindowAdapter extends WindowAdapter  {    public void windowOpened(WindowEvent e)  {      //此时Application所有静态建立的窗体都已建立完成      //取得Form1所属项目的其他窗体      Form2 = Application.Form2;    }    public void windowClosing(WindowEvent e)  {      System.exit(0);    }  }    class myActionListener implements ActionListener  {    public void actionPerformed(ActionEvent e)  {      if(e.getSource() == B1)  {        Form2.setVisible(true);      }      else if(e.getSource()==B2)  {        if(Form3 == null)  {          Application.Form3 = new myFrame3(Application);          Form3 = Application.Form3;          System.out.println("Create Form");        }        else          System.out.println("Show Form");                Form3.setVisible(true);      }    }  }}import java.awt.*;import java.awt.event.*;import javax.swing.*;public class myFrame2 extends Frame  {  private j02110501_Project1 Application;  private myFrame1 Form1;  private myFrame2 Form2;  private myFrame3 Form3;    TextField text1,text2;  Label lb1;  Button B1;  public myFrame2( j02110501_Project1 app)  {    Application = app;    Form2 = this;        this.setLayout(null);    this.setBounds(420,300,300,90);    this.setTitle("Foem2: 加法运算");    this.addWindowListener(new myWindowAdapter());        text1 = new TextField("0",6);    text1.setBounds(30,40,80,30);    this.add(text1);    lb1 = new Label("+");    lb1.setBounds(115,40,20,30);    this.add(lb1);    text2 = new TextField("0",6);    text2.setBounds(150,40,80,30);    this.add(text2);    B1 = new Button("计算");    B1.setBounds(240,40,40,30);    B1.addActionListener(new myActionListener());    this.add(B1);  }    class myActionListener implements ActionListener  {      public void actionPerformed(ActionEvent e)  {      //Form1 = Application.Form1;      double sum;      try  {        sum = Double.parseDouble( Form2.text1.getText().trim())            + Double.parseDouble(Form2.text2.getText().trim());                Form1.L1.setText(String.valueOf(sum));        if(Form3 != null)  {          System.out.println("called by Form2:n Form3.text1 = "                      + Form3.text1.getText()                      + " Form3.text2 = "                      + Form3.text2.getText() );        }      }      catch(NumberFormatException ecp)  {        JOptionPane.showMessageDialog(null,"必须输入两个“数值”！");      }    }  }    class myWindowAdapter extends WindowAdapter  {    public void windowOpend(WindowEvent e)  {      Form1 = Application.Form1;      Form3 = Application.Form3;    }        public void windowActivated(WindowEvent e)  {      if(Form3 == null)        Form3 = Application.Form3;    }    public void windowClosing(WindowEvent e)  {      Form2.setVisible(false);    }  }}import java.awt.*;import java.awt.event.*;import javax.swing.*;public class myFrame3 extends Frame  {  private j02110501_Project1 Application;  private myFrame1 Form1;  private myFrame2 Form2;  private myFrame3 Form3;    TextField text1,text2;  Label lb1;  Button B1;  public myFrame3(j02110501_Project1 app)  {    Application = app;    Form3 = this;        this.setLayout(null);    this.setBounds(420,410,300,90);    this.setTitle("Foem3: 乘法运算");    this.addWindowListener(new myWindowAdapter());        text1 = new TextField("1",6);    text1.setBounds(30,40,80,30);    this.add(text1);    lb1 = new Label("*");    lb1.setBounds(115,40,20,30);    this.add(lb1);    text2 = new TextField("1",6);    text2.setBounds(150,40,80,30);    this.add(text2);    B1 = new Button("计算");    B1.setBounds(240,40,40,30);    B1.addActionListener(new myActionListener());    this.add(B1);  }    class myActionListener implements ActionListener  {    public void actionPerformed(ActionEvent e)  {      double multiplication;      try  {        multiplication =              Double.parseDouble(Form3.text1.getText().trim())             * Double.parseDouble(Form3.text2.getText().trim());        Form1.L2.setText(String.valueOf(multiplication));        System.out.println("called by Form3:n Form2.text1 = "                                + Form2.text1.getText()                                + " Form2.text2 = "                                + Form2.text2.getText() );      }      catch(NumberFormatException ecp)  {        JOptionPane.showMessageDialog(null,"必须输入两个“数值”!");      }    }  }    class myWindowAdapter extends WindowAdapter  {    public void windowOpened(WindowEvent e)  {      Form1 = Application.Form1;      Form2 = Application.Form2;    }    public void windowClosing(WindowEvent e)  {      Form3.setVisible(false);    }  }}错误信息：Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException        at myFrame2$myActionListener.actionPerformed(myFrame2.java:46)        at java.awt.Button.processActionEvent(Unknown Source)        at java.awt.Button.processEvent(Unknown Source)        at java.awt.Component.dispatchEventImpl(Unknown Source)        at java.awt.Component.dispatchEvent(Unknown Source)        at java.awt.EventQueue.dispatchEventImpl(Unknown Source)        at java.awt.EventQueue.access$000(Unknown Source)        at java.awt.EventQueue$1.run(Unknown Source)        at java.awt.EventQueue$1.run(Unknown Source)        at java.security.AccessController.doPrivileged(Native Method)        at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)        at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)        at java.awt.EventQueue$2.run(Unknown Source)        at java.awt.EventQueue$2.run(Unknown Source)        at java.security.AccessController.doPrivileged(Native Method)        at java.security.AccessControlContext$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)

解决方案

引用在myFrame2中的actionPerformed方法下面加上Form1 = Application.Form1;就没问题，但是在windowOpened方法下面已经有了为什么还出现这种问题？这个我运行了。问题出在myFrame2中的全局变量 private myFrame1 Form1; 它是你的主窗体没有被实例化。在myFrame2你应该让他指向j02110501_Project1 中已经实例化的主窗体。在这里就是你的Application.Form1。一个对象在没有被实例化之前，对象应用是不能操作对象的方法的。会报Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 也就是你的对象引用指向了空指针。在这里Form1 指向了空指针。所以报错。你让它指向Application.Form1就不会报错了。
解决方案二：
windowOpened不是windowOpend
解决方案三：
Form1.L1.setText(String.valueOf(sum)); Form1是null，或者L1是null
解决方案四：
myFrame2.java:46
解决方案五：
空指针，有对像没有实例化啊

时间： 2024-11-01 05:38:17

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