JFrame写的计算器，可以运作，但一计算就会报错。

JFrame写的计算器，可以运行，但一计算就会报错。。

Java code

import java.applet.Applet;import java.awt.BorderLayout;import java.awt.Choice;import java.awt.GridLayout;import java.awt.Panel;import java.awt.TextField;public class Calculation extends Applet {   TextField t1 = new TextField ( "", 7 );   TextField t2 = new TextField ( "", 7 );   TextField t3 = new TextField ( "", 14 );      public void init () {    setLayout ( new BorderLayout ( ) );    Panel p = new Panel ( );    add ( "North", p );    p.setLayout ( new GridLayout ( 1 , 2 ) );    p.add ( t1 );    p.add ( t2 );    Choice ch;    ch = new Choice ();    ch.addItem("+");    ch.addItem("-");    ch.addItem("*");    ch.addItem("/");    add ( "Center", ch );    add ( "South", t3 );    ch.addItemListener ( new LL ( ));   }}import java.awt.TextField;import java.awt.event.ItemEvent;import java.awt.event.ItemListener;public class LL implements ItemListener {   public void itemStateChanged ( ItemEvent ie ) {       TextField t1 = new TextField ( "", 7 );       TextField t2 = new TextField ( "", 7 );       TextField t3 = new TextField ( "", 14 );     if ( ie.getStateChange () == ItemEvent.SELECTED )     {       String S = (String)ie.getItem ();       String S1, S2, S3 = null;       int num1, num2;       S1 = t1.getText();       num1 = Integer.parseInt(S1);       S2 = t2.getText();       num2 = Integer.parseInt(S2);        if ( S.equals( "+" ))         S3 = String.valueOf( num1 + num2 );       else if ( S.equals( "-" ) )         S3 = String.valueOf ( num1 - num2 );       else if ( S.equals ( "*" ) )         S3 = String.valueOf ( num1 * num2 );        else if ( S.equals ( "/" ) )         S3 = String.valueOf ( num1 / num2 );           t3.setText( S3 );       }      }        }

昨天学习了JFrame,这是参照书上写的一个小型计算器。
左右两边各有两个TextField，输入数字之后选择中间的计算方法（加减乘除），然后在最底下的TextField显示结果。
这是按照书上的一个类似程序写的，然后自己又加了点东西。
可以运行出界面，但一选择符号就提示出错。
请问问题出在哪里？最好帮忙修改下。十分感谢！

[解决办法]
S1 = t1.getText();
这句 s1是""，所以下面
num1 = Integer.parseInt(S1);
没办法转成Integer

因为TextField t1 = new TextField ( "", 7 );
所以t1的值是空的
[解决办法]
就像2楼说的，t1,t2是空。下面改写你的代码可以试试。

//import java.applet.Applet;
import java.awt.BorderLayout;
import java.awt.Choice;
import java.awt.GridLayout;
import java.awt.Panel;
import java.awt.TextField;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Calculation extends JFrame implements ItemListener{
JTextField t1 = new JTextField( "1", 7 );
JTextField t2 = new JTextField( "1", 7 );
JTextField t3 = new JTextField( "", 14 );
 
public Calculation () {
setSize(200,200);
setTitle("Calculation");
JPanel p = new JPanel( );
p.setLayout(new FlowLayout( ));
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//p.setLayout ( new GridLayout ( 1 , 4 ) );
p.add(t1);
p.add(t2);
Choice ch;
ch = new Choice();
ch.addItem("+");
ch.addItem("-");
ch.addItem("*");
ch.addItem("/");
p.add( "Center", ch );
p.add( "South", t3 );
ch.addItemListener( this);
setContentPane(p);
 // show();
}
public static void main(String[] arguments){
Calculation cal=new Calculation();
cal.show();

}


//import java.awt.TextField;

//import java.awt.event.ItemEvent;
//import java.awt.event.ItemListener;


public void itemStateChanged(ItemEvent ie ) {
// TextField t1 = new TextField ( "", 7 );
// TextField t2 = new TextField ( "", 7 );
// TextField t3 = new TextField ( "", 14 );
if(ie.getStateChange() == ItemEvent.SELECTED)
{
String S = (String)ie.getItem();
String S1, S2, S3 = null;
int num1, num2;
S1 = t1.getText();
num1 = Integer.parseInt(S1);
S2 = t2.getText();
num2 = Integer.parseInt(S2); 
if ( S.equals( "+" ))
S3 = String.valueOf( num1 + num2 );
else if ( S.equals( "-" ) )
S3 = String.valueOf ( num1 - num2 );
else if ( S.equals ( "*" ) )
S3 = String.valueOf ( num1 * num2 ); 
else if ( S.equals ( "/" ) )
S3 = String.valueOf ( num1 / num2 );
t3.setText( S3 );
}
}
}

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