JAva設計作業——単純計算機

設計要件:
計算機を設計し、GUIでグラフィックインタフェースを描き、2つの入力ボックス、1つの出力ボックス、4つの加減乗除ボタンがあり、簡単な加減乗除機能を実現します.
code:

import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.util.*;


public class cal extends Frame implements ActionListener
{
	private Button plus,minus,mul,div;
	private TextField Text1,Text2,Text3;
	private int a,b;
	
	public cal(){
	 super("      ");
	 
     plus=new Button("+");
     minus=new Button("-");
	 mul=new Button("*");
	 div=new Button("/");
	 
    
     Text1=new TextField("      ",20);
	 Text2=new TextField("      ",20);
	 Text3=new TextField("  ",20);
     //Text1.setEditable(false); 

      	this.setSize(300,200);
      	this.setBackground(Color.blue);
        this.setLocation(300,240);
        this.setLayout(new FlowLayout(FlowLayout.LEFT));
        
		this.add(Text1);
		this.add(Text2);
		this.add(Text3);
        this.add(plus); 
		this.add(minus); 
		this.add(mul);
		this.add(div);
		
        
		plus.addActionListener(this);
		minus.addActionListener(this);
		mul.addActionListener(this);
        div.addActionListener(this);
        this.addWindowListener(new Wadapt());
		
        this.setVisible(true); 
    }
	
	public void actionPerformed(ActionEvent e)
    {
		a = Integer.parseInt(Text1.getText());
		b = Integer.parseInt(Text2.getText());
       if(e.getSource()==plus){
		   String s = String.valueOf(a+b);
			Text3.setText(s);}
	   else if(e.getSource()==minus){
		   String s = String.valueOf(a-b);
			Text3.setText(s);}
	   else if(e.getSource()==mul){
		   String s = String.valueOf(a*b);
			Text3.setText(s);}
	   else if(e.getSource()==div){
		   if (b==0) {Text3.setText("      ！");}
		   else {
		   String s = String.valueOf(a/b);
			Text3.setText(s);}
	   }  
    }
    public static void main(String args[])
    {
    	cal mycal=new cal();
    }
}
class Wadapt extends WindowAdapter
{
    public void windowClosing(WindowEvent evt)
    {
        Frame frm=(Frame)evt.getSource();
        frm.setVisible(false);
        frm.dispose();
        System.exit(0);
    }
}