java中关于trycatch和变量类型的提问

import java.awt.*;
import java.awt.event.*;
class a extends Frame implements ActionListener{
Button 按钮1,按钮2,按钮3,按钮4;
TextField text1,text2,text3;
int a,b;
a(String s){
super(s);
按钮1=new Button("加法");
按钮2=new Button("减法");
按钮3=new Button("乘法");
按钮4=new Button("除法");
text1=new TextField(10);
text2=new TextField(10);
text3=new TextField(10);
text3.setEditable(false);
setLayout(new FlowLayout());
add(text1);add(text2);add(text3);
add(按钮1);add(按钮2);add(按钮3);add(按钮4);
按钮1.addActionListener(this);
按钮2.addActionListener(this);
按钮3.addActionListener(this);
按钮4.addActionListener(this);
setBounds(100,100,300,300);
setVisible(true);
validate();
addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent e){
System.exit(0);}});}
public void actionPerformed(ActionEvent e){