Swing 이용하여 계산기 만들기 (CalTest)

package swingTest;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;
public class CalTest extends JFrame implements ActionListener {
     private JTextField tf1, tf2, tf3;
     
     public CalTest() {
          setTitle("계산기");
          setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          setLayout(new FlowLayout());
          tf1=new JTextField(5); //숫자 1
          tf2=new JTextField(5); //숫자 2
          tf3=new JTextField(20); //결과
          
          JButton b1=new JButton("+");
          JButton b2=new JButton("-");
          JButton b3=new JButton("*");
          JButton b4=new JButton("/");
          
          b1.addActionListener(this);
          b2.addActionListener(this);
          b3.addActionListener(this);
          b4.addActionListener(this);
          
          add(new JLabel("숫자1"));
          add(tf1);
          add(new JLabel("숫자2"));
          add(tf2);
          add(b1); add(b2); add(b3); add(b4);
          add(new JLabel("결과"));
          add(tf3);
          
          setSize(700,100);
          setVisible(true);
     }
     
     public static void main(String[] args) {
          new CalTest();
     }
     
     
     @Override
     public void actionPerformed(ActionEvent e) {
          String str=e.getActionCommand();
          try {
          int a= Integer.parseInt(tf1.getText());
          int b= Integer.parseInt(tf2.getText());
          
          switch(str) {
          case "+" : tf3.setText(String.valueOf(a+b)); break;
          case "-" : tf3.setText(a-b+""); break; //문자를 더하면 자동적으로 문자로 바뀜
          case "*" : tf3.setText(a*b+""); break;
          case "/" : tf3.setText(a/b+""); break;
              }
          }catch (NumberFormatException n) {
              tf3.setText("숫자를 입력하세요");
          }catch (ArithmeticException a) {
              tf3.setText("0으로 나눌 수 없습니다.");
          }
          
     }
     
}