little error in java code

[saneha]

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class SmallCalcApp implements ActionListener{

JFrame frame;
JLabel firstOperand, secondOperand, answer;
JTextField op1, op2, ans;
JButton plus, mul;

public void initGUI(){

frame = new JFrame();

firstOperand = new JLabel("First Operand");
secondOperand = new JLabel ("Second Operand");
answer = new JLabel ("Answer");

op1 = new JTextField (15);
op2 = new JTextField (15);
ans = new JTextField (15);

mul = new JButton ("*");
mul.setPreferredSize(new Dimension(70,25));

plus = new JButton ("+");
plus.setPreferredSize(new Dimension(70,25));

Container con = frame.getContentPane();
con.setLayout(new FlowLayout());

con.add(firstOperand);
con.add(op1);

con.add(secondOperand);
con.add(op2);

con.add(plus);
con.add(ans);

plus.addActionListener(this);
mul.addActionListener(this);

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(200,220);
frame.setVisible(true);

}


public SmallCalcApp(){

initGUI();

}

public void actionPerformed(ActionEvent event){

String oper, result;
int num1, num2, res;

if (event.getSource() == plus){

oper = op1.getText();
num1 = Integer.parseInt(oper);

oper = op2.getText();
num2 = Integer.parseInt(oper);

res = num1 + num2 ;

result = res + "";
ans.setText(result);


}

else if (event.getSource() == mul){

oper = op1.getText();
num1 = Integer.parseInt(oper);

oper = op2.getText();
num2 = Integer.parseInt(oper);

res = num1 * num2;

result = res + "";

ans.setText(result);

}
}

public static void main (String args[]){

SmallCalcApp scApp = new SmallCalcApp();

}
}