高手帮我看看与什么问题

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
public class Compute implements ActionListener
{
static JFrame f=null;
JLabel l4;
JTextField t1;
JTextField t2;
JTextField t3;
public Compute()
{
f=new JFrame("简易计算器");
Container contentPane=f.getContentPane();
JPanel p1=new JPanel();
p1.setLayout(new GridLayout(2,3));
p1.setBorder(BorderFactory.createTitledBorder(BorderFactory.createLineBorder(Color.blue,2),"进行运算",TitledBorder.CENTER,TitledBorder.TOP));
JLabel l1=new JLabel("the first number:");
JLabel l2=new JLabel("运算符");
JLabel l3=new JLabel("the second number:");
t1=new JTextField();
t2=new JTextField();
t3=new JTextField();
p1.add(l1);
p1.add(t1);
p1.add(l2);
p1.add(t2);
p1.add(l3);
p1.add(t3);
JPanel p2=new JPanel();
p2.setLayout(new GridLayout(1,2));
JButton button=new JButton("结果：");
l4=new JLabel("",SwingConstants.CENTER);
p2.add(button);
p2.add(l4);
button.addActionListener(this);
contentPane.add(p1,BorderLayout.NORTH);
contentPane.add(p2,BorderLayout.SOUTH);
f.getRootPane().setDefaultButton(button);
f.pack();
f.addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent evt){
System.exit(0);
}
});
}
public void actionPerformed(ActionEvent e)
{
double a=Double.parseDouble(t1.getText());
double b=Double.parseDouble(t3.getText());
String theta=t2.getText();
double c;
int d=0;