大家帮我看看有关于JApplet的问题？


import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.TitledBorder;
import javax.swing.JApplet;
public class LoanApplet extends JApplet{
   private JTextField jftAnnualInterestRate=new JTextField();
   private JTextField jftNumberofYears=new JTextField();
   private JTextField jftLoanAmount=new JTextField();
   private JTextField jftMonthlyPayment=new JTextField();
   private JTextField jftTotalPayment=new JTextField();
   private JButton jbtComputeLoan=new JButton("Compute Payment");
   public void Init(){
     jftMonthlyPayment.setEditable(false);
     jftTotalPayment.setEditable(false);
     jftAnnualInterestRate.setHorizontalAlignment(JTextField.RIGHT);
     jftNumberofYears.setHorizontalAlignment(JTextField.RIGHT);
     jftLoanAmount.setHorizontalAlignment(JTextField.RIGHT);
     JPanel p1=new JPanel(new GridLayout(5,2));
     p1.add(new JLabel("Annual Interest Rate"));
     p1.add(jftAnnualInterestRate);
     p1.add(new JLabel("Number of Years"));
     p1.add(jftNumberofYears);
     p1.add(new JLabel("Loan Amount"));
     p1.add(jftLoanAmount);
     p1.add(new JLabel("Monthly Payment"));
     p1.add(jftMonthlyPayment);
     p1.add(new JLabel("Total Payment"));
     p1.add(jftTotalPayment);
     p1.setBorder(new TitledBorder("Enter interest rate,year and loan amount"));
     JPanel p2=new JPanel(new FlowLayout(FlowLayout.RIGHT));
     p2.add(jbtComputeLoan);
     jbtComputeLoan.addActionListener(new ButtonListener());
  }
  private class ButtonListener implements ActionListener{
     public void actionPerformed(ActionEvent e){
         double interest=Double.parseDouble(jftAnnualInterestRate.getText());
         int year=Integer.parseInt(jftNumberofYears.getText());
         double loanAmount=Double.parseDouble(jftLoanAmount.getText());
         Loan loan=new Loan(interest,year,loanAmount);
         jftMonthlyPayment.setText(String.format("%.2f",loan.getMonthlyPayment()));
         jftTotalPayment.setText(String.format("%.2f",loan.getTotalPayment()));
    }
  }
}利用浏览器和appletViewer查看后均是显示一个灰色的矩形，大侠们帮我看看怎么回事？