java socket程序出现运行时错误，帮忙看一下

//Aclient.java
import java.net.*;
import java.io.*;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.applet.*;
public class Aclient extends Applet implements Runnable,ActionListener{
JButton button;JTextField textF;JTextArea textA;
Socket socket;Thread thread;
DataInputStream in;DataOutputStream out;
public void init(){
setBackground(new Color(120,153,137));
setLayout(new BorderLayout());
button=new JButton("发送消息");
textF=new JTextField(20);
textA=new JTextArea(20,30);
setSize(450,350);
JPanel p=new JPanel();
p.add(textF);
p.add(button);
p.add(textA);
//add(textA,"Center");
add(p,"South");
button.addActionListener(this);
}
public void start(){
try{
socket=new Socket("localhost",4331);
in=new DataInputStream(socket.getInputStream());