输入超过五个字就会报异常，该怎么解决

import java.awt.*;
import javax.swing.*;
import java.sql.*;
import java.awt.event.*;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.regex.*;
import java.util.Random;
import static java.awt.BorderLayout.*;
import java.io.File;
import java.io.IOException;
import javax.sound.sampled.AudioFormat;
import javax.sound.sampled.AudioInputStream;
import javax.sound.sampled.AudioSystem;
import javax.sound.sampled.DataLine;
import javax.sound.sampled.LineUnavailableException;
import javax.sound.sampled.SourceDataLine;
import javax.sound.sampled.UnsupportedAudioFileException;
public class RobotDemo
{
final JTextArea jta=new JTextArea(30,40);
final JTextField jtf=new JTextField(10);
final JTextField jtf1=new JTextField(10);
final JTextField jtf2=new JTextField(10);
Link lk,lk1,lk2,lk3,lk4,lk5;
ResultSet rs ,rs1,rs2,rs3,rs4,rs5;
Statement sta;
String answer,res;
Random r=new Random();
public void init()
{
final JFrame jf=new JFrame();
JPanel jp=new JPanel();
JPanel jp1=new JPanel();
JPanel jp2=new JPanel();
JButton jb=new JButton("发送");
JButton jb1=new JButton("训练");
JButton jb2=new JButton("确定");
JButton jb3=new JButton("再加一条");
final JDialog jd=new JDialog(jf,"训练");
final JDialog jd1=new JDialog(jf,"提示信息");
JTextArea jta1=new JTextArea(5,10);
jd1.setLayout(new BorderLayout());
jd1.add(jta1,NORTH);
JPanel jp3=new JPanel();
jd1.add(jp3,SOUTH);
jp3.setLayout(new FlowLayout(FlowLayout.RIGHT));
JButton jb4=new JButton("确定");
jp3.add(jb4);
jta1.setText("不能发送空白内容");

jf.add(jp);
jp.setLayout(new BorderLayout());
jp.add(jta,NORTH);
jp.add(jp1,SOUTH);
jp1.add(jtf);
jp1.add(jb);
jp1.add(jb1);
jd.add(jp2);
jp2.add(jtf1);
jp2.add(jtf2);
jp2.add(jb3);
jp2.add(jb2);
jb4.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
jd1.setVisible(false);
jta.setText("");

    }
});
jb.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent e)
    {
        String out="";
        String result="";
        String s=jtf.getText();
        if(s.equals(""))
        {
            jd1.pack();
            jd1.setVisible(true);
        }
        if(s.length()<4)
        {
            result=find(s);
        }
        else
        {
            result=find1(s);
        }
        out="我："+s+"\n"+"机器人:"+result+"\n";
        jta.setText(out);
        jtf.setText("");

    }
});
jb2.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent e)
    {

        String question =jtf1.getText();
        String answer=jtf2.getText();
        try
        {
            if(question.equals("")&&answer.equals(""))
            {
                JOptionPane.showMessageDialog(null, "不能添加空白内容");
            }
            else
            {
            Statement sta = null;
            String sql="insert into chat1 values('" +question+ "','" +answer+"')";
            lk=new Link(sql);
            lk.sta.executeUpdate(sql);
            JOptionPane.showMessageDialog(null, "添加成功");
            }

        }
        catch(Exception a)
        {
            a.printStackTrace();
            JOptionPane.showMessageDialog(null, "添加失败");
        }
        finally
        {
            lk.closeConn();
        }
    }
});
jb3.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent e)
    {
        jtf2.setText("");
    }
});
jb1.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent e)
    {
        jd.pack();
        jd.setVisible(true);
    }
});
jf.pack();
jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
jf.setVisible(true);

}
public String find(String s)
{
String result="";

    try
    {
        String sql="select *from chat1";
        lk=new Link(sql);
        rs=lk.sta.executeQuery(sql);
        while(rs.next())
        {
            String string =new String(rs.getString("question"));
            if(string.equals(s)||string.contains(s)||s.contains(string))
            {
                try
                {
                    String sql1="select * from chat1 where question='"+string+"'";
                    lk1=new Link(sql1);
                    rs1=lk1.sta.executeQuery(sql1);
                    while(rs1.next())
                    {
                        result=new String (rs1.getString("answer"));
                    }

                }
                catch(Exception e)
                {
                    e.printStackTrace();
                }
                finally
                {
                    lk1.closeConn();
                }
            }
        }
    }
    catch(Exception e)
    {
        e.printStackTrace();
    }
    finally
    {
        lk.closeConn();
    }
    if(result.equals(""))
    {
        int i=3;
        int k=r.nextInt(i);
        if(k==0)
            result="asdfsdaf";
        else if(k==1)
            result="rtfdsg";
        else if(k==2)
            result="fgasdd";
    }
    return result;

}
public String find1(String s){
Boolean flag=false;
char c[];
int j=0,m=0,max=0;
String result=null,ans="";
String string[]=new String[20];
String regex="[a-zA-Z0-9，。、；‘【】、=-《》？：的“{}——+|） _（*&……%￥#@！~·哦他她你我了吗嘛呢吧 ]";
Pattern p1=Pattern.compile(regex);
Matcher m1=p1.matcher(s);
ans=m1.replaceAll("").trim();
c=new char[ans.length()];
for(int i=0;i<ans.length();i++){
c[i]=ans.charAt(i);
}

    for(j=0;j<c.length-1;j++){
        if(j==0){

            try{
                String sql="drop table if exists possible";
                lk=new Link(sql);
                lk.sta.executeUpdate(sql);
                String sql1="create  table possible(question varchar(60))";
                lk1=new Link(sql1);
                lk1.sta.executeUpdate(sql1);
            }catch(Exception e){
                e.printStackTrace();
                }
            finally{
                lk1.closeConn();
                lk.closeConn();
                }
        }                   
        try{
            String sql2="select *from chat1";
            lk2=new Link(sql2);
            rs2=lk2.sta.executeQuery(sql2);
            while(rs2.next()){
                int i=0;
                String str=new String (rs2.getString("question"));
                do{
                    flag=false;
                    int k=i+1;
                    int ij=i+j;
                    if(ij<c.length){
                    char c1=str.charAt(i);

                        if(c[ij]==c1&&str.length()==k){
                            String sql3="insert into possible values('"+str+"')";
                            lk3=new Link(sql3);
                            lk3.sta.executeUpdate(sql3);
                        }
                        else if(c[ij]==c1&&str.length()!=k){
                            flag=true;
                            i++;
                            }
                        }
                    }while(flag);
                }
            }catch(Exception e2){
                e2.printStackTrace();
                }
            finally{
                lk2.closeConn();
                if(lk3!=null){
                lk3.closeConn();
                    }
                }
    }
    try{
        String sql4="select *from possible";
        lk4=new Link(sql4);
        rs4=lk4.sta.executeQuery(sql4);
        while(rs4.next()){
            string[m]=new String(rs4.getString("question"));
            m++;
            }
        if(m!=0){
            max=string[0].length();
            ans=string[0];
            for(int k=1;k<=m;k++){
                if(max<string[k-1].length()){
                    ans=string[k-1];
                    max=string[k-1].length();
                        }
                }

                String sql5="select *from chat1 where question='"+ans+"'";
                lk5=new Link(sql5);
                rs5=lk5.sta.executeQuery(sql5);
                while(rs5.next()){
                     ans=new String(rs5.getString("answer"));
                    }
        }
        else{
            int i=4;
            int k=r.nextInt(i);
            if(k==0){
                result="rd6dfg";
            }else if(k==1){
                result="8ujhgjghg";
            }else if(k==2){
                result="dftyghf";
            }
        }
    }catch(Exception ea){
        ea.printStackTrace();
        }
    finally{
        lk4.closeConn();
        if(lk5!=null){
            lk5.closeConn();
        }
    }
    if(result.equals("")){
        result="tfyrtyrt";
        }
return result;

}
public static void main(String[] args)
{
new RobotDemo().init();
String MUSIC_FILE="相逢一笑.wav";
try
{
AudioInputStream audioInputStream= AudioSystem.getAudioInputStream(new File(MUSIC_FILE));
AudioFormat audioFormat=audioInputStream.getFormat();
DataLine.Info dataLineInfo=new DataLine.Info(SourceDataLine.class,audioFormat,AudioSystem.NOT_SPECIFIED);
SourceDataLine sourceDataLine=(SourceDataLine)AudioSystem.getLine(dataLineInfo);
sourceDataLine.open(audioFormat);
sourceDataLine.start();
int count;
byte tempBuffer[]=new byte[1024];
while ((count = audioInputStream.read(tempBuffer, 0,
tempBuffer.length)) != -1)
{
if(count>0)
{
sourceDataLine.write(tempBuffer,0,count);
}
}
sourceDataLine.drain();
sourceDataLine.close();
}
catch(Exception a)
{
a.printStackTrace();
}

}

java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.lang.String.charAt(Unknown Source)
at RobotDemo.find1(RobotDemo.java:256)
at RobotDemo$2.actionPerformed(RobotDemo.java:92)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.lang.String.charAt(Unknown Source)
at RobotDemo.find1(RobotDemo.java:256)
at RobotDemo$2.actionPerformed(RobotDemo.java:92)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at RobotDemo.find1(RobotDemo.java:325)
at RobotDemo$2.actionPerformed(RobotDemo.java:92)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

把异常信息贴出来，看看是什么异常，出现在哪行