Java Lat-6

//Nama Program:lat6.java

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

public class lat6 extends JFrame {
JTextField textHuruf=new JTextField();
JLabel jLabel1=new JLabel();

public static void main(String args[]){
  lat6 windowku=new lat6();
  windowku.setSize(500,250); 
  Dimension screen =Toolkit.getDefaultToolkit().getScreenSize();
  int lebar=(screen.width-windowku.getSize().width)/2;
  int tinggi=(screen.height-windowku.getSize().height)/2;
  windowku.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  windowku.setLocation(lebar,tinggi);  
  windowku.setVisible(true); 
  windowku.setResizable(false);  
}
public lat6(){
  this.setTitle("Saring Huruf(Sunardi,S.Kom)");
  this.getContentPane().setLayout(null);
  jLabel1.setText("Hanya huruf yang bisa dimasukkan....!"); 
  jLabel1.setFont(new java.awt.Font("Comic Sans MS",1,16)); //Jenis Font,Ketebalan,Size)
  jLabel1.setBounds(new Rectangle(4,25,450,20));  
  textHuruf.setBounds(new Rectangle(14,60,400,26));
  textHuruf.setDocument(new cekHuruf().getOnlyLetter());
  this.getContentPane().add(textHuruf,null);
  this.getContentPane().add(jLabel1,null);
}  

  class cekHuruf {
  public cekHuruf(){
}
public PlainDocument getOnlyLetter() {
  PlainDocument filterHuruf=new PlainDocument() {
  public void insertString(int offs,String str,AttributeSet a)throws  
  BadLocationException {
  StringBuffer buffer=new StringBuffer();
  int s=0;
  char[] dataInput=str.toCharArray();
  //Memeriksa semua data yang dimasukkan
  for (int i=0;i  //Menyaring Apakah data masukkan berupa LETTER?? 
  boolean isOnlyLetter=Character.isLetter(dataInput[i]);
  if (isOnlyLetter==true) {
  dataInput[s]=dataInput[i];
  s++;
}
else{
JOptionPane.showMessageDialog(null,"Angka Tidak Diijinkan.....!");
}
}
buffer.append(dataInput,0,s);
super.insertString(offs,new String(buffer),a);
}
};
return filterHuruf;
}
}
}