kita import:

import id.co.utilities.koneksi;
import java.lang.StringBuffer;
import java.util.Calendar;
import javax.swing.*;
import java.sql.*;
import java.sql.Connection;
import java.sql.SQLException;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.PlainDocument;

dan sintaknya

class onlyLetter{
public onlyLetter(){

}

public PlainDocument getOnlyLetter(){
PlainDocument filterLetter = new PlainDocument(){
public void insertString(int offs, String str, AttributeSet a)throws BadLocationException{
StringBuffer buffer = new StringBuffer();
int s = 0;
char[]dataInput = str.toCharArray();
for (int i = 0; i < dataInput.length; i++){
boolean isOnlyLetter = Character.isLetter(dataInput[i]);
if(isOnlyLetter == true){
dataInput[s] = dataInput[i];
s++;
}
}
buffer.append(dataInput, 0, s);
super.insertString(offs, new String(buffer), a);
}

};
return filterLetter;
}
}

setelah itu beri script pada txtField;

txtHuruf.setDocument(new onlyLetter().getOnlyLetter());

Namun jika kita ingin membuat kelas baru pada package, yang kita taruh hanya script dari:

public PlainDocument getOnlyLetter(){

...

return filterLetter;
}