[java] Funny Exception

Hello I am working on this program to take out digits in a sentence and replace them with the digit''s word. The program compiles but I get this funny exception. Could someone take a look at it for me? I am pasting it. import java.awt.*; import java.applet.*; import javax.swing.*; import java.util.*; import java.awt.event.*; public class DigiConverter extends Applet implements ActionListener { //use boolean isDigit(char ch) TextField enteredString; TextField result; StringBuffer temp; StringBuffer fin; Button act; char c; String[] dig = {"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"}; public void init() { fin = new StringBuffer(100); temp = new StringBuffer(100); enteredString = new TextField(50); result = new TextField(50); act = new Button("Enter"); add(result); add(enteredString); add(act); act.addActionListener(new DigiConverter()); } public void actionPerformed(ActionEvent e) { String s = enteredString.getText(); temp.append(s.trim()); for(int i=0; i < temp.length(); i++) { c = temp.charAt(i); if(Character.isDigit(c)) fin.append(dig); else fin.append(c); } result.setText(fin.toString()); } }