Input String And Test Whether Or Not It Is Palindrome

I have to write a program that inputs a string and tests whether or not it is a palindrome. This worked fine, and now I want to make it so I continually enter strings until I tell the program to stop.

The code below compiles just fine, but it doesn't do what I want. Why it doesn't work how I think it should work? (Typing in STOP does not make the program stop.)

Here is the code

import javax.swing.JOptionPane;
public class PalTest {
public static void main(String[] args){
String S="pony";
while(S!="STOP"){
S=JOptionPane.showInputDialog("Enter a string (STOP to terminate):");

[Code] ....