Java won't append a slash if missing from a String

gistfile1.java

/**
 * Tests the duplicate file finder
 * For RIT 4002-219
 * Author: David Gay
 * Spring 2012
 * CODE WRITTEN FOR USE ON A LINUX FILESYSTEM
 */

public class TestDups
{
    /**
     * Main
     * Program entry point
     * @param args[0] the starting directory
     */
    public static void main(String[] args)
    {
        String startingDir;
        try
        {
            startingDir = args[0];
            System.out.println("last char: " + startingDir.charAt(startingDir.length() - 1));
            if (startingDir.charAt(startingDir.length() - 1) != '/')
            {
                // If the user did not include a closing slash (/), append it
                startingDir.concat("/");
            }
        }
        catch (ArrayIndexOutOfBoundsException aioobe)
        {
            // If no argument is given, set a default starting dir
            startingDir = "~/.";
        }

        System.out.println("Duplicate File Finder - by David Gay");
        System.out.println("Find duplicates. Processing: " + startingDir);
    }
}