Sto se desava greshno????

Код:

public class ShortestPalindrom
{
	public static void main (String [] args)
	{
		String [] test1 = {"aba", "abba", "xxxx"};
		String [] test2 = {"ab", "aba"};
		String [] test3 = {"xyzzyx", "abc", "cde"};
		String [] test4 = {"abc", "cde"};

		System.out.println ("The shortest palindrome in test 1 is " + shortest (test1));
		System.out.println ("The shortest palindrome in test 2 is " + shortest (test2));
		System.out.println ("The shortest palindrome in test 3 is " + shortest (test3));
		System.out.println ("The shortest palindrome in test 4 is " + shortest (test4));
	}
	static String shortest(String[] words)
	{
		boolean isPalindrome = false;
		String palindromeToReturn = "";

		if (words.length > 20)
			return "The array passed exceeds the limit of 20 words!";
		for (int i = 0; i < words.length; i++)
		{
			if (words[i].length () > 1 && words[i].length () <= 50)
			{
				for (int j = 0; j < words[i].length (); j ++)
				{
					for (int z = words[i].length () ; z > 0; z--)
					{
						if (words[i].charAt (j) == words[i].charAt (z))
							isPalindrome = true;
						else
							isPalindrome = false;
					}
				}
				if (isPalindrome == true)
					palindromeToReturn = words[i];
			}
			else
				break;
		}

		return palindromeToReturn;
	}
}

mi dava error koga go run-uvam a ne koga kompajliram...vika StringIndexOutOfBouds vo linija 30...
e sea verojatno e povrzano toa nesto so charAt () momentite ama vrska neam sto???

hELP?