KMP algo for pattern search

public class KMPAlgo {

    public static void main(String[] args) {
        String txt = "ABABDABACDABABCABAB";
        String pattern = "ABABCABAB";
        int[] lps = computeLPS(pattern);
        countOccurance(txt, pattern, lps);
    }

    public static void countOccurance(String txt, String pat, int[] lps) {
        if (pat == null || pat.isEmpty() || txt == null || txt.isEmpty()) {
            return;
        }
        int M = pat.length();
        int N = txt.length();

        int indexForPattern = 0;

        int positionForTExt = 0;
        while (positionForTExt < N) {
            if (pat.charAt(indexForPattern) == txt.charAt(positionForTExt)) {
                indexForPattern++;
                positionForTExt++;
            }
            if (indexForPattern == M) {
                System.out.println("Found pattern " + "at index " + (positionForTExt - indexForPattern));
                indexForPattern = lps[indexForPattern - 1];
            }

            // mismatch after j matches
            else if (positionForTExt < N && pat.charAt(indexForPattern) != txt.charAt(positionForTExt)) {
                if (indexForPattern != 0) {
                    indexForPattern = lps[indexForPattern - 1];
                } else {
                    positionForTExt = positionForTExt + 1;
                }
            }
        }
    }

    public static int[] computeLPS(String pattern) {
        if (pattern == null || pattern.isEmpty()) {
            return null;
        }
        int[] lps = new int[pattern.length()];
        int index = 1;
        lps[0] = 0;
        int curLen = 0;
        while (index < pattern.length()) {
            if (pattern.charAt(index) == pattern.charAt(curLen)) {
                curLen++;
                lps[index] = curLen;
                index++;
            } else {
                if (curLen != 0) {
                    curLen = lps[curLen - 1];

                } else {
                    lps[index] = curLen;
                    index++;
                }
            }
        }
        System.out.println("LPS of: " + pattern);
        for (int i : lps) {
            System.out.print(i + " ");
        }
        return lps;

    }
}