weird leetcode 28 java NPE

Solution.java

class Solution {
    public int strStr(String haystack, String needle) {
        if (needle.isEmpty()) return 0;
        return new KMPOptimized(haystack,needle).kmp();
    }

    class KMPOptimized {
        String text;
        String pattern;
        int[] T;

        public KMPOptimized(String text, String pattern) {
            this.text = text;
            this.pattern = pattern;
            int[] T = new int[this.pattern.length()];
        }

        private void computeT() {
            T[0] = -1;
            int pos = 1, cnd = 0;
            while (pos < pattern.length()) {
                if (pattern.charAt(pos) == pattern.charAt(cnd))
                    T[pos] = T[cnd];
                else {
                    T[pos] = cnd;
                    cnd = T[cnd];
                    while (cnd >= 0 && pattern.charAt(pos) != pattern.charAt(cnd)) {
                        cnd = T[cnd];
                    }
                }
                pos++;
                cnd++;
            }
            T[pos] = cnd;
        }

        int kmp() {
            computeT();
            int i = 0, j = 0;
            while (i < text.length()) {
                if (text.charAt(i) == pattern.charAt(j)) {
                    i++;
                    j++;
                    if (j == pattern.length()) {
                        return i - j;
                    }
                } else {
                    //                if (j == 0)
                    //                    i++;
                    //                else
                    //                    j = T[j - 1];//wow
                    j = T[j];
                    if (j<0) {
                        i++;
                        j++;
                    }
                }
            }
            return -1;
        }
    }
}