Why there is null pointer exception but I have checked there is no null

Question

public static boolean isIsomorphic(String s, String t) {
    HashMap<Character, Character> res1 = new HashMap<Character, Character>();
    HashMap<Character, Character> res2 = new HashMap<Character, Character>();
    char[] sToArray = s.toCharArray();
    char[] tToArray = t.toCharArray();
    if (s == null && t == null)
        return true;
    if (s == null || t == null || s.length() != t.length())
        return false;
    for (int i = 0; i < sToArray.length; i++) {
        ***if ((!res1.containsKey(sToArray[i])) && (!res2.containsKey(tToArray[i])))*** {
            res1.put(sToArray[i], tToArray[i]);
            res2.put(tToArray[i], sToArray[i]);

        } else {

            if ((res1.get(sToArray[i]) != tToArray[i]) || (res2.get(tToArray[i]) != sToArray[i]))
                return false;

        }

    }
    return true;

}

Given two strings s and t, determine if they are isomorphic. When the test case is "ab" "aa"; there will be an nullpointerexception for if ((!res1.containsKey(sToArray[i])) && (!res2.containsKey(tToArray[i]))) I checked res1 does not equal to null and res2 does not equal to null. Could anybody tell the reason? Thanks very much!

Probably not the source of your exception, but you convert `s` and `t` to char arrays before checking them for null. You have to put the checks first. — Sebastian Redl, Aug 19 '16 at 14:28
Unrelated, but you probably don't want to compare those `char[]` with `==` and `!=`. — tobias_k, Aug 19 '16 at 14:29

score 1 · Answer 1 · answered Aug 19 '16 at 14:28

1

What's probably happening is that res1.get(sToArray[i]) is null. You then compare that Character with a char, which requires an unboxing operation on the Character - but it's null and a NPE is thrown.

Note that res1.containsKey(sToArray[i]) will return true if there is a null value associated with that key.

answered Aug 19 '16 at 14:28

assylias

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Thanks! You are extremely correct. – tjuli Aug 19 '16 at 18:21

Why there is null pointer exception but I have checked there is no null

1 Answers1