Ndtyjky

When I am trying to access map.get(c) in for loop (as shown in version 2), it returns null value and sets upper bound to be null, resulting in null-pointer exception. On the other hand, if I create end variable and assign it map.get(c) value (as shown in version 1), it works fine. So, would you please explain me why?

version 1: Works perfectly fine

    int count=0;

    int st=0;

    string s = "abcabcbb";



    Hashtable<Character, Integer> map = new Hashtable<Character, Integer>();



    char str = s.toCharArray();

    for(int i=0; i<str.length; i++){

        char c = str[i];

        if(map.get(c)==null){

            map.put(c, i);



            if(count < map.get(c) - st + 1){

                count = map.get(c) - st + 1;

            };

        }





        else {

            int end = map.get(c);     // End variable --> returns int value as expected



            for(int j=st; j<=end; j++){

                map.remove(str[j]);

                st = j+1;

            }

            map.put(c,i);

        }



    }



    System.out.println(count);

Version 2: Gives null-pointer exception

    int count=0;

    int st=0;

    string s = "abcabcbb";



    Hashtable<Character, Integer> map = new Hashtable<Character, Integer>();



    char str = s.toCharArray();

    for(int i=0; i<str.length; i++){

        char c = str[i];

        if(map.get(c)==null){

            map.put(c, i);



            if(count < map.get(c) - st + 1){

                count = map.get(c) - st + 1;

            };

        }





        else {

            //int end = map.get(c);     // End variable commented



            for(int j=st; j<=map.get(c); j++){   // replaced end w map.get(c) --> returns null instead of int

                map.remove(str[j]);

                st = j+1;

            }

            map.put(c,i);

        }



    }



    System.out.println(count);

Thanks for your help in advance!
Rohan.

A for-loop runs until its condition is not met (in your case, until j <= map.get(c) is false). This condition is also not cached, as shown by the output of the code below:

public static void main(String args) {

    for (int i = 0; i < getCondition(); i++) {



    }

}



private static int getCondition() {

    System.out.println("Test");

    return 3;

}

Output:

Test

Test

Test

Test

For that reason, map.get(c) will be called for each iteration of the for-loop. If you happen to remove the entry with key c from map, then the value returned from Map#get is null, which is what causes the NullPointerException.