Myujth

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.