[Java] String replaceAll method from space to %20

Here is a simple method to replace all occurrences of space in a string with '%20' without using the replaceAll method.

The idea is to first count how many spaces are there in the string, then allocate a char array of appropriate size and fill it up correctly.

You can check my Gist for the code (replaceSpace) with tests:

 public static String replaceSpace(String s){  
     if(s == null || s.length() == 0) return null;  
   
     int space_count = 0;  
     char c;  
   
     //count how many white spaces we have  
     for(int i = 0; i < s.length(); i++){  
       c = s.charAt(i);  
       if(c == ' ') space_count++;  
     }  
   
     if(space_count == 0) return s;  
   
     //allocate space for new string replacement. CAREFUL: we replace 1 char with 3 so we need 2 extra spaces!  
     char[] res = new char[s.length() + space_count * 2];  
   
     //prepare result string. CAREFUL: can't use same index otherwise we skip characters in either string or char[]  
     for(int i = 0, j = 0; j < s.length(); i++, j++){  
       c = s.charAt(j);  
   
       if(c == ' '){  
         res[i] = '%';  
         res[++i] = '2';  
         res[++i] = '0';  
       }  
       else res[i] = c;  
     }  
   
     return new String(res);  
   }