Replacing whitespace characters in a string

Here is an example of method overloading - same method name with different number of parameters. The complexity of this algorithm is O(n) because the input string is scanned character by character.

But what if there is a input with a massive size? I would want to partition out the string into several portion depending on the size threshold which can be set in the method and the hash the partitioned texts into a hashmap. Threading is possible in this context because the texts will be processed synchronously instead of sequentially.

public static StringBuffer removeSpace(String input){
  return removeSpace(input, "%20");
}

public static StringBuffer removeSpace(String input, String fillerString){
  StringBuffer sb = new StringBuffer();
  int strLength = input.length();
  int index = 0;
  while(index != (strLength)){
    if (input.charAt(index) != ' ') {
      sb.append(input.charAt(index));
    } else {
      sb.append(fillerString);
    }
    index++;
  }
  return sb;
}

You may test this code with this sample method calls:

System.out.println(removeSpace("Hello World, I am Nicholas Key"));
System.out.println(removeSpace("Hello World, I am Nicholas Key", "[SPACE]"));