public class Solution {

    public String replaceSpace(StringBuffer str) {

        return str.toString().replaceAll(" ","%20");

    }

}

private static String replaceSpace(String input) {

    if (input == null) {
        return null;
    }

    if (input.isEmpty()) {
        return "";
    }

    StringBuilder sb = new StringBuilder();

    for (int i = 0; i < input.length(); i++) {
        if (input.charAt(i) != ' ') {
            sb.append(input.charAt(i));
        } else {
            sb.append("%20");
        }
    }

    return sb.toString();
}