This is not very memory-efficient. Can you please suggest a better way or improvement?

    public class Test{

        public static void main(String[] args) {
            System.out.println(replaceSpace("All men must die"));
        }

        public static String replaceSpace(String s) {
            String[] stringArray = s.split(" ");
            StringBuffer sb = new StringBuffer();
            for(String s3 : stringArray) {
                sb.append(s3);
                sb.append("%20");
            }
            // if the last character is not space then, don't append %20.
            if(s.charAt(s.length()-1) != ' ') {
                return sb.substring(0, sb.length()-3).toString();
            }

            return sb.toString();
        }
    }