How can I replace space with 20 java?

 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(" ");
        }
        // if the last character is not space then, don't append  .
        if(s.charAt(s.length()-1) != ' ') {
            return sb.substring(0, sb.length()-3).toString();
        }
        return sb.toString();
    }
}