No, that's not what Benji meant.  You appear to be struggling with the
very basics of the Java programming language.  If so, it would be wise
to back off and spend more time getting familiar with objects and
methods before worrying about learning APIs.

What Benji meant was:

   new File(pathName).mkdir();

Where pathName is a String variable containing the path of the directory
you want to create.

import java.io.File;
import java.io.PrintWriter;

public class Mkdir {
        static PrintWriter err = new PrintWriter(System.err, true);

        public static void main(String[] args) {
                if (args.length != 1) {
                        err.println("usage:  java Mkdir <dirname>");
                        System.exit(1);
                }

                new File(args[0]).mkdir();
        }
}