code: public RAMDirectory mkdir(String name) throws IOException
    {
      RAMEntry entry = getEntry(name);
      if(entry instanceof RAMDirectory)
      {
        RAMDirectory ramDirectory = (RAMDirectory) entry;
        return ramDirectory;
      }
      else
      {
        if(entry == null)
        {
          RAMDirectory directory = new RAMDirectory(_clock, name);
          return (RAMDirectory) add(directory);
        }
        else
        {
          throw new IOException("File exists: " + name);
        }
      }
    }
docs: Creates an empty directory and add it to this directory with this name. If the directory
  already exists, it does not recreate it.
  
  @param name
  @return the created directory
  @throws IOException if the name represents a file