/*
* The interface for the telephone directory.
  @author Koffman & Wolfgang
*/

public interface PhoneDirectory {

  /** Load the data file containing the directory, or
        establish a connection with the data source.
        @param sourceName The name of the file (data source)
        with the phone directory entries
   */
  void loadData(String sourceName);

  /** Look up an entry.
        @param name The name of the person to look up
        @return The number or null if name is not in the directory
   */
  String lookupEntry(String name);

  /** Add an entry or change an existing entry.
        @param name The name of the person being added or changed
        @param number The new number to be assigned
        @return The old number or, if a new entry, null
   */
  String addOrChangeEntry(String name, String number);

  /** Remove an entry from the directory.
        @param name The name of the person to be removed
        @return The current number. If not in directory, null is
                returned
   */
  String removeEntry(String name);

  /** Method to save the directory.
        pre:  The directory has been loaded with data.
        post: Contents of directory written back to the file in the
              form of name-number pairs on adjacent lines
              modified is reset to false.
   */
  void save();
}