/** 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
   */
  public String addOrChangeEntry(String name, String number) {
    String oldNumber = null;
    int index = find(name);
    if (index > -1) {
      oldNumber = theDirectory[index].getNumber();
      theDirectory[index].setNumber(number);
    }
    else {
      add(name, number);
    }
    modified = true;
    return oldNumber;
  }

  /** Look up an entry.
      @param name The name of the person
      @return The number. If not in the directory, null is returned
   */
  public String lookupEntry(String name) {
    int index = find(name);
    if (index > -1) {
      return theDirectory[index].getNumber();
    }
    else {
      return null;
    }
  }

  /** 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.
   */
  public void save() {
    if (modified) { // If not modified, do nothing.
      try {
        // Create PrintWriter for the file.
        PrintWriter out = new PrintWriter(new FileWriter(sourceName));

        // Write each directory entry to the file.
        for (int i = 0; i < size; i++) {
          out.println(theDirectory[i].getName());
          out.println(theDirectory[i].getNumber());
        }
        // Close the file.
        out.close();
        modified = false;
      }
      catch (Exception ex) {
        System.err.println("Save of directory failed");
        ex.printStackTrace();
        System.exit(1);
      }
    }
  }

  /** Find an entry in the directory.
      @param name The name to be found
      @return The index of the entry with the requested name.
              If the name is not in the directory, returns -1
   */
  private int find(String name) {
    for (int i = 0; i < size; i++) {
      if (theDirectory[i].getName().equals(name)) {
        return i;
      }
    }
    return -1; // Name not found.
  }

  /** Add an entry to the directory.
      @param name The name of the new person
      @param number The number of the new person
   */
  private void add(String name, String number) {
    if (size >= capacity) {
      reallocate();
    }
    theDirectory[size] = new DirectoryEntry(name, number);
    size++;
  }

  /** Allocate a new array to hold the directory. */
  private void reallocate() {
    capacity *= 2;
    DirectoryEntry[] newDirectory = new DirectoryEntry[capacity];
    System.arraycopy(theDirectory, 0, newDirectory, 0,
                     theDirectory.length);
    theDirectory = newDirectory;
  }

  /** 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
   */
  public String removeEntry(String name) {
    int itemindex = find(name);
    if (itemindex > -1) {
      theDirectory[itemindex] = theDirectory[size - 1];
      size--;
      modified = true;
      return name;
    }
    else {
      return null;
    }
  }  // end remove Entry