Hints for Project 6a.

1. Don't define your own Comparable interface from your project.  Your should be 
   using the Comparable interface from the Java Class Library.  This interface is 
   in the java.lang package, so you don’t need to import it.

2. Implement the Comparable interface like this in the LibaryItem class header:

   public class LibraryItem implements Comparable<LibraryItem> {
       // body of LibraryItem class goes here. 
   }
   
   In Project 6b, the LibraryItem class will also need to implement the
   Serializable interface like this:
   
   public class LibraryItem implements Comparable<LibraryItem>, Serializable {
       // body of LibraryItem class goes here. 
   }
   
   The Serializable interface is in the java.io package, so import it like this:
   
   import java.io.Serializable;

3. In LibaryItem, define the compareTo method like this:

   @Override
   public int compareTo(LibraryItem other) {
      return this.title.compareTo(other.title);
   }

   You can’t use < to compare String objects; use compareTo instead.