How to compare 2 Objects( and )?

Question

Should I write an interface?

Answer 1

Java has 2 interfaces that would be of interest to you.
They both are designed to return an integer that describes the relationship between two objects.
Suppose you have two objects: objectA and objectB
The return value is as following:
if (returnValue < 0) then objectA precedes objectB. (A < B)
if (returnValue > 0) then objectA follows objectB. (A > B)
if (returnValue == 0) then objectA and objectB are equivalent (A == B).
It is up to you to do the comparing and determine which is the correct return value...
Here are the two interfaces:
1) Comparable interface (java.lang.Comparable)
You would use this interface when you have a class that exhibits some natural order..
You add this interface to the class, and, within the class, implement the following method:
   public int compareTo( Object obj )
   The variable obj in this case refers to the "other" object that you want to compare.
   public class MyClass implements Comparable // <--- notice the last two words!!
          other methods.
      public boolean isTallerThan( MyClass mc )
         // just some method i made up to compare...
      public int compareTo( Object objectB )
         // You are comparing "this" object with the "otherObject"...
         // Note that objectA is "this".
         MyClass otherObject = (MyClass)objectB;
         // Do whatever you need to compare the two.
         if (this.isTallerThan(otherObject) )
             return -1;
         else if ( otherObject.isTallerThan(this) )
             return +1;
         else
             return 0; // they are of equal height...whatever...
   2) Comparator interface (java.util.Comparator)
You could use this interface if you have a different set of standards with which to compare two different objects. Usually, it is it's own class, designed for only this purpose. You implement the following method:
   public int compare( Object objA, Object objB )
   Notice that in this case you have to explicitly pass in both objects to be compared, whereas in the Comparable interface, objA is implicitly assumed to be the object on which the method is called (or as I like to call it, this).
Here is an example:
   public class MyComparator implements Comparator
      public int compare( Object objA, Object objB )
         do whatever comparisons you need to determine whether
         objA < objB (-1 is returned)
         objA > objB (+1 is returned)
         or
         objA == ObjB (0 is returned)