Not everyone is fortunate enough to have the money to pay professors
to think up homework problems for them.  For the rest of you who are
new, and think you need the exercise, here is a problem for you to
tackle:

There are even simpler ones at:
http://mindprod.com/projects/beginner.html

Predator Prey

This exercise teaches basic Collections and generics. Pick an
ecosystem. Create a set of Species objects that contain some facts
about each species such as name and average adult weight in k.g. Now
accept facts about predator and prey relationship encoded with a
method:

public static void eats( String predator, String prey );

There should be a method:

public static Species[] whoEats( String prey )

and there should be a method:

public static Species[] whoIsEatenBy( String predator )

and finally:

// All Species heaviest first
public static Species[] speciesByWeight()

Implement the predator->prey and prey->predator lookups with
one-to-many mappings using a HashMap keyed by species name whose value
is an ArrayList of Species that you maintain in sorted order by
species name. When you insert, use Collections. binarySearch to find
the new insertion point.

Implement speciesByWeight using a Collections.sort rather than a
TreeSet. Make sure you include all species, without duplicates.

For bonus points, implement:

public staticSpecies[] whoIsIndirectlyEatenBy( String predator )
Watch out for endless recursion.

Note that I have not told you all the pieces you will need to create
to solve the puzzle. I am not totally spoon feeding you in this
exercise.

see http://mindprod.com/jgloss/comparable.html
http://mindprod.com/jgloss/hashmap.html
http://mindprod.com/jgloss/binarysearch.html
http://mindprod.com/jgloss/sort.html
http://mindprod.com/jgloss/arraylist.html