25a - JUnit test case to assert that CanOfDogFood correctly returns
false when ingredients are Beef (Introduces JUnit, Assert, ...).
25b - JUnit test case to verfiy that Dogs do and Sheeps do not accept
non-vegetarian CanOfDogFood composites (Introduces test driven
development).
25c - Fix in Mammal so that non-Omnivore instances do not accept
composite IFood that is non-vegetarian.
25d - Mammals can be Prey which are non-vegetarian IFood instances.
Refactor Sheep to not override feed() anymore.
26 - Dolly the clone sheep (Introduces equality and identity).
27 - Print Mammal instance's stomach content as well food calories
(Introduces Object's toString method used for stdout)
28 - Reduce field visibility (Introduces access modifiers private and
protected).
29 - Build a barn and greenhouse (Introduces access modifiers on
method/constructor level and package).