Because an list of animals can do more than a list
 of cats (i.e., accept an animal), so that a list
 of cats is not a subtype of it.

 Assume,

java.util.List<Cat> catList = new java.util.ArrayList<Cat>();

 Now assume, the following assignement would be allowed:

java.util.List<animal> animalList = catList;

 Then one could add an animal to this »animalList«:

animalList.add( new animal(){} );

 However, this would break the type of »catList«, because
 the following call to the get-Operation of the catList now
 will not return a cat, because we have been allowed to add
 an animal above:

Cat cat = catList.get( 0 );

    Because your method might add a Dog to the List<Animal>.
Since it's actually a List<Cat>, you would then have trouble.

    The Hydrogen Dog and the Cobalt Cat
    Side by side in the armory sat.
    Nobody talked about fusion or fission;
    Everyone spoke of their peacetime mission
    'Til somebody came in and opened the door ...

    There they were in a neutron fog:
    The Codrogen Cat and the Hybalt Dog.
    They mushroomed up with a terrible roar,
    And nobody never was there, no more.

    -- "A Space Child's Mother Goose"
    Winsor & Parry
    (from possibly inaccurate memory)

Will Cheeder a écrit :

Make the method parameter signature be

public void myMethod(List<? extends Animal> myAnimalList)

and it will work.

See also for this thread for links to tutorials I found useful
http://groups.google.com/group/comp.lang.java.programmer/browse_thread/thread/bc
bd48d4566a7575

Phil