What’s the purpose behind wildcards and how are they different from generics?

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The wildcards do not make a lot of sense when you declare local variables, however they are really important when you declare a parameter for a method.

Imagine you have a method:

int countLegs ( List< ? extends Animal > animals )
{
   int retVal = 0;
   for ( Animal cur : animals )
   {
      retVal += cur.countLegs( );
   }

   return retVal;
}

With this signature you can do this:

List<Dog> dogs = ...;
countLegs( dogs );

List<Cat> cats = ...;
countLegs( cats );

List<Animal> zoo = ...;
countLegs( zoo );

If, however, you declare countLegs like this:

int countLegs ( List< Animal > animals )

Then in the previous example only countLegs( zoo ) would have compiled, because only that call has a correct type.

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