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

Question

I\'d never heard about wildcars until a few days ago and after reading my teacher\'s Java book, I\'m still not sure about what\'s it for and why would I need to use it.

Answer 1

You can store Dogs and Cats in a List<Animal>. That is not where wildcards are needed.

Let's say you have a method that takes a list of animals:

void foo(List<Animal> animals) {
    ...
}

Now you can't pass the method a List of Dogs -- it only takes an argument of type List<Animal>. You need a wildcard to make the method accept all kinds of Lists of Animals: List<Animal>, List<Dog>, List<Cat>, ...

void foo(List<? extends Animal> animals) {
    ...
}

See

http://java.sun.com/docs/books/tutorial/extra/generics/wildcards.html

Answer 2

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.

Answer 3

Java generics are invariant.

Suppose we have B extends A:

• B is a subtype of A
• an instanceof B is also an instanceof A

Since Java arrays are covariant:

• B[] is a subtype of A[]
• an instanceof B[] is also an instanceof A[]

However, Java generics are invariant:

• List<B> is NOT a subtype of List<A>
• a instanceof List<B> is NOT an instanceof List<A>.

Wildcards are used to make it more flexible while preserving type safety.

• a List<B> is a List<? extends A>

References

• Java Tutorials/Generics
  • Subtyping
  • More fun with wildcards

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Answer 4

The difference between your 2 examples is simply that the first one is a list of generic/general animals - you can therefore add any type of animal to it, and any instance of a subclass of type Animal. (e.g. It can contain some dogs, some cats, some porcupines...) Whereas the second - List <? extends Animal> - will be a list of one specific subtype of class animal. It can be any one you choose (this is set each time at runtime), but only one. It will either be a list of dogs, or a list of cats, or a list of turtles... etc.