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.

Let's say I have a super class Animal and few sub classes like Dog, Cat, Parrot, etc... Now I need to have a list of animals, my first thought would be something like:

List<Animal> listAnimals 

Instead, my colleagues are recommending something like:

List<? extends Animal> listAnimals 

Why should I use wildcards instead of simple generics?

Let's say I need to have a get/set method, should I use the former or the later? How are they so different?

Answer 1

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 2

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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