Tutorials :Flexible Object Operation with Java Generics :

Java Generics and Subtyping

1: import java.util.List;
2: import java.util.LinkedList;
3: 
4: class subtypingGenerics {    
5:    public static void main(String[] args) {
6:    
7:    List<Integer> numbers = new LinkedList<Integer>();
8:    List<Object> objects = numbers;   
9:    }
10: }

Most of the time, the answer is yes—but your compiler will disagree. The compiler knows allowing this code to pass will most likely result in a List containing elements that are not String, down the road.

Furthering this example, you will come across a very important rule—and one that's worth remembering for future sections: If you have X (sub class or sub interface), a subtype of Y, and a generic type declaration (Z), then Z<X> it is not a subtype of Z<Y>.

Wildcards

To demonstrate, suppose you want to create a method that displays the elements of a collection. Without Java Generics, your method might look like Listing 1 (the displayCollection method).

Notice how you call the displayCollection method to display a collection of floats and a collection of strings. Now, rewrite Listing 1 using Java Generics—but without wildcards (Listing 2).

Now, you have a serious problem, because Listing 2 compiles with errors! Why?! Because Object is not the supertype of all kind of collections (as you already know from the previous section). This is where wildcards solve the problem.

By using a question mark between angle brackets ("?"), instead of Object type, you provide the real supertype of all kind of collections, which is Collection<?>. This may sound like a minor change, but it is so important that it deserves showing the entire code again (Listing 3).

Author's Note: The Collection<?> formula is known as the collection of unknown and it supports elements of any type.

Now it's time to see how to include bounds in wildcards. First, there are two kinds of bounds:

• Upper Bound: When a wildcard includes an upper bound, the generic type is a subtype of the bounding class. An upper bound is marked by the extends keyword, as you can see in the below prototype which is one of the ArrayList’s constructors:

  public ArrayList(Collection<? extends E> c)
  

• Lower Bound: When a wildcard includes a lower bound, the generic type is a super-type of the bounding class. Keep in mind that the super-type relation is reflexive. A lower bound is marked by the super keyword, as you can see in the below prototype, which is the comparator method of the SortedSet class:

  Comparator<? super E> comparator()
  

public abstract class abstractCar {    
   public abstract void carInfo();    
}

public class loganCar extends abstractCar {    
   public void carInfo(){
     System.out.println("-- Logan Info --");
   }
}

public class sanderoCar extends abstractCar {    
   public void carInfo(){
     System.out.println("-- Sandero Info --");
   }
}

Everything seems fine, but a closer look will reveal a big problem with your application. What happens if you have a collection of loganCar or sanderoCar, not a collection of abstractCar?

Obviously, the application will not pass compile-time because of what you learned in a previous section. To fix this, you can use an upper bound to indicate that the showInfo method can be called with a collection of abstractCar and any other subtype of abstractCar, like loganCar or sanderoCar. Listing 5 shows the application again.

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