Whats the use of saying

by 
List<SomeObject> l;

In this you cannot say List<SomeObject> l = new ArrayList<SubClassOfSomeObjectClass>;(not allowed)
wheres for 

List<? extends SomeObject> l;

you can say

List<? extends SomeObject> l = new ArrayList<SubClassOfSomeObject>;(allowed)

But note that in List<? extends SomeObject> l = new ArrayList<SubClassOfSomeObject>; you cannot add anything to your list l because ? represents unknown class (Except null of-course).

Update: For your question in the comment What could I possibly do with a list if I cannot add anything to it?

Now consider a case in which you have to write a function to print your list but mind you it must only accept a List having objects which are subclasses of your SomeObject. In this case as I stated above you cannot use

public void printList(List<SubClassOfSomeObjectClass> someList)

So what would you do? You would do something like

    public void printList(List<? extends SomeObject> someList) {
        for(SomeObject myObj : someList) {
             //process read operations on  myObj
        }

More Related Contents:

  1. Is List a subclass of List? Why are Java generics not implicitly polymorphic?
  2. Java- The meaning of ?
  3. Higher-kinded generics in Java
  4. How do I correctly implement this code segment without getting a logic error? [closed]
  5. Why don’t Java Generics support primitive types?
  6. How to create a generic array in Java?
  7. Jackson and generic type reference
  8. Why does javac complain about generics unrelated to the class’ type arguments? [duplicate]
  9. How to avoid type safety warnings with Hibernate HQL results?
  10. Cast Object to Generic Type for returning
  11. How to cast List to List
  12. How do I clone a generic List in Java?
  13. Using Spring RestTemplate in generic method with generic parameter
  14. Lists with wildcards cause Generic voodoo error
  15. Cannot convert from List to List
  16. Static Binding and Dynamic Binding
  17. Generics, arrays, and the ClassCastException
  18. Why do some claim that Java’s implementation of generics is bad?
  19. Using Mockito to mock classes with generic parameters
  20. Understanding upper and lower bounds on ? in Java Generics
  21. Java generics – Make Generic to extends 2 interfaces
  22. Difference Between Object Type and Reference Type
  23. Initialize Java Generic Array of Type Generic
  24. Heterogeneous container to store genericly typed objects in Java
  25. How can elements be added to a wildcard generic collection?
  26. What does List mean in java generics?
  27. Java getMethod with subclass parameter
  28. Collections.emptyList() returns a List?
  29. Cannot use Java 8 method with lambda arguments without specifying type arguments
  30. Java generic type

Leave a Comment