Java generics, objects and wildcards differences & clarifications

Question

I wish to understand this concept:

1. T object - generic, will be erased into actual type.
2. ? object - will be erased into what?

Answer 1

I will list the main differences between T and ?:

• Basic: T is a type parameter and ? is a wildcard.

• Meaning: T is used as a type parameter when defining a generic class. T will be replaced by a concrete type when you instantiate the generic class. On the other hand, we use ? when we want to refer to an unknown type argument.

• Place of definition: You need to declare T on the top of the class, or method if you define a generic method. You can use ? everywhere.
• Mapping: Every use of T is mapped to the same type (in the same class). Every use of ? can be mapped to a different type.
• Object instantiation: You can create objects with the generic parameter T like new ArrayList<T>(). You cannot instantiate objects but only pointers with ?.

• Collections updating: You can add objects to a collection of type T. You cannot add object to a collection of type ? (since you don't know its type).

• Type erasures: With generics, type erasure applies to the use of generics. when generics are used, they are converted into compile-time checks and execution-time casts. So if you have this code for example: List<String> myList = new ArrayList<String>(); and then you wish to add to your list so you do myList.add("Hello World"); and then you want to get the item you just added by performing String myString = myList.get(0); then the compiler will compile your code to List myList = new ArrayList(); and String myString = (String) myList.get(0); (the add stays the same for obvious reasons).
  So basically, at execution time there is no way of finding out that T is essentially String for the list object (that information is gone).
  
  Now for wildcards the story is different. A wildcard (?) is replaced by an Object (since it's unbounded). This is not very useful. At build-time the compiler will check you are only calling Object's behaviours. If you have something like ? extends Foo, then the ? is replaced with its bound Foo (at build-time the compiler will check you are only passing Foo or any of its subtypes (types that inherit from Foo) as an argument).

For differences between ? and Object & T and Object you may read here and here respectively.