What is the difference between List and List<? extends Number>?

Question

I have a very basic question ragarding Java generics . I thought that both List and List are

Answer 1

Generic types are more pedantic.

<? extends Number> means Number or an unknown a sub class. If you obtain such a value it will be a Number, but you cannot give a value of this type because you don't know which is valid.

The difference is in arguments and return values.

List<Number> numbers = new ArrayList<Number>();
Number n = 1;
numbers.add(n); // ok.
n = numbers.get(0); // ok
numbers.add(1); // ok.

List<? extends Number> numbers2 = new ArrayList<Double>();
numbers2.add(n); // not ok
n = numbers2.get(0); // ok

List<? super Number> numbers3 = new ArrayList<Serializable>();
numbers3.add(n); // ok
n = numbers3.get(0); // not ok.

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super is used in a few places to signify the type can be a super type. e.g.

In Collections, this method says the Comparator needs to be able to compare the same type or any super type.

public static <T> void sort(List<T> list, Comparator<? super T> c)

This means you can have

Comparator<Number> comparesAnyNumbers = ...
List<Integer> ints = ...
Collections.sort(ints, comparesAnyNumbers);

Answer 2

Generics are compile time language features, means, they don't exist in run-time. In generic mechanism, for compile-time checks, they are not homogeneous, i.e. if you want to use polymorphism in generic type.

Following gives you a compile time error, although it seems a valid definition:

 List<Number> list = new ArrayList <Integer>();

whereas

 List<? extends Number> list = new ArrayList <Integer>();

is valid. Moreover, you can't use wildcard types on the right side:

 List list = new ArrayList <? extends Integer>();

won't be compiled.

Answer 3

List<Number> --> List of Numbers (or instances of Number)

List<? extends Number> --> List any type which extends Number