Question

Why do generics in Java work with classes but not with primitive types?

For example, this works fine:

List<Integer> foo = new ArrayList<Integer>();

but this is not allowed:

List<int> bar = new ArrayList<int>();

Answer 1

Generics in Java are an entirely compile-time construct – the compiler turns all generic uses into casts to the right type. This is to maintain backwards compatibility with previous JVM runtimes.

This:

List<ClassA> list = new ArrayList<ClassA>();
list.add(new ClassA());
ClassA a = list.get(0);

gets turned into (roughly):

List list = new ArrayList();
list.add(new ClassA());
ClassA a = (ClassA)list.get(0);

So, anything that is used as generics has to be convertable to Object (in this example get(0) returns an Object), and the primitive types aren’t. So they can’t be used in generics.