Why is it not possible to use a primitive type as an argument?

Generic classes cannot be parameterized to primitive types. To solve this problem, the language defines wrapper classes of the primitive types: Integer, Float, Double, Character, Boolean, etc.

The compiler automatically transforms primitive types into wrapper classes and vice versa:

List<Integer> list = ...
list.add(12)
list.add(new Integer(12))

What you are trying to do is not possible because in Java, the type of any variable is either a primitive type or a reference type. Generic type arguments must be reference types. Since primitives do not extend from Object, they cannot be used as generic type arguments to a parameterized type.

That's why with Integerworks, because it does extend from Object, while it intis primitive.

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

And, since Java 7 you can do this:

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

However it should be noted that with autoboxing in Java , the primitive type intwill be converted to a Integerwhen needed.

Autoboxing is the automatic conversion that the Java compiler performs between primitive types and their corresponding object wrapper classes.

So this is valid:

int myInt = 1;
List <Integer> list = new ArrayList <Integer> ();
list.add (myInt);

System.out.println (list.get (0)); // imprime 1

• Source: Create a List of primitive int?

You can use arrayList for that purpose as follows:

Arraylist <Integer> enterosLista = new Arraylist<>();

You cannot declare a List as an array, for example:

int []UnArray  = {1,2,3}

You must do it as follows:

Arraylist <Integer> enterosLista = new Arraylist<>();
List <Integer> enterosLista = new Arraylist<>();

Generic classes in java expect an object to extend from Object, primitive data types such as int, long, char, etc, do not extend from it.

Integeris a wrapper of the primitive typeint

An object is a different thing than a primitive type, even though they contain the same information. Always keep in mind that objects in Java ( Integer ) have one treatment type and primitive types ( int ), another. That at a given moment they contain the same information does not mean in any case that they are the same.

Here a read about the difference between Primitive Data Types and Classes