Explain the syntax of Collections.emptyList()

Question

I just studied about generic programming, the List interface, and ArrayList, so I can understand the statement below.

Arra         


        

Answer 1

That line creates an empty list of strings by calling a static method with a generic type parameter.

Inside the Collections class, there is a static method emptyList declared like:

public static final  List emptyList() {
    return (List) EMPTY_LIST;
}

This has a generic type parameter T. We call call this method by using:

List list = Collections.emptyList();

and T is infered to be a String because of the type of list.

We can also specify the type of T by putting it in angle brackets when calling emptyList. This may be needed if we want a more specific type than is inferred:

List list = Collections.emptyList();

emptyList() is not correct because that placement is only valid when creating instances of generic classes, not calling methods. When using new there are two possible type parameters, the ones before the class name are for the constructor only, and the ones after the class name are for the whole instance, so with the class:

class MyClass {
    public  MyClass(A a, B b) {
        System.out.println(a + ", " + b);
    }
}

We can call its constructor where A is String and B is Integer like:

MyClass a = new MyClass("a", 3);

or by using type inference:

MyClass a = new MyClass<>("a", 3);

See also:

• Generic Methods
• Type Inference