What is a natural return type in java?

Question

I am reading Joshua Blochs "Effective Java", and it says

A third advantage of static factory methods is that, unlike constructors, they can return an object of any subtype of their return type. This gives you great flexibility in choosing the class of the returned object. One application of this flexibility is that an API can return objects without making their classes public. Hiding implementation classes in this fashion leads to a very compact API. This technique lends itself to interface-based frameworks, where interfaces provide natural return types for static factory methods.

Can anyone explain what "natural return types are"? Thanks!

Answer 1

In this context, "natural" simply means natural for the context of the factory method; i.e. what is appropriate or what you would expect. Intuitive would be another synonym.

This is just normal English usage ... not IT or Java-specific terminology.

Answer 2

A concrete example of this would be something like Guava's ImmutableList factory methods.

This has 13 overloads of the of method, with increasing numbers of parameters. If you look at the source of the first 3:

@SuppressWarnings("unchecked")
public static <E> ImmutableList<E> of() {
  return (ImmutableList<E>) EMPTY;
}

public static <E> ImmutableList<E> of(E element) {
  return new SingletonImmutableList<E>(element);
}

public static <E> ImmutableList<E> of(E e1, E e2) {
  return construct(e1, e2);
}

The zero-arg and two-arg methods actually return an instance of RegularImmutableList, which is a subclass of ImmutableList; the one-arg method returns an instance of SingletonImmutableList.

But is this detail about the subclass relevant to you, the caller? No. It is "natural" that if you call a method that constructs an ImmutableList, you get back a reference to an ImmutableList.

"Natural" in this sense perhaps means "at an appropriate level of abstraction".