public static factory method

Question

First of all please forgive me if its a really dumb question, I am just trying to learn this language to its core. I am reading Effective Java and the very first chapter tal

Answer 1

Both of your questions can be answered by looking at some code that makes use of both of these properties of static factory methods. I suggest looking at Guava's ImmutableList.

Note how the no-arg factory method of() always returns the same instance (it doesn't create a new instance each time). If you look carefully, you'll also notice that its copyOf(Iterable) factory method actually returns the object that is passed to it if that object is itself an ImmutableList. Both of these are taking advantage of the fact that an ImmutableList is guaranteed to never change.

Notice also how various factory methods in it return different subclasses, such as EmptyImmutableList, SingletonImmutableList and RegularImmutableList, without exposing the types of those objects. The method signatures just show that they return ImmutableList, and all subclasses of ImmutableList have package-private (default) visibility, making them invisible to library users. This gives all the advantages of multiple implementation classes without adding any complexity from the user's perspective, since they are only allowed to view ImmutableList as a single type.

In addition to ImmutableList, most instantiable classes in Guava utilize static factory methods. Guava also exemplifies a lot of the principles set forth in Effective Java (not surprising, given that it was designed by those principles and with guidance from Josh Bloch himself), so you may find it useful to take a look at it more as you're working through the book.