Question

I am reading the Effective Java by Joshua Bloch and I have question about Item1 Static Factory Method.

Quote[Bloch, p.7]

Interfaces cant have static methods,
so by convention, static factory
methods for an interface named Type
are put in non-instantiable class
named Types. For example, the Java
Collections Framework, provide
unmodifiable collections, synchronized
collections, and the like. Nearly all
of these implementations are export
via static factory methods in one
noninstantiable class
(java.util.Collections). The classes
of the returned objects are all
non-public.

Ok. When look at the sources code, I see java.util.Collection interface and java.util.Collections class with private constructor (non-instantiable class). and I see that the non-instantiable class Collections has all static methods, just like what Bloch said. But i fail to see the connection between the two classes as Bloch said

Interfaces cant have static methods, so by convention, static factory methods for an interface named Type are put in non-instantiable class named Types.

1. Can anyone point out the obvious to me?

2. what is it mean when he said

The classes of the returned objects are all non-public

Here is where I obtain the java sources: http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/Collection.java?av=f

Answer 1

1. Interfaces cant have static methods, so by convention, static factory methods for an interface named Type are put in non-instantiable class named Types.

   The point is just the plural ‘s’ on “Type[s]”. So if your interface is called Foo and you want to create some implementation called MyFoo then your factory with the methods to instantiate should be called Foos by convention.

2. The classes of the returned objects are all non-public

   This means that the classes of objects returned from the factory methods have a private or default visibility modifier as in private class MyFoo{} so that they can not be instantiated by any other means but their factory methods. Since you can’t construct an Object using the new operator from private inner or package private class out of their scope (reflection aside).

e.g.:

 public interface Foo{ //interface without plural 's' (question 1)
     public void bar();
 }
 public abstract class Foos(){ // abstract factory with plural 's' (question 1)
    public static Foo createFoo(){
        return new MyFoo();
    }
    private class MyFoo implements Foo{ // a non visible implementation (question 2)
       public void bar(){}
    }
 }