(This post is an excerpt of “Effective Java, 3rd Edition”)

Super straight-forward example of a factory method

An example of a static factory method that returns Boolean, which is a boxed primitive class of boolean:

public static Boolean valueOf(boolean b) {
    return b ? Boolean.TRUE : Boolean.FALSE;

(By the way, it’s nothing to do with the so-called “Factory Method pattern” of GoF)

Merits of static factory methods compared with conventional constructors

  • They have names
  • They are not required to create a new object each time they’re invoked
    • For example: Boolean.TRUE described above doesn’t create an instance
  • They can return an object of any subtype of their return type
    • The class of the returned object can vary from call to call as a function of the input parameters.
      • EnumSet provides only static factories: In OpenJDK, the factory method returns either RegularEnumSet or JumboEnumSet based on the number of elements

Demerits of static factory methods compared with conventional constructors

  • Static factory methods without public or protected constructors cannot be subclassed
  • Static factory methods do not stand out in API documentation

Actually, “Being unable to create subclasses without public or protected constructors” can be thought as a merit as well, because composition is preferred to inheritance generally.


In many cases, static factories are preferred to public constructors. Let’s think about the differences between them and make a right choice!