Effective Java

By Joshua Bloch

…

• Static factory method : static method that returns an instance of a class.

Advantages of static factory methods over constructors:

1. static methods have names
2. static methods must not create a new object (example: Boolean.valueOf(boolean))
   • instance-controlled classes
3. static methods can return instances of any subtype of their return type
   • possibility to hide subtypes from public → inteface-based frameworks
4. class of the returned object can vary from call to call as a function of the input parameters
5. class of the returned object need not exist when the class containing the method is written
   • service provider frameworks
     • → dependency injection (default implementation: java.util.ServiceLoader)

Limitations/Shortcomings of static factory methods:

1. the main limitation of providing only static factory methods is that classes without public or protected constructors cannot be subclassed
2. hard for programmers to find
   • to “fix” it use common names like
     • from - makes object from one parameter, e.g. Date.from(instant)
     • of - makes “aggregated” object, e.g. Set<Rank> faceCards = EnumSet.of(JACK, QUEEN, KING);
     • valueOf - more verbose alternative to from and of
     • instance or getInstance - returns maybe not new instance
     • create or newInstance - returns new instance
     • getType - like getInstance but is in a different class
     • newType - like createInstance but is in a different class
     • type - concise alternative to getType and newType, e.g. Collections.list()

Problem: many fields where some of them are not always necessary.

• telescoping constructor pattern - many constructors with growing parameter number
  • Disadvantages
    • telescoping contructor works but it is hard to write client code, and harder to read it.

• JavaBeans pattern - parameterless constructor and many setters
  • Disadvantages
    • construction is split across multiple calls, so the bean can be in inconsistent state partway through it construction
    • JavaBeans precludes the possibility to make a class immutable

Solution: Builder pattern

• Don't create the object directly, but with the help of a builder object (typically static member of the class it creates)

NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8).calories(100).sodium(35).carbohydrate(27).build();

• The Builder pattern is well suited to class hierarchies.

• Covariant return typing - subclass method is declared to return a subtype of the return type declared in the superclass
  • allows clients to use these builder without the need for casting

Disadvantages of Builder:

• more verbose than telescoping constructor
  • so it should be used if we really have more than a handful of parameters

…

…