What are static factory methods?

What's a "static factory" method?

The static factory method pattern is a way to encapsulate object creation. Without a factory method, you would simply call the class's constructor directly: Foo x = new Foo(). With this pattern, you would instead call the factory method: Foo x = Foo.create(). The constructors are marked private, so they cannot be called except from inside the class, and the factory method is marked as static so that it can be called without first having an object.

There are a few advantages to this pattern. One is that the factory can choose from many subclasses (or implementers of an interface) and return that. This way the caller can specify the behavior desired via parameters, without having to know or understand a potentially complex class hierarchy.

Another advantage is, as Matthew and James have pointed out, controlling access to a limited resource such as connections. This a way to implement pools of reusable objects - instead of building, using, and tearing down an object, if the construction and destruction are expensive processes it might make more sense to build them once and recycle them. The factory method can return an existing, unused instantiated object if it has one, or construct one if the object count is below some lower threshold, or throw an exception or return null if it's above the upper threshold.

As per the article on Wikipedia, multiple factory methods also allow different interpretations of similar argument types. Normally the constructor has the same name as the class, which means that you can only have one constructor with a given signature. Factories are not so constrained, which means you can have two different methods that accept the same argument types:

Coordinate c = Coordinate.createFromCartesian(double x, double y)

and

Coordinate c = Coordinate.createFromPolar(double distance, double angle)

This can also be used to improve readability, as Rasmus notes.

NOTE! "The static factory method is NOT the same as the Factory Method pattern" (c) Effective Java, Joshua Bloch.

Factory Method: "Define an interface for creating an object, but let the classes which implement the interface decide which class to instantiate. The Factory method lets a class defer instantiation to subclasses" (c) GoF.

"Static factory method is simply a static method that returns an instance of a class." (c) Effective Java, Joshua Bloch. Usually this method is inside a particular class.

The difference:

The key idea of static factory method is to gain control over object creation and delegate it from constructor to static method. The decision of object to be created is like in Abstract Factory made outside the method (in common case, but not always). While the key (!) idea of Factory Method is to delegate decision of what instance of class to create inside Factory Method. E.g. classic Singleton implementation is a special case of static factory method. Example of commonly used static factory methods:

• valueOf


• getInstance


• newInstance

We avoid providing direct access to database connections because they're resource intensive. So we use a static factory method getDbConnection that creates a connection if we're below the limit. Otherwise, it tries to provide a "spare" connection, failing with an exception if there are none.

public class DbConnection{
   private static final int MAX_CONNS = 100;
   private static int totalConnections = 0;

   private static Set<DbConnection> availableConnections = new HashSet<DbConnection>();

   private DbConnection(){
     // ...
     totalConnections++;
   }

   public static DbConnection getDbConnection(){

     if(totalConnections < MAX_CONNS){
       return new DbConnection();

     }else if(availableConnections.size() > 0){
         DbConnection dbc = availableConnections.iterator().next();
         availableConnections.remove(dbc);
         return dbc;

     }else {
         throw new NoDbConnections();
     }
   }

   public static void returnDbConnection(DbConnection dbc){
     availableConnections.add(dbc);
     //...
   }
}

Readability can be improved by static factory methods:

Compare

public class Foo{
  public Foo(boolean withBar){
    //...
  }
}

//...

// What exactly does this mean?
Foo foo = new Foo(true);
// You have to lookup the documentation to be sure.
// Even if you remember that the boolean has something to do with a Bar
// you might not remember whether it specified withBar or withoutBar.

to

public class Foo{
  public static Foo createWithBar(){
    //...
  }

  public static Foo createWithoutBar(){
    //...
  }
}

// ...

// This is much easier to read!
Foo foo = Foo.createWithBar();