Factory Methods

Restricting the visibility of the instance constructor only makes sense if somewhere within the class (or its subclasses, in case of CREATE PROTECTED) the instantiation is done.

Often, such classes provide a public static method that creates an instance of this class and returns the reference to this instance to the caller. Such methods are sometimes called factory methods. Factory methods are executed when an instance of the class is created and has some advantages as compared to the instance constructor.

Advantages of factory methods

• A factory method can have coding that executes before the actual instantiation.
  For example, the method can perform checks, read data from the database, or set locks, before actually creating the instance. If an error occurs, no instance is created; and, no memory is allocated for it.

 

• Classes can have more than one factory method. Each of the methods can have a different implementation and signature as well.
  The different factory method makes it possible to create instances of the same class based on different information. For example, one factory method creates a new instance that offers input parameters for all attributes, while another factory method only imports key information and then retrieve the other attribute values from the database.

 

• Use factory methods to administrate the instances of a class within the class itself.
  The administration of the instance within the class is often used to avoid the creation of many identical instances. A reference of every new instance is stored in a private static attribute of the class (an internal table). Whenever the factory method is called, it first checks whether an instance with this key already exists. If so, it returns the existing instance instead of creating a new one.

 

• Use a factory method to instantiate one of the subclasses rather than the class itself.
  As an example, the factory method of an airplane class can check the plane type, then instantiate either a cargo or a passenger plane.

Example for a Class with Factory Method

The example illustrates a class with a Factory Method and using a static attribute to reference all its instances. The instantiation is restricted to the class and is only done in static method FACTORY. When the method is called, the factory method checks whether an identical instance has already been created. If so, it returns the existing instance instead of creating a new one.

Hint: In this example, it is not necessary to make attributes public for the READ TABLE statement. As the factory method lies within the class, it has full access to all private attributes.

The Singleton Pattern

Use the singleton concept to prevent a class from being instantiated more than once for the same program context.

There are various different ways to implement a singleton class. The first singleton pattern is a special case of the factory concept. Instead of storing several instances, the class stores only one instance. When the factory method GET_INSTANCE is called for the first time, it instantiates the class. For every subsequent call, it returns the reference to the existing object.

Singleton Class - Using the Static Constructor

The class uses its static constructor to create the single instance in advance. The GET_INSTANCE method does not create the instance but only returns a reference to the already existing instance.

Hint: In a third variant of the singleton pattern, the GO_INSTANCE static attribute is made public and read-only. The GET_INSTANCE method is not required.