Software Design Patterns

Ensure class has only 1 instance, and provide global access to it (static class in java).

Many cases where we need only 1 instance of a class (ex. most factories)

Must have only 1 instance of a class & have a well-known access point to it. When only instance should be extended by subclassing - clients should not have to modify code to use extended instance.

The Singleton itself. It defines an instance operation that lets clients access its unique instance. It may be responsible for creating its own instance via a private constructor.

Controlled access to a sole instance. Reduced namespace. Permits refinement of operations & representation. Permits variable number of instances. More flexible than class operations.

Can hide operation that creates the instance behind a class operation to guarantee creation occurs successfully. Since it has a pointer to a singleton object, the actual singleton can be changed at run-time via subtyping. Allows instantiation of object at time client specifies.

Define an interface for creating an object but let the subclasses decide which class to instantiate.

Encapsulates the knowledge of which subclass to create. Lets the application using the Factory Method be ignorant of which subclass is needed.

Class cannot anticipate class of objects it must create. Class wants its subclasses to specify the object it creates. Classes delegate responsibility to one of several helper subclasses & want to localize knowledge of which subclass is the delegate

Product - defines the interface of objects created by factory method. Concrete Product - Implements the Product interface. Creator - Declares the factory method. Concrete Creator - Overrides the factory method to return an instance of a Concrete Product

Creator relies on its subclasses (Concrete Creators) to define factory method that will return an instance of the appropriate Concrete Product class

Eliminates need to bind application specific class into code. Potential disadvantage - large overhead to create just 1 object. Provides hooks for subclasses which can make for complicated object creation. Connects parallel class hierarchies.

2 major varieties - Creator can be an abstract class, or it can be a concrete class that provides a default implementation. There can also be parameterized factory methods that take a parameter dictating which type of object to create. All objects created share common interface.

Decouple an abstraction from its implementation.

Abstraction could have several possible implementations. It is difficult to modify, extend, or reuse abstraction & implementation independently. The bridge puts abstraction and its implementation in separate class hierarchies.