A class which has only one instance.

• private ctor
• public static getInstance method
  
  

Serialization, concurrency, garbage collection can introduce problems.

An object's state should never change.

• member variables may be defined as final
• ctor initializes member variables (or internal states)
• no menthod modifies member variables (or internal states)
  
  

Related pattern: Read-Only Interface

To mark semantic attribute with no specific members.

• a class implements an empty interface
• a utility operation can recognize the marker with instanceof keyword
  
  

java.io.Serializable and java.rmi.Remote are markers.

Access an object's service indirectly through another object called proxy, generally without letting the client know.

• a service class provides the service
• through proxy class, a client can access the service
• there is a COMMON INTERFACE of both the service class and the proxy class
  
  

Virtual Proxy is a proxy that creates the service object on demand.

Provides an interface for creating family of objects without specifying their concrete classes.

• AbstractFactory AbstractProduct ConcreteFactory ConcreteProduct Client
• call methods to create objects instead of the new keyword with a concrete class names
  
  

Factory that has a creator method which creates a product composed of several components.

• an abstract class declares a creator method returning the product
• subclass usually overrides the methods creating the components but use the same creator for the product.
  
  

Factory Method is like a virtual ctor.

Factory that clones an existing object rather than creating it.

• classes: Reusable, ReusablePool, Client
• ReusablePool methods: acquire, release