1. Façade Pattern Jeff Schott CS590L Spring 2004

2. What is a façade? 1) The principal face or front of a building 2) A false, superficial, or artificial appearance

3. What is the intent? n Provide a unified interface to a set of interfaces in a subsystem n Define a higher-level interface that makes a subsystem easier to use

4. Motivation n Structuring a system into subsystems helps reduce complexity n Minimize communication dependencies between subsystems n Introduce a façade that provides a single simplified interface

5. Provides a single interface to clients

6. Example - Compiler n Scanner n Parser n ProgramNode n BytecodeStream n ProgramNodeBuilder Compiler subsystem contains classes:

7. n Most clients don’t care about details like parsing and code generation n Compiler subsystem includes a Compiler class - unified interface

8. Applicability n Provide a simple interface to a complex system n Reduce dependencies between clients and implementation classes n Layered subsystems - façade provides an entry point to each layer

9. Structure

10. Benefits n Shields clients from subsystem components n Promotes weak coupling between the subsystem and clients n Doesn’t prevent clients from accessing subsystem classes if necessary

11. Implementation Considerations n Coupling between clients and the subsystem can be further reduced by making the façade an abstract class n Public v. Private subsystem classes

12. Sample Code

17. Related Patterns n Abstract Factory n Mediator n Singleton n Adapter n Decorator

18. Questions?