Design Patterns & Refactoring

Home , Bridge pattern, Dependency injection

Oliver Haase

HTWG Konstanz

‘A computer once beat me at chess, but it was no match to me at kick-boxing’ — Emo Philips

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 1 / 11 Description I

Classiﬁcation: object based structural pattern Purpose: decouple abstraction (interfaces) and implementation, such that both can evolve independently

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 2 / 11 Motivation

Scenario: Provide more comfortable RemoteOpComm interface without modiﬁcation of existing type hierarchy.

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 3 / 11 Motivation

Problem: Implementation classes TCPCommunication and UDPCommunication must be duplicated.

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 4 / 11 Motivation

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 5 / 11 Structure using Bridge Pattern

Client

Bridge Communication impl CommImpl send(op, params): void sendImpl() receive(): Result receiveImpl()

impl.sendImpl(op,params);

return impl.receiveImpl();

RemoteOpComm TcpCommunication UdpCommunication invoke(op, params): Result sendImpl(op, params): void sendImpl(op, params): void receiveImpl(): Result receiveImpl(): Result

send(op, params); return receive();

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 6 / 11 Implications

Please note that the abstractions (left hand side) use only the functionality provided by class CommImpl. Additional functionality in the subclasses cannot be taken advantage of; additional functionality in sub-interfaces must be achieved only through usage of the general functionality as contained in the root interface.

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 7 / 11 General Structure

Client

Bridge Abstraction impl Implementor operation() operationImpl()

impl.operationImpl();

ConcreteImplementorA ConcreteImplementorB Specialization operationImpl() operationImpl()

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 8 / 11 Description II

Members: Abstraction (Communication): defines interface of the abstraction maintains reference to Implementor instance Specialization (RemoteOpComm): extends Abstraction interface through usage of its operations Implementor (CommImpl): defines interface for implementation classes (can differ from Abstraction interface) ConcreteImplementor (TcpCommunication): provides implementation of Implementor interface

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 9 / 11 Description III

Interactions: Abstraction delegates requests to Implementor object Consequences: loose coupling of abstraction and implementation Implementor object can be replaced at run-time

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 10 / 11 Implementation

Who decides when and how, what particular Implementor object to use?

1 Specialization object, based on the params passed in at construction time. Example: Collection object that gets initialized with small initial size uses LinearList implementation. Large Collection object uses BTree implementation. (Note: Implementation can be dynamically replaced as the collection grows.) 2 Others decide → creational patterns! Specialization object gets fed with factory or prototype Dependency injection → Specialization object gets passed in Implementor object by Client.

Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 11 / 11