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One-and-only-one object, shared among others

One-and-only-one object, shared among others Problem: how to avoid multiple objects?

One-and-only-one object, shared among others Solution: Logic +Logic( ) + getInstance( ) : Logic Logic - Logic( ) + getInstance( ) : Logic

One-and-only-one object, shared among others Solution: Logic - theOne : Logic - Logic( ) + getInstance( ) : Logic

One-and-only-one object, shared among others Solution: public static Logic getInstance(){ if (theOne == null) theOne = new Logic(); return theOne; } Logic - theOne : Logic - Logic( ) + getInstance( ) : Logic //somewhere else in the system… Logic logic = Logic.getInstance(); //instead of … Logic logic = new Logic();

Using patterns to solve recurring problems Part I Déjà vu: Using patterns to solve recurring problems Part I CS2103/T, Lecture 9, Part 2, [March, 2018]

Using patterns to solve recurring problems Déjà vu: Using patterns to solve recurring problems

Using patterns to solve recurring problems experience Déjà vu: Using patterns to solve recurring problems

experience … is what you get when you didn’t get what you wanted.

experience … is valuable.

Learning from experience … is too costly.

experience Learning from Note to self: never volunteer to be the headstand guy

Learning from experience Patterns

Context: One-and-only-one Logic object, but shared by others Problem: Need to control the construction of the Logic object, but allow sharing Solution: Use private constructor and maintain a static reference for the only copy of the Logic object Logic - theOne : Logic - Logic( ) + getInstance( ) : Logic

Name: Singleton Pattern Context: One-and-only-one object, but shared by others Problem: Need to control the construction, but allow sharing Solution: Use private constructor and maintain a static reference for the only copy of the object Logic - theOne : Logic - Logic( ) + getInstance( ) : Logic Singleton - theOne : Singleton - Singleton( ) + getInstance( ) : Singleton

Name: Singleton Pattern Context: One-and-only-one object, but shared by others Problem: Need to control the construction, but allow sharing Solution: Use private constructor and maintain a static reference for the only copy of the object [An elegant solution to a recurring problem] Pattern Singleton - theOne : Singleton - Singleton( ) + getInstance( ) : Singleton

Name: Singleton Pattern Context: One-and-only-one object, but shared by others Problem: Need to control the construction, but allow sharing Solution: Use private constructor and maintain a static reference for the only copy of the object [An elegant solution to a recurring problem] Pattern <<Singleton>> - theOne : <<Singleton>> - Singleton( ) + getInstance( ) : <<Singleton>>

Name: Singleton Pattern … for that, I used the Name: Singleton Pattern Show off ! Context: One-and-only-one object, but shared by others Problem: Need to control the construction, but allow sharing Solution: Use private constructor and maintain a static reference for the only copy of the object <<Singleton>> - theOne : <<Singleton>> - Singleton( ) + getInstance( ) : <<Singleton>>

Patterns = a high-level vocabulary

[A stupid solution to a recurring problem] Anti-Pattern Context: One-and-only-one object, but shared by others Problem: Need to control the construction, but allow sharingSolution: Represent abstraction and occurrences as different classes.

[A stupid solution to a recurring problem] Anti-Pattern Context: One-and-only-one object, but shared by others Problem: Need to control the construction, but allow sharingSolutionSolution: Represent abstraction and occurrences as different classes.

1. Singleton 2. Façade 3. Command

1. Singleton 2. Façade 3. Command

1. Singleton 2. Façade 3. Command Logic UI ATD

1. Singleton 2. Façade 3. Command Logic TextUI MSLogic ATD

<<Façade>> 1. Singleton 2. Façade 3. Command <<Client1>> <<Façade>> <<Client2>>

User commands are kept as objects 1. Singleton 2. Façade 3. Command Edit User commands are kept as objects Sort Delete

Explain this design in less than 5 words, without diagrams or code Command cmd = createCommand(commandString); cmd.execute(); history.add(cmd); … cmd.undo(); {abstract} Command execute() {abstract} undo() {abstract} Edit execute() undo() Delete Add History add(Command) *

1. Singleton 2. Façade 3. Command

Learning → Applying

Learning → Applying

Learning → Applying

GoF patterns Learning → Applying Creational: Behavioral : Abstract Factory Builder Factory Method Prototype Singleton Behavioral : Chain of Responsibility Command Interpreter Template Method Iterator Memento State Visitor Mediator Observer Strategy Structural: Adapter Bridge Composite Decorator Façade Flyweight Proxy GoF patterns

Learning → Applying

Learning → Applying Pattern So many patterns, so little time…

Learning → Applying Pattern So many patterns, so little time…

Learning → Applying Pattern So many patterns, so little time…

Which pattern can you apply in your project? Learning → Applying Which pattern can you apply in your project?

Learning → Applying

1. Singleton 2. Façade 3. Command 4. More to come…. Identify patterns in sample code Use more patterns if it helps Should be able to relate each pattern to the project