Design Patterns and Refactoring

1. Design Patterns and Refactoring

2. History • A Pattern Language: Towns, Buildings, Construction, Christopher Alexander, 1977 • The Timeless Way of Building, Christopher Alexander, 1979 • Using Pattern Languages for Object-Oriented Programs (a paper at the OOPSLA-87 conference), Ward Cunningham and Kent Beck, 1987 • Design Patterns, Erich Gamma, Richard Helm, John Vlissides, and Ralph Johnson (known as the “Gang of Four”, or GoF), 1994 • Refactoring: Improving the Design of Existing Code, Martin Fowler, 2000

3. Buzzwords • Design Patterns describe the higher-level organization of solutions to common problems • UML is a diagramming language designed for Object-Oriented programming • Design Patterns are always described in UML notation • Refactoring is restructuring code in a series of small, semantics-preserving transformations (i.e. the code keeps working) in order to make the code easier to maintain and modify • Refactoring often modifies or introduces Design Patterns • Extreme Programming is a form of Agile Programming that emphasizes refactoring • Unit testing is testing classes in isolation • Unit testing is an essential component of Extreme Programming • Unit testing is supported by JUnit

4. Design Patterns • Design Patterns describe the higher-level organization of solutions to common problems • Design Patterns are a current hot topic in O-O design • UML is always used for describing Design Patterns • Design Patterns are used to describe refactorings

5. UML • UML stands for Unified Modeling Language • UML is a big, complicated diagramming language designed for Object-Oriented programming • UML comprises at least seven or eight different kinds of diagrams that can be used to describe: • the organization of a program • how a program executes • how a program is used • how a program is deployed over a network • …and more • This talk will cover just a tiny bit of one kind of diagram, the class diagram

6. Example: Card cardId:int -copy:boolean=false «constructor» Card(int id) +isKind(desiredKind:int) +isSharable():boolean +toString():String Name of the class Variables [optional] Methods UML class diagrams • Key: • + means public visibility • # means protected visibility • - means private visibility • <blank> means default (package) visibility • static variables are underlined

7. A B C D Factory Product A B creates 1..4 Class Ccontains 1 to 4 objectsof class D Other kinds ofrelations Class Bextendsclass A Class Bimplementsinterface A UML relationships

8. Sample Design Patterns • There are a few dozen Design Patterns described in the GoF book • I’ll only talk about a couple, to give some of the flavor of what they are all about • I’ll try to use examples that are relevant to problems you have dealt with in this class (plus some I’m particularly fond of) • I’m using UML diagrams in only a few examples, because they just take too much time to draw in PowerPoint • Specialized tools, such as Rational Rose, Together, and ArgoUML are much better for this

9. Problem: Uncertain delegation • Much of the point of polymorphism is that you can just send a message to an object, and the object does the right thing, depending on its type • However, if the object might be null, you have to be careful not to send it any message • if (myObject != null) myObject.doSomething(); • Examples: • You have an Ocean, represented by a sparse array containing a few Fish • You have a TrafficGrid, some of which contains Cars and Trucks • You want to send output to somewhere, possibly to /dev/null • If you do a lot with this object, you code can end up cluttered with tests for null

10. Solution: Null Object • Create another kind of object, a “null object,” representing the absence of any other kind of object • Example: An Ocean might contain Inhabitants, where Inhabitant is subclassed by BigFish, LittleFish, Algae, and NothingButWater • This way, no location in the Ocean is null • If Inhabitant contains a method reproduce(), the subclass NothingButWater could implement this method with an empty method body • If appropriate, some methods of the null object could throw an Exception • Ideally, the superclass (Inhabitant, in this example) should be abstract

11. Refactoring: Introduce Null Object • The general idea is simple: Instead of having some variables (locations in the array) be null, have them be “null objects” • However, this requires numerous changes in the code • It’s hazardous to change working code—you introduce bugs that it can take days to find • Refactoring is all about: • doing an operation like this in small steps, • having an automated set of unit tests, and • running unit tests frequently, so that if an error occurs you can pinpoint it immediately • This approach makes refactoring much safer and protects against hard-to-find bugs • As a result, programmers are far more willing to refactor

12. Introduce Null Object: In detail, I • Create a subclass of the source class to act as a null version of the class. Create an isNull operation on the source class and the null class. For the source class it should return false, for the null class it should return true. • You may find it useful to create an explicitly nullable interface for the isNull method. • As an alternative you can use a testing interface to test for nullness • Compile. • Find all places that can give out a null when asked for a source object. Replace them to give out a null object instead.

13. Introduce Null Object: In detail, II • Find all places that compare a variable of the source type with null and replace them with a call to isNull. • You may be able to do this by replacing one source and its clients at a time and compiling and testing between working on sources. • A few assertions that check for null in places where you should no longer see it can be useful. • Compile and test. • Look for cases in which clients invoke an operation if not null and do some alternative behavior if null. • For each of these cases override the operation in the null class with the alternative behavior. • Remove the condition check for those that use the overridden behavior, compile, and test.

14. Refactoring details • The details of Introduce Null Object were copied directly from Fowler, pages 261-262 • I am not going into this much detail in any of the remaining examples • Notice, however, that with this list of “baby steps” in front of you, you can do the refactoring a little at a time, with well-marked places to do testing, so that it’s very easy to catch and correct errors • Note also that to do this, you need a good set of totally automated tests—otherwise the testing you have to do is just too much work, and you won’t do it • Unless, that is, you have a superhuman amount of discipline • JUnit (now built in to BlueJ) is a great start

15. Fish <<abstract>>move() BigFish LittleFish move() move() Scenario: Big fish and little fish • The scenario: “big fish” and “little fish” move around in an “ocean” • Fish move about randomly • A big fish can move to where a little fish is (and eat it) • A little fish will not move to where a big fish is

16. Problem: Similar methods in subclasses • Here we have a Fish class with two subclasses, BigFish and LittleFish • The two kinds move the same way • To avoid code duplication, the move method ought to be in the superclass Fish • However, a LittleFish won’t move to some locations where a BigFish will move • The test for whether it is OK to move really ought to be in the move method • More generally, you want to have almost the same method in two or more sibling classes

17. Solution: Template method • Note: The Design Pattern is called “Template Method”; the refactoring is called “Form Template Method” • We won’t bother making this distinction in the remainder of the lecture • In the superclass, write the common method, but call an auxiliary method (such as okToMove) to perform the part of the logic that needs to differ • Write the auxiliary method as an abstract method • This in turn requires that the superclass be abstract • In each subclass, implement the auxiliary method according to the needs of that subclass • When a subclass instance executes the common method, it will use its own auxiliary method as needed

18. The move() method • General outline of the method: • public void move() {choose a random direction; // same for bothfind the location in that direction; // same for bothcheck if it’s ok to move there; // differentif it’s ok, make the move; // same for both} • To refactor: • Extract the check on whether it’s ok to move • In the Fish class, put the actual (template) move() method • Create an abstract okToMove() method in the Fish class • Implement okToMove() in each subclass

19. Fish Fish move() <<abstract>>okToMove(locn):boolean <<abstract>>move() BigFish BigFish LittleFish BigFish okToMove(locn):boolean move() okToMove(locn):boolean move() The Fish refactoring • Note how this works: When a BigFish tries to move, it uses the move() method in Fish • But the move() method in Fish uses the okToMove(locn) method in BigFish • And similarly for LittleFish

20. Problem: Constructors create objects • Constructors make objects. Only constructors can make objects. When you call a constructor of a class, you will get an instance of that class. • Sometimes you want more flexibility than that— • You may want to guarantee that you can never have more than one object of a given class • You may want to create an object only if you don’t already have an equivalent object • You may want to create an object without being sure exactly what kind of object you want • The key insight is that, although only constructors make objects, you don’t have to call constructors directly—you can call a method that calls the constructor for you • Several “creational” Design Patterns are based on this observation

21. Singleton • A Singleton is a class that can have only one instance • You may want just one instance of a null object, which you use in many places • You may want to create just one AudioStream, so you can only play one tune at a time • class Singleton { private static Singleton instance = new Singleton();// don’t let Java give you a default public constructor private Singleton() { } Singleton getInstance() { return instance; } ...}

22. The Factory Method Design Pattern • Suppose you write a class that works with several different kinds of objects • You can do this if the classes all have a common interface • You may want to be able to create objects, without being dependent on the kind of object • A factory methods can create instances of different classes, depending (say) on its parameters • Example: • Image createImage (String ext) { if (ext.equals("gif")) return new GIFImage(); if (ext.equals("jpg")) return new JPEGImage(); ...}

23. Problem: Reducing interdependencies • Suppose you have an application that provides multiple services • Suppose further that the application consists of a large number of classes • You want to provide access to those services, without requiring the user to know all the internal details • For example, you have a simulation of an ocean, containing assorted kinds of fish, and you want to provide access to that simulation • Just to make the problem interesting, suppose that you have two or more such applications (say, Oracle, MySql, and Access 2000) and you want to write a program that works with any of them • Solution: Use the Façade Design Pattern

24. The FaçadeDesign Pattern • Create a class that accepts many different kinds of requests, and “forwards” them to the appropriate internal class • If the back ends vary, you man need to write a separate Interface class for each (all implementing the same interface), but the users of your Facade class don’t need to change • Example: • class Ocean { public void setBigFishGestationPeriod(int period) { BigFish.setGestationPeriod(period); } public int getNumberOfAlgae() { return Algae.getCount(); } ...} • Of course, the Facade class can do other things as well

25. The Immutable Design Pattern • There are many benefits to objects that cannot be changed after they have been created • Such objects are called immutable • Objects that refer to an immutable object never have to worry about whether that object has been changed • Immutable objects are thread-safe—this is a significant efficiency concern, because synchronization is expensive • Example: Strings in Java • It’s easy to make immutable objects in Java: • Make all instance variables private, and • Provide no methods that change those variables

26. Delegation (or, when not to use inheritance) • When you create a subclass, you agree to inherit all its (non-private) fields and methods • What if you don’t want them all? • Example: A Vector can do everything that a Stack should be able to do, and much, much more • You may want to inherit just some of the functionality, and probably add some of your own • Inheritance doesn’t let you do that—at least, not easily • If your class wants to hide variables or methods inherited from a superclass, it shouldn’t inherit from that superclass • If an object needs to be a different subclass at different times (say, a LittleFish turning into a BigFish), then it shouldn’t be a subclass of that class in the first place • Instead of inheriting, just use an instance of that class, and delegate to it

27. Example: Stacks • class Stack { Vector contents = new Vector(); public void push(Object o) { contents.add(o); // delegate to the Vector } public Object pop() { return contents.remove(contents.size() – 1); } ...}

28. Some time ago I was working on code to evalute expressions Expressions can be parsed into a tree structure Now what? You could walk the tree and, at each node, use a switch statement to do the right thing I “discovered” a better solution (basically, a simple form of the Command Design Pattern) + 2 * Command lhs:Command rhs:Commandvalue:int 5 x 0..2 Tree for 2 + 5 * x evaluate():int An example

29. class Add extends Command { int evaluate( ) { int v1 = lhs.evaluate().value; int v2 = rhs.evaluate().value; value = v1 + v2; return value; }} To evaluate the entire tree, evaluate the root node This is just a rough description; there are a lot of other details to consider Some operands are unary You have to look up the values of variables Etc. Command lhs:Command rhs:Commandvalue:int 0..2 evaluate():int Using my “Command” pattern

30. The Command Design Pattern • Reasons for using the Command Design Pattern: • You want to control if,when, and in what order the commands are executed • You want to keep a log of commands executed • Popular reason: You want to manage undo and redo operations • Possible class organization (from GoF): • AbstractCommand with doIt() and undoIt() methods • ConcreteCommand subclasses of AbstractCommand • Invoker is a class that creates ConcreteCommand objects if it needs to invoke a command • CommandManager to decide what, when, and how to execute and undo commands

31. Refactoring • Refactoring is: • restructuring (rearranging) code... • ...in a series of small, semantics-preserving transformations (i.e. the code keeps working)... • ...in order to make the code easier to maintain and modify • Refactoring is not just any old restructuring • You need to keep the code working • You need small steps that preserve semantics • You need to have unit tests to prove the code works • There are numerous well-known refactoring techniques • You should be at least somewhat familiar with these before inventing your own

32. When to refactor • You should refactor: • Any time that you see a better way to do things • “Better” means making the code easier to understand and to modify in the future • You can do so without breaking the code • Unit tests are essential for this • You should not refactor: • Stable code (code that won’t ever need to change) • Someone else’s code • Unless you’ve inherited it (and now it’s yours)

33. Design vs. coding • “Design” is the process of determining, in detail, what the finished product will be and how it will be put together • “Coding” is following the plan • In traditional engineering (building bridges), design is perhaps 15% of the total effort • In software engineering, design is 85-90% of the total effort • By comparison, coding is cheap

34. The refactoring environment • Traditional software engineering is modeled after traditional engineering practices (= design first, then code) • Assumptions: • The desired end product can be determined in advance • Workers of a given type (plumbers, electricians, etc.) are interchangeable • “Agile” software engineering is based on different assumptions: • Requirements (and therefore design) change as users become acquainted with the software • Programmers are professionals with varying skills and knowledge • Programmers are in the best position for making design decisions • Refactoring is fundamental to agile programming • Refactoring is sometimes necessary in a traditional process, when the design is found to be flawed

35. A personal view • In my opinion, • Design, because it is a lot more creative than simple coding, is also a lot more fun • Admittedly, “more fun” is not necessarily “better” • ...but it does help you retain good programmers • Most small to medium-sized projects could benefit from an agile programming approach • We don’t yet know about large projects • Most programming methodologies attempt to turn everyone into a mediocre programmer • Sadly, this is probably an improvement in general • These methodologies work less well when you have some very good programmers

36. Back to refactoring • When should you refactor? • Any time you find that you can improve the design of existing code • You detect a “bad smell” (an indication that something is wrong) in the code • When can you refactor? • You should be in a supportive environment (agile programming team, or doing your own work) • You should have an adequate set of automatic unit tests

37. Example 1: switch statements • switch statements are very rare in properly designed object-oriented code • Therefore, a switch statement is a simple and easily detected “bad smell” • Of course, not all uses of switch are bad • A switch statement should not be used to distinguish between various kinds of object • There are several well-defined refactorings for this case • The simplest is the creation of subclasses

38. Example 1, continued • class Animal { final int MAMMAL = 0, BIRD = 1, REPTILE = 2; int myKind; // set in constructor ... String getSkin() { switch (myKind) { case MAMMAL: return "hair"; case BIRD: return "feathers"; case REPTILE: return "scales"; default: return "integument"; } }}

39. Example 1, improved • class Animal { String getSkin() { return "integument"; }}class Mammal extends Animal { String getSkin() { return "hair"; }}class Bird extends Animal { String getSkin() { return "feathers"; }}class Reptile extends Animal { String getSkin() { return "scales"; }}

40. How is this an improvement? • Adding a new animal type, such as Amphibian, does not require revising and recompiling existing code • Mammals, birds, and reptiles are likely to differ in other ways, and we’ve already separated them out (so we won’t need more switch statements) • We’ve gotten rid of the flags we needed to tell one kind of animal from another • Basically, we’re now using Objects the way they were meant to be used

41. JUnit tests • As we refactor, we need to run JUnit tests to ensure that we haven’t introduced errors • public void testGetSkin() { assertEquals("hair", myMammal.getSkin()); assertEquals("feathers", myBird.getSkin()); assertEquals("scales", myReptile.getSkin()); assertEquals("integument", myAnimal.getSkin());} • This should work equally well with either implementation • The setUp() method of the test fixture may need to be modified • A (beta) version of JUnit is now included in BlueJ

42. Bad Smell Examples • We should refactor any time we detect a “bad smell” in the code • Examples of bad smells include: • Duplicate Code • Long Methods • Large Classes • Long Parameter Lists • Multi location code changes • Feature Envy • Data Clumps • Primitive Obsession

43. Structure of Observer Pattern

44. Three Types of Patterns • Creational patterns: • Deal with initializing and configuring classes and objects • Structural patterns: • Deal with decoupling interface and implementation of classes and objects • Composition of classes or objects • Behavioral patterns: • Deal with dynamic interactions among societies of classes and objects • How they distribute responsibility

45. Creational Patterns • Abstract Factory: • Factory for building related objects • Builder: • Factory for building complex objects incrementally • Factory Method: • Method in a derived class creates associates • Prototype: • Factory for cloning new instances from a prototype • Singleton: • Factory for a singular (sole) instance

46. Structural patterns • Describe ways to assemble objects to realize new functionality • Added flexibility inherent in object composition due to ability to change composition at run-time • not possible with static class composition • Example: Proxy • Proxy: acts as convenient surrogate or placeholder for another object. • Remote Proxy: local representative for object in a different address space • Virtual Proxy: represent large object that should be loaded on demand • Protected Proxy: protect access to the original object

47. Structural Patterns • Adapter: • Translator adapts a server interface for a client • Bridge: • Abstraction for binding one of many implementations • Composite: • Structure for building recursive aggregations • Decorator: • Decorator extends an object transparently • Facade: • Simplifies the interface for a subsystem • Flyweight: • Many fine-grained objects shared efficiently. • Proxy: • One object approximates another

48. Behavioral Patterns • Chain of Responsibility: • Request delegated to the responsible service provider • Command: • Request or Action is first-class object, hence re-storable • Iterator: • Aggregate and access elements sequentially • Interpreter: • Language interpreter for a small grammar • Mediator: • Coordinates interactions between its associates • Memento: • Snapshot captures and restores object states privately Which ones do you think you have seen somewhere?

49. Behavioral Patterns (cont.) • Observer: • Dependents update automatically when subject changes • State: • Object whose behavior depends on its state • Strategy: • Abstraction for selecting one of many algorithms • Template Method: • Algorithm with some steps supplied by a derived class • Visitor: • Operations applied to elements of a heterogeneous object structure

50. The End