Refactoring II Jan 14 2019 Books I Design Patterns is the classic book by the Gang of Four Erich Gamma Richard Helm Ralph Johnson and John Vlissides Basically a catalog of program organizations Reading it is rather like reading a dictionary Although equally applicable to Java the examples are in C Head First Design Patterns by Elizabeth Freeman Fun irreverent and entertaining IMHO the best book for developing an understanding of what design patterns are all about Books II AntiPatterns Refactoring Software Architectures and Projects in Crisis by William J Brown Raphael C Malveau Hays W Skip McCormick Thomas J Mowbray Describes bad patterns and how to fix them More fun to read than the Gang of Four book Refactoring Improving the Design of Existing Code by Martin Fowler with contributions from Kent Beck and others Very clear plenty of useful ideas very readable Much of this lecture will be based on this book Design Patterns and Refactoring Design Patterns describe good solutions to common or at least not extremely rare design problems An Antipattern is a common but poor Design Pattern Design Patterns are always specified in UML Model View Controller is a very common Design Pattern Antipatterns can be refactored into better designs Refactoring is rearranging existing code while maintaining the same functionality Refactoring is usually done in terms of applying some Design Pattern UML review I Name of the class Variables optional Methods Key Example Card cardId int copy boolean false constructor Card int id isKind desiredKind int isSharable boolean toString String means public visibility means protected visibility means private visibility blank means default package visibility static variables are underlined UML review II A A C Factory 1 4 B Class B extends class A B D Class C Class B contains implements interface A 1 to 4 objects of class D creates Product Other kinds of relations Bad smells Where did this term come from If it stinks change it Grandma Beck discussing child rearing philosophy The basic idea is that there are things in code that cause problems Duplicated code Long methods Etc A common reason for refactoring is that you need to add features that don t fit well with the existing design Any time you change working code you run the risk of breaking it A good test suite makes refactoring much easier and safer An example Some time ago I was working on code to evaluate 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 Adding a new operator requires modifying existing code I discovered a better solution 2 5 x Tree for 2 5 x Command lhs Command rhs Command 0 2 value int evaluate int Using my Command pattern class Add implements Command int evaluate int v1 lhs evaluate value int v2 rhs evaluate value value v1 v2 return value Command lhs Command rhs Command 0 2 value int evaluate int To evaluate the entire tree evaluate the root node To add an operator add another class that extends Command 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 Duplicated code I If the same code fragment occurs in more than one place within a single class you can use Extract Method Turn the fragment into a method whose name explains the purpose of the method Any local variables that the method requires can be passed as parameters if there aren t too many of them If the method changes a local variable see whether it makes sense to return that as the value of the method possibly changing the name of the method to indicate this If the method changes two or more variables you need other refactorings to fix this problem Duplicated code II If the same code fragment occurs in sibling classes you can use Extract Method in both classes then use Pull Up Method Use ExtractMethod in each class Be sure the code is identical If necessary adjust the method signatures to be identical Copy the extracted method to the common superclass Delete one subclass method Compile and test Delete the other subclass method Compile and test Duplicated code III If the same code fragment occurs in unrelated classes you can use Extract Method in one class then Use this class as a component in the other class or Invoke the method from the other class or Move the method to a third class and refer to it from both of the original classes In any case you need to decide where the method makes most sense and put it there Eclipse has an Extract method command that does most of this work for you Duplicated code IV If almost the same code fragment occurs in sibling classes use Extract Method to separate the similar bits from the different bits and use Form Template Method I ll give an example in a moment The Template Method Template Methods lead to an inverted control structure A superclass calls methods in its subclass Template methods are so fundamental that they can be found in almost every abstract class Template Method uses inheritance A similar pattern Strategy Pattern uses delegation rather than inheritance 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 Here s the initial code before refactoring abstract Fish abstract move BigFish LittleFish move move The move method General outline of the method public void move choose a random direction find the location in that direction check if it s ok to move there if it s ok make the move same for both same for both different same for both To do the Form Template Method 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 The Fish refactoring abstract Fish Note how this works When a BigFish tries to move it uses the BigFish LittleFish move method in Fish move move But the move method in Fish uses the abstract Fish okToMove locn move abstract okToMove locn boolean method in BigFish And similarly for LittleFish abstract move BigFish BigFish okToMove locn boolean okToMove locn boolean The resultant code In Fish public void move choose a random direction same for both find the location in that direction same for both check if it s ok to move there same for both if it s ok make the move same for
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