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12-Dec-14 Refactoring IV. Previously discussed bad smells Duplicated code — and other forms of redundancy Long method — use short methods that delegate

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12-Dec-14 Refactoring IV Slide 2 Previously discussed bad smells Duplicated code and other forms of redundancy Long method use short methods that delegate work Large class trying to do too much Long parameter list hard to use and remember Divergent change changes in one class for different reasons Shotgun surgery a change requires little changes all over Feature envy method uses too much from some other class Data clumps variables that frequently occur together Primitive obsession being afraid of making small objects Switch statements probably should use polymorphism instead Slide 3 More bad smells Parallel inheritance hierarchies cant make just one subclass Lazy class too few responsibilities Speculative generality code that isnt needed Temporary field an object doesnt use all its variables Message chains asking for objects to ask for objects Middle man too much responsibility passed along Inappropriate intimacy classes accessing each other too much Alternative classes with different interfaces similar work but with different signatures Incomplete library class inadequate for reuse Data class just data, maybe getters and setters Refused bequest subclass dont use much of their inheritance Comments when used as a substitute for good code Slide 4 Parallel inheritance hierarchies When you make a subclass of one class, you have to make a corresponding subclass of another class General strategy: Use Move Method and Move Field to make instances of one hierarchy refer to instances of the other Slide 5 Lazy class Small classes are fine, but sometimes a class just doesnt do enough If a class is very similar to its superclass, you can try to use Collapse Hierarchy to merge the two classes Eliminate the subclass by using Pull Up Field and Pull Up Method; or, Eliminate the superclass by using Push Down Field and Push Down Method If a class just isnt doing very much, move all its features into another class with Move Field and Move Method Slide 6 Speculative generality One of the principles of Extreme Programming (XP) is that you shouldnt write code until you need it XP assumes that code will change frequently, and tries to make change as fast and easy as possible If you try to make things too general, you may have unnecessary code that just gets in the way If the only users of a class or method are test cases, the code should be thrown away Since I hate to throw away good code, I usually move unused code to a discards directory Slide 7 Temporary field We expect an object to use all its fields Its confusing when an instance variable is used only in certain cases Use Extract Class to create a home for these variables Eliminate conditional code with Introduce Null Object Sometimes programmers will add instance variables to avoid long parameter lists between communicating methods Use Extract Class to create a new method object Slide 8 Message chains A message chain is a sequence such as BazObject b =foo.getBar().getBaz() Here we are asking foo for a bar object so that we can ask it for a baz object Often, but not always, these are getter methods Message chains can be abbreviated or eliminated by Hide Delegate As an example of Hide Delegate, we can introduce the following method into foo s class: BazObject getBaz() { return bar.getBaz(); } And then we can just call BazObject b = foo.getBaz(); Slide 9 Middle man Delegationproviding methods to call methods in another classis often useful for hiding internal details Example: BazObject getBaz() { return bar.getBaz(); } However, too much delegation isnt good You can: Use Remove Middle Man and talk to the object that really knows what is going on Use Inline Method to absorb a few small methods into the caller Use Replace Delegation With Inheritance to turn the middle man into a subclass of the real object Slide 10 Inappropriate intimacy Classes may make too much use of each others fields and methods Use Move Method and Move Field to reduce the association Try to Change Bidirectional Association to Unidirectional The idea here is to take the class that is less dependent on the other class and remove the remaining dependencies If the classes have common needs, try Extract Class Use Hide Delegate to let another class act as a middle man If a subclass knows too much about its superclass, use Replace Inheritance With Delegation (see later comments on Refused Bequest) Slide 11 Replace Inheritance With Delegation Sometimes a subclass inherits more from its superclass than you want it to have Example: Suppose class Sub extends Super, inherits desired methods int foo() and void bar(), along with other methods it does not want, and adds method int baz() Replace class Sub extends Super {...} with class Sub { // class name should also be changed Super s = new Super(); int foo() { return; } // delegate to s void bar() {; } // delegate to s int baz() {...} // new method } Slide 12 Alternative classes, different interfaces You end up with two essentially equivalent classes (example: Javas Enumeration and Iterator classes) Java cant eliminate Enumeration because that would break old code Even in this situation, the functionality can be moved into a single class Use Rename Method on methods that do the same thing but have different signatures Use Move Method until classes are doing the same things You may want to use Extract Superclass Slide 13 Incomplete library class Library classes (such as those supplied by Sun) dont always do everything we want them to do Its usually impossible to modify these library classes Use Introduce Foreign Method: Write the method you want, as if it were in the library class Make an instance of the library class the first parameter Add a comment that describes the method as a foreign method Example: private static Date nextDay(Date arg) { // foreign method, should be in Date return new Date(arg.getYear(), arg.getMonth(), arg getDate() + 1); } Slide 14 Data class Classes that just hold data, and maybe setters and getters for that data, are undesirable If a field is public, use Encapsulate Method to make it private and add setters and getters (if it isnt too late) If a method returns a collection, use Encapsulate Collection Make the method return a read-only view ( java.util.Collections supplies methods such as unmodifiableSet(Set) and unmodifiableMap(Map) ) Provide add and remove methods as appropriate Try to use Move Method to move the setters and getters to the class that needs them Slide 15 Refused bequest Subclasses may inherit unwanted methods from their superclasses This suggests that the hierarchy may be wrong Create a new subclass and use Push Down Method and Push Down Field on the unused methods Fowler feels that this isnt a severe problem, unless the subclass is reusing behavior but not the interface In a few places, Suns packages override an inherited method with one that just throws an UnsupportedOperationException A better solution is to Replace Inheritance With Delegation Slide 16 Comments Fowler says comments often are used as a deodorant If you need a comment to explain what a block of code does, use Extract Method If you need a comment to explain what a method does, use Rename Method If you need to describe the required state of the system, use Introduce Assertion This should not discourage the use of comments (especially javadoc comments)the point is that code should be self-explanatory, so that comments are not necessary Slide 17 Advice from Kent Beck Stopping is the strongest move in the refactorers repertoire. The idea is to take small goals, refactor in small steps, so that you can quit in a stable state, having made progress Somewhere your code smells bad. Resolve to get rid of the problem. Then march toward that goal. Stop when you are unsure If the code is already better, go ahead and release it If you arent sure of your changes, discard them and go back to the previous stable state Backtrack. If the code quits working, and you havent tested in a while, dont debugthrow away the changes since the last working tests Emotionally difficult, but rewriting is faster and easier than debugging Work with someone. There are many advantages to working in pairs. Slide 18 Soundbites I When you find you have to add a feature to a program, and the programs code is not structured in a convenient way to add the feature, first refactor the program to make it easy to add the feature, then add the feature Before you start refactoring, check that you have a solid suite of tests. These tests must be self-checking. Refactoring changes the programs in small steps. If you make a mistake, it is easy to find the bug. Slide 19 Soundbites II Any fool can write code that a computer can understand. Good programmers write code that humans can understand. Refactoring (noun): a change made to the internal structure of software to make it easier to understand and cheaper to modify without changing the observable behavior of the software. Refactor (verb): to restructure software by applying a series of refactorings without changing the observable behavior of the software. Slide 20 Soundbites III Three strikes and you refactor. The first time you do something, you just do it. The second time you do something similar, you wince at the duplication, but you do the duplicate thing anyway. The third time you do something similar, you refactor. Dont publish interfaces prematurely. Modify your code ownership policies to smooth refactoring. When you feel the need to write a comment, first try to refactor the code so that any comment becomes superfluous. Slide 21 Soundbites IV Make sure all tests are fully automatic and that they check their own results. A suite of tests is a powerful bug detector that decapitates the time it takes to find bugs. Run your tests frequently. Localize tests whenever you compileevery test at least every day. When you get a bug report, start by writing a unit test that exposes the bug. Slide 22 Soundbites V It is better to write and run incomplete tests than not to run complete tests. Think of the boundary conditions under which things might go wrong and concentrate your tests there. Dont forget to test that exceptions are raised when things are expected to go wrong. Dont let the fear that testing cant catch all bugs stop you from writing the tests that will catch most bugs. Slide 23 Conclusions A couple of the more important goals of Agile Programming (including XP) are to: Keep code flexible and easily changed Avoid long debugging sessions Always have something that works Refactoring depends heavily on: Taking small steps Having a fully automated test suite that you use frequently Slide 24 The End