1. Designing Classes 2 How to write classes in a way that they are easily understandable, maintainable and reusable

3. Code Quality Two important concepts for quality of code: Coupling – as little as possible please Coupling – as little as possible please Cohesion – as much as possible please Cohesion – as much as possible please

4. Coupling Coupling refers to links between separate units of a program.Coupling refers to links between separate units of a program. If two classes depend closely on many details of each other, we say they are tightly coupled.If two classes depend closely on many details of each other, we say they are tightly coupled. We aim for loose coupling.We aim for loose coupling.

5. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Loose Coupling Loose coupling makes it possible to: Loose coupling makes it possible to: understand how a class works without having to understand how the classes it uses work;understand how a class works without having to understand how the classes it uses work; change the implementation of a class without affecting those classes that use it.change the implementation of a class without affecting those classes that use it. This improves maintainability … makes change easier.

6. Cohesion Cohesion relates to: the the number and diversity of tasks for which a single unit is responsible. If a programming unit is responsible for one logical task, we say it has high cohesion. Cohesion applies both at the level of classes and of methods. We aim for high cohesion.

7. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling High Cohesion High cohesion makes it easier to: High cohesion makes it easier to: understand what a class or method does (because it only does one thing);understand what a class or method does (because it only does one thing); Choose and use descriptive names;Choose and use descriptive names; reuse classes or methods.reuse classes or methods. This improves usability … and makes change easier.

8. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Cohesion of Classes Classes should represent just one, and only one, well defined entity. Cohesion of Methods A method should be responsible for one, and only one, well defined task.

9. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Code Duplication This is an indicator of bad design. It makes maintenance harder. It can lead to introduction of errors, especially during maintenance.

10. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling World of Zuul Chapter 7: zuul-bad

11. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Chapter 7: zuul-bad Problems Fields in Room are all public. Fields in Room are all public. The fact that a Room has four exits is burnt into the interface of its setExits method. The fact that a Room has four exits is burnt into the interface of its setExits method. The Game class references Room fields. The Game class references Room fields. So, the implementation of the Game and Room classes are coupled. A change in one (e.g. adding more exits to a Room ) means changing the other (e.g. the createRooms, printWelcome and goRoom methods of Game ). So, the implementation of the Game and Room classes are coupled. A change in one (e.g. adding more exits to a Room ) means changing the other (e.g. the createRooms, printWelcome and goRoom methods of Game ). This is BAD! This is BAD!

12. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Chapter 7: zuul-bad Problems There are duplicated code fragments in the Game class (in createRooms and printWelcome methods). There are duplicated code fragments in the Game class (in createRooms and printWelcome methods). Changing the details of one of these fragments means changing all. Changing the details of one of these fragments means changing all. This is BAD! This is BAD! Factorise the duplicated code into a single method (see Code 7.2, p209, of the book: printLocationInfo ) and duplicate its invocation instead. Factorise the duplicated code into a single method (see Code 7.2, p209, of the book: printLocationInfo ) and duplicate its invocation instead. Now, there is only one fragment of code to maintain. This is Good! Now, there is only one fragment of code to maintain. This is Good!

13. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Chapter 7: zuul-bad There are duplicated code fragments in the Game class (in createRooms and printWelcome methods). There are duplicated code fragments in the Game class (in createRooms and printWelcome methods). Changing the details of one of these fragments means changing all. Changing the details of one of these fragments means changing all. This is BAD! This is BAD! Factorise the duplicated code into a single method (see Code 7.2, p209, of the book: printLocationInfo ) and duplicate its invocation instead. Factorise the duplicated code into a single method (see Code 7.2, p209, of the book: printLocationInfo ) and duplicate its invocation instead. Now, there is only one fragment of code to maintain. This is Good! Now, there is only one fragment of code to maintain. This is Good!  Chapter 7: zuul-better

14. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Chapter 7: zuul-bad  Make fields in Room private and provide an accessor method to get them (Code 7.4, p213). Make fields in Room private and provide an accessor method to get them (Code 7.4, p213). In Game, can no longer reference directly the exit fields (all Room s) of Room. In Game, can no longer reference directly the exit fields (all Room s) of Room. In Game. printLocationInfo (which does so reference), we could just change it to use the accessor method, rather than the field. In Game. printLocationInfo (which does so reference), we could just change it to use the accessor method, rather than the field. However, it’s neater to invoke a (new) method of Room to get the exit information it needs to print (Code 7.6, p218). Each Room object has that information, so it is really its responsibility. However, it’s neater to invoke a (new) method of Room to get the exit information it needs to print (Code 7.6, p218). Each Room object has that information, so it is really its responsibility. For Game. goRoom (which also does so reference), thirteen lines of code collapse to one (see p214). For Game. goRoom (which also does so reference), thirteen lines of code collapse to one (see p214). Chapter 7: zuul-better

15. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Chapter 7: zuul-bad  Make fields in Room private and provide an accessor method to get them (Code 7.4, p213). Make fields in Room private and provide an accessor method to get them (Code 7.4, p213). In Game, can no longer reference directly the exit fields (all Room s) of Room. In Game, can no longer reference directly the exit fields (all Room s) of Room. In Game. printLocationInfo (which does so reference), we could just change it to use the accessor method, rather than the field. In Game. printLocationInfo (which does so reference), we could just change it to use the accessor method, rather than the field. However, it’s neater to invoke a (new) method of Room to get the exit information it needs to print (Code 7.6, p218). Each Room object has that information, so it is really its responsibility. However, it’s neater to invoke a (new) method of Room to get the exit information it needs to print (Code 7.6, p218). Each Room object has that information, so it is really its responsibility. For Game. goRoom (which also does so reference), thirteen lines of code collapse to one (see p214). For Game. goRoom (which also does so reference), thirteen lines of code collapse to one (see p214). Chapter 7: zuul-better Look carefully at this accessor method … Look carefully at this accessor method …

16. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling public Room getExit (String direction) { if (direction.equals ("north")) { return northExit; } if (direction.equals ("east")) { return eastExit; } if (direction.equals ("south")) { return southExit; } if (direction.equals ("west")) { return westExit; } if (direction.equals ("south")) { return southExit; } if (direction.equals ("west")) { return westExit; } return null; return null;} public Room getExit (String direction) { if (direction.equals ("north")) { return northExit; } if (direction.equals ("east")) { return eastExit; } if (direction.equals ("south")) { return southExit; } if (direction.equals ("west")) { return westExit; } if (direction.equals ("south")) { return southExit; } if (direction.equals ("west")) { return westExit; } return null; return null;} Chapter 7: zuul-bad Chapter 7: zuul-better 

17. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Chapter 7: zuul-bad  Make fields in Room private and provide an accessor method to get them (Code 7.4, p213). Make fields in Room private and provide an accessor method to get them (Code 7.4, p213). In Game, can no longer reference directly the exit fields (all Room s) of Room. In Game, can no longer reference directly the exit fields (all Room s) of Room. In Game. printLocationInfo (which does so reference), we could just change it to use the accessor method, rather than the field. In Game. printLocationInfo (which does so reference), we could just change it to use the accessor method, rather than the field. However, it’s neater to invoke a (new) method of Room to get the exit information it needs to print (Code 7.6, p218). Each Room object has that information, so it is really its responsibility. However, it’s neater to invoke a (new) method of Room to get the exit information it needs to print (Code 7.6, p218). Each Room object has that information, so it is really its responsibility. For Game. goRoom (which also does so reference), thirteen lines of code collapse to one (see p214). For Game. goRoom (which also does so reference), thirteen lines of code collapse to one (see p214). Chapter 7: zuul-better It maps a String (direction) into an exit Room. It maps a String (direction) into an exit Room. But that’s just the function of a: But that’s just the function of a: So, replace the four exit fields with one of the above (private of course) – Code 7.5. Now, we can have as many exits as we like. So, replace the four exit fields with one of the above (private of course) – Code 7.5. Now, we can have as many exits as we like. Finally, replace the setExits method (which burns the message “four exits” into its definition) with a single setExit method (which we can invoke for as many exits as we like) – p216/217. Finally, replace the setExits method (which burns the message “four exits” into its definition) with a single setExit method (which we can invoke for as many exits as we like) – p216/217. Now, Room is independent of its number of exits. Now, Room is independent of its number of exits. HashMap HashMap

18. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling World of Zuul Chapter 7: zuul-bad Chapter 7: zuul-better MORAL: class diagrams are not enough to tell whether the design is good.

19. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Responsibility-Driven Design Where should we add new fields and new methods (and to which class)? Where should we add new fields and new methods (and to which class)? The class that holds the data (fields) processes (methods) the data. The class that holds the data (fields) processes (methods) the data. Responsibility-driven design leads to low coupling. Responsibility-driven design leads to low coupling.

20. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Localising Change One aim of reducing coupling and responsibility-driven design is to localise change. One aim of reducing coupling and responsibility-driven design is to localise change. When a change is needed, as few classes as possible should be affected. When a change is needed, as few classes as possible should be affected. Improve maintainability … make change easier.

21. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Think Ahead When designing a class, think what changes are likely to be made in the future. When designing a class, think what changes are likely to be made in the future. Aim to make those changes easy. Aim to make those changes easy. Improve maintainability … make change easier.

22. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Refactoring When classes are maintained, code usually needs to be added. When classes are maintained, code usually needs to be added. Lists of fields, lists of methods and the code inside methods tend to become longer. Lists of fields, lists of methods and the code inside methods tend to become longer. Every now and then, classes and methods should be refactored to maintain cohesion and low coupling. Every now and then, classes and methods should be refactored to maintain cohesion and low coupling.

23. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Refactoring and Testing When maintaining code, separate the refactoring from making other changes. When maintaining code, separate the refactoring from making other changes. Do the refactoring first, without adding to the functionality. Do the refactoring first, without adding to the functionality. Test before and after refactoring to ensure that nothing gets broken. Test before and after refactoring to ensure that nothing gets broken. e.g. zuul-bad to zuul-better.

24. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Design Questions Common questions: Common questions: How long should a class be?How long should a class be? How long should a method be?How long should a method be? Answered with regard to cohesion and coupling.

25. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Design Guidelines A method is too long if it does more then one logical task. A class is too complex if it represents more than one logical entity. Note: these are guidelines … everything is still open to the designer.

26. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Review Programs are continuously changed. Programs are continuously changed. It is important to anticipate change and make it as easy as possible. It is important to anticipate change and make it as easy as possible. Quality of code requires much more than just performing correctly! Quality of code requires much more than just performing correctly! Code must be understandable and maintainable. Code must be understandable and maintainable.

27. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Good quality code avoids duplication, displays high cohesion, low coupling. Good quality code avoids duplication, displays high cohesion, low coupling. Coding style (commenting, naming, layout, etc.) is also very important. Coding style (commenting, naming, layout, etc.) is also very important. There is a big difference in the amount of work required to maintain poorly structured and well structured code. There is a big difference in the amount of work required to maintain poorly structured and well structured code. In fact, unless it is well structured, the code is doomed … it will not be used for long. In fact, unless it is well structured, the code is doomed … it will not be used for long. Review