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

2. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Main concepts to be covered Responsibility-driven design Responsibility-driven design Coupling Coupling Cohesion Cohesion Refactoring Refactoring Enumerations Enumerations 2

3. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Software changes Software is not like a novel that is written once and then remains unchanged. Software is not like a novel that is written once and then remains unchanged. Software is extended, corrected, maintained, ported, adapted … Software is extended, corrected, maintained, ported, adapted … The work is done by different people over time (often decades). The work is done by different people over time (often decades). 3

4. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Change or die There are only two options for software: There are only two options for software: Either it is continuously maintained...Either it is continuously maintained... Or it diesOr it dies Software that cannot be maintained will be thrown away. Software that cannot be maintained will be thrown away. 4

5. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling World of Zuul 5

6. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling An Example Add two new directions to the World of Zuul: Add two new directions to the World of Zuul: upup downdown What do you need to change to do this? What do you need to change to do this? 6

7. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Code Quality Two important concepts for quality of code: 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 7

8. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling 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. 8

9. 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 to use a class without having to understand how that class works;understand how to use a class without having to understand how that class works; 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. 9

10. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Cohesion Cohesion relates to: the the number and diversity of tasks for which a single unit is responsible. 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. 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. Cohesion applies both at the level of classes and of methods. We aim for high cohesion. We aim for high cohesion. 10

11. 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. 11

12. 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. 12

13. 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. 13

14. 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. 14

15. 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. 15

16. 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. 16

17. 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. 17

18. 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. 18

19. 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. 19

20. 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 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. A class is too complex if it represents more than one logical entity. Note: these are guidelines … everything is still open to the designer. 20

21. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Enumerated Types A new language feature.A new language feature. Uses enum instead of class to introduce a new type name and a set of named constants of that type.Uses enum instead of class to introduce a new type name and a set of named constants of that type. This is a better (safer) alternative to a set of named static int constants.This is a better (safer) alternative to a set of named static int constants. For the World of Zuul, we shall use names for command words, rather than string literals (which will appear only in one place).For the World of Zuul, we shall use names for command words, rather than string literals (which will appear only in one place). 21 java.sun.com/docs/books/tutorial/java/javaOO/enum.html

22. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling A basic enumerated type public enum CommandWord { // A value for each command word, // A value for each command word, // plus one for unrecognised commands. // plus one for unrecognised commands. GO, QUIT, HELP, UNKNOWN GO, QUIT, HELP, UNKNOWN} Enumerated objects are constructed implicitly. 22 Each name represents an object of the enumerated type, e.g. CommandWord.HELP. zuul-with-enum-v1

23. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Equivalent class type public class CommandWord {} 23 The constructor is declared private so that no CommandWord objects, other than the ones declared here, can be constructed. zuul-without-enum-v1 public class CommandWord { // A object for each command word, // A object for each command word, // plus one for unrecognised commands. // plus one for unrecognised commands. public final static Commandword GO = new CommandWord (); public final static Commandword GO = new CommandWord (); public final static Commandword QUIT = new CommandWord (); public final static Commandword QUIT = new CommandWord (); public final static Commandword HELP = new CommandWord (); public final static Commandword HELP = new CommandWord (); public final static Commandword UNKNOWN = new CommandWord (); public final static Commandword UNKNOWN = new CommandWord ();} public class CommandWord { private CommandWord () { // nothing to do } // nothing to do }}

24. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling public enum CommandWord { // A value for each command word, // A value for each command word, // plus one for unrecognised commands. // plus one for unrecognised commands. GO("go"), QUIT("quit"), HELP("help"), UNKNOWN("unknown"); } GO("go"), QUIT("quit"), HELP("help"), UNKNOWN("unknown"); } Another enumerated type public enum CommandWord { private String commandString; private CommandWord (String commandString) { this.commandString = commandString; } private String commandString; private CommandWord (String commandString) { this.commandString = commandString; }} 24 zuul-with-enum-v2 public enum CommandWord { public String toString () { return commandString; } }

25. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling public class CommandWord {} { // A object for each command word, // A object for each command word, // plus one for unrecognised commands. // plus one for unrecognised commands. public final static Commandword GO = new CommandWord ("go"); public final static Commandword GO = new CommandWord ("go"); public final static Commandword QUIT = new CommandWord ("quit"); public final static Commandword QUIT = new CommandWord ("quit"); public final static Commandword HELP = new CommandWord ("help"); public final static Commandword HELP = new CommandWord ("help"); public final static Commandword UNKNOWN = new CommandWord ("?"); public final static Commandword UNKNOWN = new CommandWord ("?"); } public class CommandWord {... private final field (String)... private constructor (to construct above constants)... private final field (String)... private constructor (to construct above constants)} Equivalent class type 25 public class CommandWord {... public toString (returns String field)... public toString (returns String field)} public class CommandWord {... public static values (returns array of above constants)... public static values (returns array of above constants)} zuul-without-enum-v2 Automatically provided by an enum

26. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling public class CommandWord { // A object for each command word, // plus one for unrecognised commands. // plus one for unrecognised commands. public final static Commandword GO = new CommandWord ("go"); public final static Commandword GO = new CommandWord ("go"); public final static Commandword QUIT = new CommandWord ("quit"); public final static Commandword QUIT = new CommandWord ("quit"); public final static Commandword HELP = new CommandWord ("help"); public final static Commandword HELP = new CommandWord ("help"); public final static Commandword UNKNOWN = new CommandWord ("?"); private final String commandString; private CommandWord (String commandString) { this.commandString = commandString; } public final static Commandword UNKNOWN = new CommandWord ("?"); private final String commandString; private CommandWord (String commandString) { this.commandString = commandString; } Equivalent class type 26 zuul-without-enum-v2

27. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling public class CommandWord {... public final statics (GO, QUIT, HELP, UNKNOWN) private final String commandString; private CommandWord (String commandString) { this.commandString = commandString; } }... public final statics (GO, QUIT, HELP, UNKNOWN) private final String commandString; private CommandWord (String commandString) { this.commandString = commandString; } } Equivalent class type 27 zuul-without-enum-v2 public class CommandWord { public String toString () { return commandString; } } public class CommandWord { public static CommandWord[] values () { return new CommandWord[] {GO, QUIT, HELP, UNKNOWN}; } } Automatically provided by an enum

28. Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling See also … 28 zuul-with-enum-v1 zuul-with-enum-v2 zuul-without-enum-v1 zuul-without-enum-v2 zuul-without-enum-v3 zuul-without-enum-v4 projects\chapter07\projects\chapter07\courses\co882\projects-phwcourses\co882\projects-phw zuul-better

29. 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. 29

30. 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 30