Object-Oriented Programming Classes, Objects, Variables

Transcription

1 Object-Oriented Programming Classes, Objects, Variables Ewan Klein School of Informatics Inf1 :: 2009/10 Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 1 / 37

2 1 Admin 2 State and Objects 3 Objects in Java 4 Methods 5 Coding Standards and Documentation 6 Finally Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 2 / 37

3 Admin Stuff Tutorials start tomorrow sheets will be available, but no extra preparation required Advanced (optional) exercise published this week (link from course web page) Next Week: Assessed assignment; published Mon 1st Feb, due back noon Mon 8th Feb to be submitted using examsubmit (like submit but checks correctness of file names) Approx 3 4 exercises Worth 5% of total marks for course Scheduled Labs: mainly Eclipse Tutorials: OO stuff Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 3 / 37

4 Google Group 48 members when I counted this morning Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 4 / 37

5 Motivation for Object Oriented Approach One way of achieving modularization decoupling behaviour of different program components parallel, independent code development re-usability of components Useful when a group of functions need to share information Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 5 / 37

6 In a State, 1 Real world objects have properties which change different states Weather in Edinburgh Monday temperature = 7 o C Tuesday sky = cloudy wind = 15 mph temperature = 5 o C sky = drizzle wind = 1 mph Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 6 / 37

7 In a State, 2 Dogs Max Brindle coatcolour = black coatlength = short thirsty = true coatcolour = cream coatlength = shaggy thirsty = false Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 7 / 37

8 Changing State Making Carrot Soup Carrots at time t 0 colour = orange texture = hard shape = whole Carrots at time t 1 colour = orange texture = hard shape = diced Carrots at time t 2 colour = orange texture = soft shape = mushy Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 8 / 37

9 Max is a Labrador Max is a black Labrador Max is an object Max belongs to the class of dogs (is an instance of the class) The breed black Labrador is a subclass of the class of dogs Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 9 / 37

10 Subclasses and Instances Dog superclass Black Lab Tibetan Terrier subclasses Max instance of a class Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 10 / 37

11 Behaviour Dogs Max runs barks drinks Max does these things by virtue of being a dog Running, barking and drinking are dog behaviours Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 11 / 37

12 Objects in Java Instance variables hold the state of the object Methods specify the possible behaviours of the object An object inherits attributes and methods from its superclass Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 12 / 37

13 Class Diagram inherited Dog size name coatcolour isthirsty run() bark() drink() subclass instance variables methods Black Labrador swim() extending behaviour Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 13 / 37

14 Methods Methods are comparable to functions in Haskell Often used to change state rather than to return a value Haskell function: type declaration max :: Int -> Int -> Int Java method: type declaration int max(int x, int y) Java method: example with no return value void setname(string name) Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 14 / 37

15 Creating and Using Objects Create a new Dog object Dog mydog = new Dog(); 1 new Dog() create space in memory for the object 2 Dog mydog declare a variable of type Dog Dog is a reference / nonprimitive type; mydog is a reference variable 3 Dog mydog = new Dog() assign the new Dog to the reference variable The Dot Operator mydogname = Brindle ; mydogisthirsty; mydogrun(); Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 15 / 37

16 Deja vu We ve seen something similar in the lab exercises Scanner Scanner input = new Scanner(); String name = inputnext(); Random Random generator = new Random(); int num = generatornextint(mysize) Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 16 / 37

17 Java Class Example The Dog Class public class Dog { String name; String coatcolour; boolean isthirsty; public void run() { Systemoutprintln( Running ); isthirsty = true; public void drink() { Systemoutprintln( Drinking ); isthirsty = false; Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 17 / 37

18 Launch the Class, 1 Changing the Dog s state public class DogLauncher { public static void main(string[] args) { Dog d = new Dog(); disthirsty = false; Systemoutprintln( Thirsty? + disthirsty); drun(); Systemoutprintln( Thirsty? + disthirsty); ddrink(); Systemoutprintln( Thirsty? + disthirsty); Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 18 / 37

19 Launch the Class, 2 Console Output: Dog behaviour Thirsty? false Running Thirsty? true Drinking Thirsty? false Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 19 / 37

20 Arrays and Objects Create a Dog array Dog[] mydogs = new Dog[7]; Elements? fido = mydogs[0]; // doesn t give us a dog Create Dog objects as elements mydogs[0] = new Dog(); // mydogs[0] is a reference variable mydogs[1] = new Dog(); Do something with a Dog mydogs[0]run(); Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 20 / 37

21 Terminology Parameters Arguments Signature Return value Local variables vs instance variables Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 21 / 37

22 Scenario Assume we are using the class Reviewer to model someone who assigns ratings (1 to 5 stars) to movies One task is to assign a list of movies to a reviewer We can model the list as an array Movie[] Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 22 / 37

23 Reviewer Example, 1 Variables public class Reviewer { Movie[] movies; //instance variable // newmovies is a parameter public void setmovies(movie[] newmovies) { //assign the value of a parameter to instance variable movies = newmovies; Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 23 / 37

24 Reviewer Example, 2 Alternative Naming Convention public class Reviewer { Movie[] movies; // instance variable // movies is a parameter public void setmovies(movie[] movies ) { // this is the name of the current class thismovies = movies; Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 24 / 37

25 ReviewerLauncher Arguments public class ReviewerLauncher { public static void main(string[] args) { Movie[] movies = new Movie[5]; movies[0] = new Movie(); /* Set the movie titles * The Matrix is the argument to method call */ movies[0]settitle( The Matrix ); Reviewer r = new Reviewer(); // Tell the Reviewer which movies to rate rsetmovies(movies); \ Pass movies as the arg Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 25 / 37

26 Reviewer Example, 3 Method with Two Parameters public class Reviewer { public void ratemovie( String title, int rating ) { for (int i = 0; i < movieslength; i++) { if (movies[i]gettitle() == title) { movies[i]setrating(rating); Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 26 / 37

27 ReviewerLauncher Reviewer r = new Reviewer(); 1: Correct Call rratemovie( The Matrix, 3); 2: Incorrect Call rratemovie(5, 3); 3: Incorrect Call rratemovie( The Matrix ); 4: Incorrect Call rratemovie(3, The Matrix ); Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 27 / 37

28 Signature The signature of a method consists of its name and its parameter list Example signature ratemovie(string title, int rating) Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 28 / 37

29 Reviewer Example, 4 Local Variables public class Reviewer { public String favourite() { int max = 0; /* Local variables int index = 0; * must be initialized! */ for (int i = 0; i < movieslength; i++) { if (movies[i]getrating() > max) { max = movies[i]getrating(); index = i; return movies[index]gettitle(); Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 29 / 37

30 Return Return type of a method is stated in the method declaration A method declared void doesn t return a value Any method with a non-void return type rtype must contain a statement of the form return returnvalue; where the data type of returnvalue matches the type rtype Return value public class Reviewer { public String favourite() { return movies[index]gettitle(); Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 30 / 37

31 Sun Code Conventions ( Motivation for coding standards: 80% of the lifetime cost of a piece of software goes to maintenance Hardly any software is maintained for its whole life by the original author Code conventions improve the readability of the software, allowing engineers to understand new code more quickly and thoroughly Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 31 / 37

32 Comments Implementation: for commenting out code or for comments about the particular implementation Documentation: describe the specification of the code, from an implementation-free perspective Documentation comments (aka doc strings ) can be extracted to HTML files using the javadoc tool Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 32 / 37

33 Comment Syntax: Implementation Single line, 1 if (condition) { /* Handle the condition */ Single line, 2 if (condition) { // Handle the condition // if (foo) { // // Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 33 / 37

34 Comment Syntax: Documentation Block Comment /** * Set the title of this movie */ public void settitle(string title) { // TODO: implement method stub NB starts with /** Occurs just before a declaration / definition Doc comment tags etc) treated specially by javadoc Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 34 / 37

35 Sun Comment Guidelines Give overviews of code, provide additional information not readily available in the code itself Contain only information that is relevant to reading and understanding the program Discuss nontrivial or nonobvious design decisions Avoid duplicating information that is present in (and clear from) the code Avoid any comments that are likely to get out of date as the code evolves Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 35 / 37

36 Summary OOP helps you to program in a modular way All Java code is located in a class Objects are instances of a class Objects have state, specified by instance variables Two kinds of variable: primitive and reference NB instance variable vs local variable is more about use; can be both primitive and reference A reference variable contains information about how to get to an object, not the object itself Objects have behaviour, specified by methods Use the dot operator to call a method on a reference variable (eg, dogbark()) Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 36 / 37

37 Reading Chapters 2, 4 of HFJ Ewan Klein (School of Informatics) Object-Oriented ProgrammingClasses, Objects, Variables Inf1 :: 2009/10 37 / 37