Introduc)on to Classes and Objects. Dr. Bowen Hui Computer Science University of Bri)sh Columbia Okanagan

Transcription

1 Introduc)on to Classes and Objects Dr. Bowen Hui Computer Science University of Bri)sh Columbia Okanagan

2 Previously Java constructs: Variables (int, double, String) Methods Condi)onal (if, if-else) Java library: Scanner Random Basic programs: Calculator Guess number game 2

3 More Interes)ng Programs Programs about Places People Pets? Not many things are int, double, String! Maybe composite of basic Java types? 3

4 More Interes)ng Programs Programs about Places People Pets? Not many things are int, double, String! Maybe composite of basic Java types? Need: bigger objects with interac)on abili)es 4

5 Example Abili)es Objects reac)ng to environment

6 Example Abili)es Objects reac)ng to user feedback 6

7 Example Abili)es Objects reac)ng to other objects Share Cookie 7

8 Today s Objec)ves 1. To be able to specify a generic object 2. To create objects 3. To have object remember informa)on 4. To have object react to environment/user 8

9 1. Specify Generic Object How? Blueprint (class) A generic descrip)on of those types of objects What goes into a blueprint? Traits (instance data) Things we want to keep track of about these objects Behaviours (methods) Things that these objects can do 9

10 1. Specify Generic Object How? Blueprint (class) A generic descrip)on of those types of objects What goes into a blueprint? Traits (instance data) Things we want to keep track of about these objects Behaviours (methods) Things that these objects can do 10

11 Example What is the blueprint for a dog? Example traits? Name, size, colour Hunger level, happiness Example behaviours? Eat, sleep, play, bark Shake a paw, roll over, sit 11

12 Example What is the blueprint for a dog? Example traits? Name, size, colour Hunger level, happiness Example behaviours? Eat, sleep, play, bark Shake a paw, roll over, sit 12

13 Example What is the blueprint for a dog? Example traits? Name, size, colour Hunger level, happiness Example behaviours? Eat, sleep, play, bark Shake a paw, roll over, sit 13

14 Defining Traits Type of informa)on Number? Phrase? Truth value? Other? may create your own later Declare as variables inside your class 14

15 Defining Behaviours Special method: constructor Main purpose: ini)alize traits Dog( String petname ) name = petname; no return type stomach = 0; isfull = false; // starts off empty Other behaviours: define as regular methods Example: void eatsnacks( int cookies ) stomach = stomach + cookies; if( stomach > 5 ) isfull = true; 15

16 Defining Behaviours Special method: constructor Main purpose: ini)alize traits Dog( String petname ) name = petname; no return type stomach = 0; isfull = false; // starts off empty Other behaviours: define as regular methods Example: void eatsnacks( int cookies ) stomach = stomach + cookies; if( stomach > 5 ) isfull = true; 16

17 Visual Representa)on Dog class: Data declarations Dog() bark() Method declarations eatsnacks() 17

18 Transla)ng into Java Generic dog Traits Behaviours Braces have to match Steps: 1. Create class template 2. Declare instance data 3. Define methods Capital lefer class ClassName // instance data // methods 18

19 Create Class Template class Dog // instance data // methods Dog() bark() eatsnacks() 19

20 Declare Instance Data class Dog // instance data Dog() // methods bark() eatsnacks() 20

21 Declare Instance Data class Dog // instance data Dog() // methods bark() Scope for instance data eatsnacks() 21

22 Define Methods class Dog // instance data // methods Dog( String petname ) name = petname; stomach = 0; isfull = false; Constructor must have the same name as the class Dog() bark() eatsnacks() 22

23 Define Methods class Dog // instance data // methods Dog( String petname ) name = petname; stomach = 0; isfull = false; How come the constructor can use name, stomach, isfull? Dog() bark() eatsnacks() 23

24 Define Methods class Dog // instance data // methods Dog( String petname ) name = petname; stomach = 0; isfull = false; Note: any method in the class can reference the class s instance data Dog() bark() eatsnacks() 24

25 Define Methods class Dog // instance data // methods Dog( String petname ) name = petname; stomach = 0; isfull = false; // next column // continue void bark() System.out.println( woof ); Dog() bark() eatsnacks()

26 Define Methods class Dog // instance data // continue void bark() System.out.println( woof ); // methods Dog( String petname ) name = petname; stomach = 0; isfull = false; // next column void eatsnacks( int cookies ) stomach += cookies; if( stomach > 5 ) isfull = true; Dog class is completed Dog() bark() eatsnacks()

27 Today s Objec)ves 1. To be able to specify a generic object 2. To create objects 3. To have object remember informa)on 4. To have object react to environment/user 27

28 2. Create Objects Purpose of blueprint Describes a generic object of that class Need to create object Use reserved keyword: new Call constructor to specify traits of individual objects Do this inside main() Note: Behaviours don t change 28

29 Visual Representa)on new instance class template: Dog() bark() eatsnacks() object instance: Dog() bark() eatsnacks() Casper 0 false 29

30 Using new Recall variable defini)ons: int num = 5; Crea)ng new objects require different syntax Changes: Use new Call constructor method with unique info Example: Dog pet = new Dog( Casper ); Calls constructor method with a unique name 30

31 Transla)ng into Java Test class: main() Create unique objects Call its methods class TestClass public static void main() // create object // call its methods 31

32 Transla)ng into Java Test class: main() Create unique objects Call its methods class TestClass public static void main() // create object // call its methods Same as before Refers to another class in another file 32

33 Example Test Class class TestDog public static void main() Dog pet = new Dog( Casper ); pet.eatsnacks( 2 ); Use new 33

34 Example Test Class class TestDog public static void main() Dog pet = new Dog( Casper ); pet.eatsnacks( 2 ); Dog() bark() eatsnacks() Casper 0 false Pass in info unique to object upon crea)on 34

35 Example Test Class class TestDog public static void main() Dog pet = new Dog( Casper ); pet.eatsnacks( 2 ); Invoke object s method using dot operator Dog() bark() eatsnacks() Casper 2 false 35

36 Example Test Class class TestDog public static void main() Dog pet = new Dog( Casper ); pet.eatsnacks( 2 ); pet.eatsnacks( 5 ); Dog() bark() Casper 7 true eatsnacks() 36

37 Example Test Class class TestDog public static void main() Dog petboy = new Dog( Casper ); petboy.eatsnacks( 2 ); petboy.eatsnacks( 5 ); Dog petgirl = new Dog( Bitzy ); petgirl.eatsnacks( 2 ); Create as many Dog objects as needed 37

38 Example Test Class class TestDog public static void main() Dog petboy = new Dog( Casper ); petboy.eatsnacks( 2 ); petboy.eatsnacks( 5 ); Dog petgirl = new Dog( Bitzy ); petgirl.eatsnacks( 2 ); Could I have called both Dog objects with same variable name pet? 38

39 Example: One class template to create five Dog objects 39

40 Today s Objec)ves 1. To be able to specify a generic object 2. To create objects 3. To have object remember informa)on 4. To have object react to environment/user 40

41 3. Object Remembering Info Already know how to define instance data How instance data works: Each object maintains its own variables Gives object a state at any given )me When first created Casper 0 false Dog() bark() eatsnacks() 41

42 3. Object Remembering Info Already know how to define instance data How instance data works: Each object maintains its own variables Gives object a state at any given )me When first created Later Casper 0 false Casper 2 false Dog() Dog() bark() bark() eatsnacks() eatsnacks() 42

43 3. Object Remembering Info Already know how to define instance data How instance data works: Each object maintains its own variables Gives object a state at any given )me When first created Later Much later Casper 0 false Casper 2 false Casper 7 true Dog() Dog() Dog() bark() bark() bark() eatsnacks() eatsnacks() eatsnacks() 43

44 Objects Independence Instance data across objects do not interact When first created Casper 0 false Dog() bark() eatsnacks() Bitzy 0 false Dog() bark() eatsnacks() 44

45 Objects Independence Instance data across objects do not interact When first created Later Casper 0 false Casper 2 false Dog() Dog() bark() bark() eatsnacks() eatsnacks() Bitzy 0 false Bitzy 1 false Dog() Dog() bark() bark() eatsnacks() eatsnacks() 45

46 Objects Independence Instance data across objects do not interact When first created Later Much later Casper 0 false Casper 2 false Casper 7 true Dog() Dog() Dog() bark() bark() bark() eatsnacks() eatsnacks() eatsnacks() Bitzy 0 false Bitzy 1 false Bitzy 2 false Dog() Dog() Dog() bark() bark() bark() eatsnacks() eatsnacks() eatsnacks() 46

47 Today s Objec)ves 1. To be able to specify a generic object 2. To create objects 3. To have object remember informa)on 4. To have object react to outside 47

48 4. Object Reac)ng to Surrounding Already know how to define methods How to get object to be responsive? Change state over )me Change state based on interac)on with user Call different methods as indicated 48

49 Responding to User When called its name, dog barks back How to add this behaviour? New method Call it in test class 49

50 Responding to User class Dog // continue from previous template void respond( String dogname ) if( dogname.equals( name ) ) bark(); else ignoreowner(); 50

51 Responding to User class TestDog Dog casper = new Dog( Casper ); // user interaction Scanner console = new Scanner( System.in ); String petname = console.nextline(); casper.respond( petname ); // try again console = new Scanner( System.in ); petname = console.nextline(); casper.respond( petname ); Respond to user Respond to user 51

52 Responding to User class TestDog Dog casper = new Dog( Casper ); // user interaction Scanner console = new Scanner( System.in ); String petname = console.nextline(); casper.respond( petname ); // try again console = new Scanner( System.in ); petname = console.nextline(); casper.respond( petname ); 52

53 Special Method: Accessor Outside world cannot see object s instance data Solu)on: use accessors to get data value back Also called gefers Any instance data can be retrieved this way Template: returntype getvar() return var; How to write accessor method for name? 53

54 Example: getname() class Dog // instance data // methods continue from before String getname() return name; 54

55 Using getname() class TestDog Dog casper = new Dog( Casper ); String petname = casper.getname(); System.out.println( Just created a dog named + petname ); 55

56 Using getname() class TestDog Dog casper = new Dog( Casper ); String petname = casper.getname(); System.out.println( Just created a dog named + petname ); 56

57 Special Method: Mutator Outside world cannot change object s instance data Solu)on: use mutators to change data value Also called sefers Any instance data can be changed this way Template: void setvar( vartype newvalue ) var = newvalue; How to write mutator method for name? 57

58 Example: setname() class Dog // instance data // methods continue from before void setname( String newname ) name = newname; 58

59 Using setname() class TestDog Dog casper = new Dog( Casper ); String petname = casper.getname(); System.out.println( Just created a dog named + petname ); casper.setname( Big Boy ); petname = casper.getname(); System.out.println( Dog named is now + petname ); 59

60 Using setname() class TestDog Dog casper = new Dog( Casper ); String petname = casper.getname(); System.out.println( Just created a dog named + petname ); casper.setname( Big Boy ); petname = casper.getname(); System.out.println( Dog name is now + petname ); 60

61 Modeling Other Applica)ons How might you define these classes? Houses Students Recall: lab s FeedMe game What are the classes and objects? 61

62 Recall: FeedMe Game What are the classes and objects? 62

63 Modeling Other Applica)ons How might you define these classes? Houses Students Recall: lab s FeedMe game What are the classes and objects? Dog class: one dog object Apple class: five falling apples 63

64 Student s Customized Lab Games 64

65 Student s Customized Lab Games 65

66 Summary A class is a blueprint to describe group of objects Instance data represents traits Methods represents behaviours Special methods: Constructor sets up a new object Accessor retrieves trait s data value Mutator changes trait s data value Individual objects created from a class An object is an instance of a class Use reserved word: new in test class Call object s methods using. dot operator An object s instance data do not interact with another s 66

67 Summary A class is a blueprint to describe group of objects Instance data represents traits Methods represents behaviours Special methods: Constructor sets up a new object Accessor retrieves trait s data value Mutator changes trait s data value Individual objects created from a class An object is an instance of a class Use reserved word: new in test class Call object s methods using. dot operator An object s instance data do not interact with another s 67

68 Summary A class is a blueprint to describe group of objects Instance data represents traits Methods represents behaviours Special methods: Constructor sets up a new object Accessor retrieves trait s data value Mutator changes trait s data value Individual objects created from a class An object is an instance of a class Use reserved word: new in test class Call object s methods using. dot operator An object s instance data do not interact with another s 68

69 Next Class Interac)on among objects Share Cookie 69