CS Programming I: Classes

Transcription

1 CS Programming I: Classes Marc Renault Department of Computer Sciences University of Wisconsin Madison Fall 2017 TopHat Sec 3 (PM) Join Code: TopHat Sec 4 (AM) Join Code:

2 Classes

3 1/23 Class Defining Classes Classes provide a way of defining new types. They are data structures that: can group member variables. can group member methods.

4 1/23 Class Defining Classes Classes provide a way of defining new types. They are data structures that: can group member variables. can group member methods. public members are accessible outside the class. private members are only accessible inside the class.

5 1/23 Class Defining Classes Classes provide a way of defining new types. They are data structures that: can group member variables. can group member methods. public members are accessible outside the class. private members are only accessible inside the class. Creating instances of classes MyClass varname = new MyClass(arg1,...) Creating instances of your own classes work just as the predefined Java classes.

6 2/23 TopHat Question 1 What is the output when executed? public class MyClass { public int a; public MyClass () { a = 5; public static void main ( String [] args ) { MyClass b = new MyClass (); b.a = 10; System. out. print (b.a);

7 3/23 TopHat Question 2 What is the output when executed? public class MyClassBis { private int a; public MyClassBis () { a = 5; public static void main ( String [] args ) { MyClassBis b = new MyClassBis (); b.a = 10; System. out. print (b.a);

8 4/23 TopHat Question 3 What is the output when UseMyClass is executed? public class UseMyClass { public static void main ( String [] args ) { MyClass b = new MyClass (); b. a = 10; System. out. print (b.a); public class MyClass { public int a; public MyClass () { a = 5;

9 5/23 TopHat Question 4 What is the output when UseMyClassBis is executed? public class UseMyClassBis { public static void main ( String [] args ) { MyClassBis b = new MyClassBis (); b. a = 10; System. out. print (b.a); public class MyClassBis { private int a; public MyClassBis () { a = 5;

10 6/23 Class Interface Class Interface Public view of the class. public methods and members that can be accessed from outside.

11 6/23 Class Interface Class Interface Public view of the class. public methods and members that can be accessed from outside. Good Practice: Keep Things private Code smell: indecent exposure. private unless reason to make public. Encapsulation is one of the fundamental principals in object-oriented programming (OOP).

12 7/23 Member Variables public class HockeyPlayer { private String name ; private int goals ; private int assists ; private int gamesplayed ; private double toi ; private double penaltymin ; public HockeyPlayer () { name = ""; initmembervars (); Fields Fields are member variables. Non-static member variables are instance variables. public HockeyPlayer ( String name ) { this. name = name ; initmembervars ();

13 7/23 Member Variables public class HockeyPlayer { private String name ; private int goals ; private int assists ; private int gamesplayed ; private double toi ; private double penaltymin ; public HockeyPlayer () { name = ""; initmembervars (); Fields Fields are member variables. Non-static member variables are instance variables. Properties Fields that can be set by those using the class. public HockeyPlayer ( String name ) { this. name = name ; initmembervars ();

14 Member Variables public class HockeyPlayer { private String name ; private int goals ; private int assists ; private int gamesplayed ; private double toi ; private double penaltymin ; public HockeyPlayer () { name = ""; initmembervars (); Fields Fields are member variables. Non-static member variables are instance variables. Properties Fields that can be set by those using the class. Default Values public HockeyPlayer ( String name ) { this. name = name ; Uninitialized member initmembervars (); variables are given default values. 7/23

15 Member Instance Methods private void initmembervars () { goals = 0; assists = 0; gamesplayed = 0; toi = 0.0; penaltymin = 0.0; Instance Methods Non-static methods whose behaviour depends on the particular object instance. public String getname () { return name ; public void setname ( String name ) { this. name = name ; public int getgoals () { return goals ; 8/23

16 Constructors public class HockeyPlayer { private String name ; private int goals ; private int assists ; private int gamesplayed ; private double toi ; private double penaltymin ; Constructor Special method with no return type. Same name as the class. Called when creating an instance. Can be overloaded. public HockeyPlayer () { name = ""; initmembervars (); public HockeyPlayer ( String name ) { this. name = name ; initmembervars (); private void initmembervars () { 9/23

17 Constructors public class HockeyPlayer { private String name ; private int goals ; private int assists ; private int gamesplayed ; private double toi ; private double penaltymin ; public HockeyPlayer () { name = ""; initmembervars (); public HockeyPlayer ( String name default ) { constructor with this. name = name ; no parameters. initmembervars (); private void initmembervars () { Constructor Special method with no return type. Same name as the class. Called when creating an instance. Can be overloaded. Default Constructor If no constructor is defined, Java provides a It is good practice to define a constructor. 9/23

18 10/23 What is this? public HockeyPlayer ( String name ) { this. name = name ; initmembervars (); public void setname ( String name ) { this. name = name ; this keyword this is a reference to the object (or the current instance). The object is an implicit parameter of all instance methods.

19 10/23 What is this? this keyword this is a reference to the object (or the current instance). The object is an implicit parameter of all instance methods. Can also be used to call other constructors: public HockeyPlayer () { name = ""; goals = 0; assists = 0; gamesplayed = 0; toi = 0.0; penaltymin = 0.0; public HockeyPlayer ( String name ) { this (); this. name = name ;

20 11/23 Mutators, Accessors, and Helpers private void initmembervars () { goals = 0; assists = 0; gamesplayed = 0; toi = 0.0; penaltymin = 0.0; public int getpoints () { return goals + assists ; public void addgoal () { goals ++; Mutators Methods that change the member variables.

21 11/23 Mutators, Accessors, and Helpers private void initmembervars () { goals = 0; assists = 0; gamesplayed = 0; toi = 0.0; penaltymin = 0.0; public int getpoints () { return goals + assists ; public void addgoal () { goals ++; Accessors Methods that access the member variables without changing them. Mutators Methods that change the member variables.

22 11/23 Mutators, Accessors, and Helpers private void initmembervars () { goals = 0; assists = 0; gamesplayed = 0; toi = 0.0; penaltymin = 0.0; public int getpoints () { return goals + assists ; public void addgoal () { goals ++; Helpers private method used within the class. Accessors Methods that access the member variables without changing them. Mutators Methods that change the member variables.

23 12/23 Getters and Setters public String getname () { return name ; Getter Accessor method that retrieves a property. public void setname ( String name ) { this. name = name ;

24 12/23 Getters and Setters public String getname () { return name ; Getter Accessor method that retrieves a property. public void setname ( String name ) { this. name = name ; Setter Mutator method to change a property.

25 Getters and Setters public String getname () { return name ; Getter Accessor method that retrieves a property. public void setname ( String name ) { this. name = name ; Setter Mutator method to change a property. Best Practice Good for properties. Violates OOP encapsulation when used for all fields. Why getter and setter methods are evil https: // why-getter-and-setter-methods-are-evil.html 12/23

26 13/23 TopHat Question 5 What is the output? public class UseHockeyPlayer { public static void main ( String [] args ) { HockeyPlayer h = new HockeyPlayer (" Austin Matthews "); System. out. print (h); a. Austin Matthews b. HockeyPlayer@5e265ba4

27 14/23 TopHat Question 6 What is the output when executed? public class SimpleClass { public int x; public SimpleClass ( int x) { this.x = x; public String tostring () { return "" + x; public static void main ( String [] args ) { SimpleClass a = new SimpleClass (10); SimpleClass b = new SimpleClass (10); b. x = 20; System. out. print (a + " " + b);

28 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack Heap

29 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack Heap class StaticA { public static int x; public int y; public class StaticEx { public static void main ( String [] args ){ y: 0 StaticA s = new StaticA (); s StaticA: x: 0

30 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack Heap class StaticA { public static int x; public int y; public class StaticEx { public static void main ( String [] args ){ y: 0 StaticA s = new StaticA (); s. y = 1; s StaticA: x: 0

31 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack Heap class StaticA { public static int x; public int y; public class StaticEx { public static void main ( String [] args ){ y: 1 StaticA s = new StaticA (); s. y = 1; s StaticA: x: 0

32 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack Heap class StaticA { public static int x; public int y; public class StaticEx { public static void main ( String [] args ){ y: 1 StaticA s = new StaticA (); s. y = 1; s. x = 2; s StaticA: x: 0

33 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack Heap class StaticA { public static int x; public int y; public class StaticEx { public static void main ( String [] args ){ y: 1 StaticA s = new StaticA (); s. y = 1; s. x = 2; s StaticA: x: 2

34 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack t s Heap class StaticA { public static int x; public int y; public class StaticEx { public static void main ( String [] args ){ y: 1 StaticA s = new StaticA (); s. y = 1; s. x = 2; StaticA t y: 0 StaticA: x: 2 = new StaticA ();

35 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack t s Heap class StaticA { public static int x; public int y; public class StaticEx { public static void main ( String [] args ){ y: 1 StaticA s = new StaticA (); s. y = 1; s. x = 2; y: 0 StaticA: x: 2 StaticA t = new StaticA (); t. x = 3;

36 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack t s Heap class StaticA { public static int x; public int y; public class StaticEx { public static void main ( String [] args ){ y: 1 StaticA s = new StaticA (); s. y = 1; s. x = 2; y: 0 StaticA: x: 3 StaticA t = new StaticA (); t. x = 3;

37 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack t s Heap class StaticA { public static int x; public int y; public class StaticEx { public static void main ( String [] args ){ y: 1 StaticA s = new StaticA (); s. y = 1; s. x = 2; y: 0 StaticA: x: 3 StaticA t = new StaticA (); t. x = 3; t. y = 4;

38 15/23 Static Member Variables Static Member Variables Class variables Stored in a special part of the heap. Stack t s Heap class StaticA { public static int x; public int y; public class StaticEx { public static void main ( String [] args ){ y: 1 StaticA s = new StaticA (); s. y = 1; s. x = 2; y: 4 StaticA: x: 3 StaticA t = new StaticA (); t. x = 3; t. y = 4;

39 16/23 TopHat Question 7 What is the output when executed? public class SimpleStaticClass { public static int x = 10; public String tostring () { return "" + x; public static void main ( String [] args ) { SimpleStaticClass a = new SimpleStaticClass (); SimpleStaticClass b = new SimpleStaticClass (); b. x = 20; System. out. print (a + " " + b);

40 17/23 TopHat Question 8 What is the output? public class StaticEx2 { private static int i = 0; public static int f() { return ++i; public static void main ( String [] args ) { System. out. print (f() + " " + f() + " " + f ());

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42 18/23 Unified Modeling Language () A modeling language used in software engineering. A visual way to describe a software system.

43 18/23 Unified Modeling Language () A modeling language used in software engineering. A visual way to describe a software system. Some Diagrams Use Case Diagram Class Diagram Instance Diagram

44 18/23 Unified Modeling Language () A modeling language used in software engineering. A visual way to describe a software system. Some Diagrams Use Case Diagram Class Diagram Instance Diagram Resources Online Drawing Tool:

45 19/23 Use Case Diagram BP1 Mine Sweeper BP1 Mine Sweeper Configure size of the map Choose locations to sweep Player Choose to play again Use Case Diagram Simple graphical representation of the system. Describes how the external actors (users) interact with the system.

46 Class Diagram HockeyPlayer name: String goals: int assists: int addgoal() setname(name: String) saves: int shots: int Goalie getsavepct(): double plays for HockeyTeam * plays for 1..* 0..1 belongs to * HockeyLeague Class Diagram Describes the structure of the system at the class level. 20/23

47 Instance Diagram HockeyLeague name = NHL HockeyTeam name = Maple Leafs location = Toronto HockeyTeam wild name = Wild location = Minnesota HockeyTeam hawks name = Blackhawks location = Chicago HockeyPlayer name = Austin Matthews Goals = 13 Assists = 12 Goalie name = Frederik Andersen saves = 681 shots = 741 Object / Instance Diagram Describes the state of the system at the object level at a given instance in time. 21/23

48 22/23 Exercise Draw the use-case and class diagrams for the following Used Car Sale system. Vehicles are identified by a VIN, colour, engine type, gas type, and transmission type. Each vehicle has certificate recording the number of kilometres, date of last inspection, number of accidents. Vehicles are sub-divided into: cars, noting the number of passengers trucks, noting the weight class vans, noting the number of passenger and the weight class Clients are registered, indicating their name, address and phone number. Clients can search for and buy the used vehicles. Clients sign purchase contracts indicating the purchase date, sale price, number of payments.

49 23/23 Further Reading COMP SCI 200: Programming I zybooks.com, zybook code: WISCCOMPSCI200Fall2017 Chapter 12. Creating Classes Chapter 13. More Classes

50 Appendix References Appendix

51 Appendix References References

52 Appendix References 24/23 Image Sources I