CS Programming I: Inheritance

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1 CS Programming I: Inheritance 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 Inheritance

3 Derived Classes public class Goalie extends HockeyPlayer { private int save ; private int shots ; public Goalie ( String name ){ super ( name ); public void addshot (){ shots ++; public void addsave (){ save ++; extend Keyword Used to create a class based on another class. A class can only extend one class. public double getsavepct (){ return (( double ) save )/ shots ; 1/11

4 Derived Classes public class Goalie extends HockeyPlayer { private int save ; private int shots ; public Goalie ( String name ){ super ( name ); public void addshot (){ Subclass shots ++; The class that extends another class. extend Keyword Used to create a class based on another class. A class can only extend one class. public Alsovoid called: addsave derived (){ class, extended class, or child class. save ++; public double getsavepct (){ return (( double ) save )/ shots ; 1/11

5 Derived Classes public class Goalie extends HockeyPlayer { private int save ; private int shots ; public Goalie ( String name ){ super ( name ); extend Keyword Used to create a class based on another class. A class can only extend one class. public void addshot (){ Subclass shots ++; The class that extends another class. public Alsovoid called: addsave derived (){ class, extended class, or child class. save ++; Superclass public The double class that getsavepct is extended. (){ Also return called: (( double base class, ) save or )/ parent shots ; class. 1/11

6 Derived Classes public class Goalie extends HockeyPlayer { private int save ; private int shots ; public Goalie ( String name ){ super ( name ); public void addshot (){ shots ++; public void addsave (){ save ++; extend Keyword Used to create a class based on another class. A class can only extend one class. public double getsavepct (){ return (( double ) save )/ shots ; 1/11

7 2/11 What is super? super Keyword First line in constructor to specify the superclass constructor to use. Also can be used called superclass overridden methods.

8 TopHat Question 1 What happens when this constructor is called? public class Goalie extends HockeyPlayer { private int save ; private int shots ; public Goalie ( String name ){ public void addshot (){ shots ++; public void addsave (){ save ++; public double getsavepct (){ return (( double ) save )/ shots ; 3/11

9 4/11 What is super? super Keyword First line in constructor to specify the superclass constructor to use. Also can be used called superclass overridden methods. Constructor Chaining 1 If there is no explicit call to the superclass constructor in subclass constructor, then super() is called implicitly. 2 If super() doesn t exist and no explicit call to the superclass constructor, then compilation error.

10 5/11 More on Access Modifiers Access Modifiers public members are accessible outside the class. private members are only accessible inside the class.

11 5/11 More on Access Modifiers Access Modifiers public members are accessible outside the class. private members are only accessible inside the class. Packages Classes can be bundled into packages. Grouping of related types. First line in every file: package somepackage;

12 5/11 More on Access Modifiers Access Modifiers public members are accessible outside the class. private members are only accessible inside the class. default (no access modifier specified; also called package access modifier) members are accessible to all classes in the package. Packages Classes can be bundled into packages. Grouping of related types. First line in every file: package somepackage;

13 5/11 More on Access Modifiers Access Modifiers public members are accessible outside the class. private members are only accessible inside the class. default (no access modifier specified; also called package access modifier) members are accessible to all classes in the package. protected members have the same accessibility as the default plus accessible to subclasses. Packages Classes can be bundled into packages. Grouping of related types. First line in every file: package somepackage;

14 6/11 Overriding Methods Overriding Subclass method that overrides a superclass method must have: the same name, the same parameters, the same return type (or subtype), have a no more restrictive access level.

15 6/11 Overriding Methods Overriding Subclass method that overrides a superclass method must have: the same name, the same parameters, the same return type (or subtype), have a no more restrictive access level. Overriding vs overloading Key difference: overriding has the same parameters whereas over overloading has different parameters.

16 6/11 Overriding Methods Overriding Subclass method that overrides a superclass method must have: the same name, the same parameters, the same return type (or subtype), have a no more restrictive access level. Overriding vs overloading Key difference: overriding has the same parameters whereas over overloading has different annotation indicates to the compiler that a method is suppose to be overriding a superclass method.

17 7/11 Polymorphism TopHat Question 2 What is the output? ArrayList < Object > arrl = new ArrayList < Object >(); arrl. add (" foo "); arrl. add (1); arrl. add (2.5); System. out. print ( arrl );

18 7/11 Polymorphism TopHat Question 2 What is the output? ArrayList < Object > arrl = new ArrayList < Object >(); arrl. add (" foo "); arrl. add (1); arrl. add (2.5); System. out. print ( arrl ); Virtual method invocation The JVM determines the method to call based on the actual object and not the reference type variable.

19 public String speak () { return " GLUB!"; 8/11 TopHat Question 3 What is the output? public class UseAnimal { public static void main ( String [] main ) { Animal a = new Fish (); System. out. print (a. speak ()); class Animal { public String speak () { return " ---"; class Fish extends Animal {

20 9/11 Is-a Relationship Object HockeyPlayer instanceof Operator someobj instanceof someclass true if someobj is the same class or a subclass of someclass. name: String Goalie

21 9/11 Is-a Relationship Object HockeyPlayer name: String Goalie instanceof Operator someobj instanceof someclass true if someobj is the same class or a subclass of someclass. Is-a vs Has-a Is-a: inheritance Goalie is a HockeyPlayer Has-a: contains HockeyPlayer has a String

22 10/11 TopHat Question 4 What is the output? Object g = new Goalie (" Brodeur "); System. out. print ( g instanceof Goalie ); System. out. print (" "); System. out. print ( g instanceof HockeyPlayer ); System. out. print (" "); System. out. print ( g instanceof Object ); System. out. print (" "); System. out. print ( null instanceof Goalie );

23 11/11 Further Reading COMP SCI 200: Programming I zybooks.com, zybook code: WISCCOMPSCI200Fall2017 Chapter 12. Creating Classes Chapter 13. More Classes

24 Appendix References Appendix

25 Appendix References References

26 Appendix References 12/11 Image Sources I

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