1. Copyright © 1999-2015 Curt Hill Inheritance and Polymorphism A Powerful Technique

2. Copyright © 1999-2015 Curt Hill Biology Example Class is a loan word from biology –Means group of related orders, families and species Animals (a kingdom) –Vertebrates (a phylum) –Mammals (a class) –Rodents (an order) –Squirrels (a family) –Thirteen line ground squirrel (a specie)

3. Copyright © 1999-2015 Curt Hill Relationship The prior list shows an is a relationship A thirteen line ground squirrel is a squirrel A squirrel is a rodent A thirteen line ground has all the characteristics of any of the above

4. Copyright © 1999-2015 Curt Hill Composition and Inheritance Composition expresses a has a relationship –Fraction has a numerator (integer) Inheritance expresses an is a relationship –A student is a person

5. Copyright © 1999-2015 Curt Hill Inheritance Deriving new classes from old classes New class may retain any properties or methods of old New class may add new properties, new methods New class may replace old properties or methods

6. Copyright © 1999-2015 Curt Hill Inheritance example Person has a name and age Student is a person with GPA and major Employee is a person with a wage Grad is a student with degree

7. Copyright © 1999-2015 Curt Hill person employeestudent grad set_name set_wageset_major set_degree

8. Copyright © 1999-2015 Curt Hill Syntax details The Java extends keyword indicates subclass: class student extends person { The person constructor will be called as part of the student constructor If we want a particular constructor from person we call with super

9. Copyright © 1999-2015 Curt Hill Super Constructor Example class student extends person{... public student (String nme, char m_or_f, date d){ super(nme,m_or_f,d); hours=0;... }

10. Copyright © 1999-2015 Curt Hill Properties and methods All classes have properties (eg. name and age) and methods (eg. set_name) –Person explicitly declares it –All the rest inherit it –Can access if public or protected

11. Copyright © 1999-2015 Curt Hill Using superclass methods Use super as a class name to get the immediate super class method super.x(y)

12. Copyright © 1999-2015 Curt Hill The print routine In person: public void print(){ System.out.print(name); System.out.print(' ');... In student: public void print() { super.print(); System.out.print(" hrs: "); System.out.print(hours);

13. Copyright © 1999-2015 Curt Hill Polymorphism Literally: many shapes Classes that are related are interchangable In Java any container, such as an array can have related contents An array of person can also contain students

14. Copyright © 1999-2015 Curt Hill Polymorphism Example person ptr[] = new person[max]; // person must be super class ptr[0] = new person(); ptr[1] = new student(); ptr[2] = new employee();

15. Copyright © 1999-2015 Curt Hill Polymorphic methods What can be done to an array of some super class? Anything that can be done to all of them –Methods defined only in super class Such as get_name –Methods redefined in subclasses Such as print

16. Copyright © 1999-2015 Curt Hill Inherited functions Execute the set_name function against any variable of these four types –The same effect occurs –Defined in the base type –Variable is upcast, that is converted to a base type

17. Copyright © 1999-2015 Curt Hill Polymorphism Execute the print function against any variable of these four types –The different effects occur based on the actual type of the variable –It just calls print knowing that all person derived classes have a print function A function can accept a person type and receive either a person, student, employee or grad without problem

18. Copyright © 1999-2015 Curt Hill Polymorphic Example for (int i=0;i<max;i++) ptr[i].print(); Prints them without knowing in advance which are students and which are persons Not knowing which routine to call until runtime is dynamic binding

19. Copyright © 1999-2015 Curt Hill Polymorhism Again Polymorphism requires no effort by the programmer All that needs to be done is point a handle at a method and execute The system determines what the pointer that it has points at From a type perspective it could be what it is declared or any descendent

20. Copyright © 1999-2015 Curt Hill Object Every class is a subclass of Object Thus: class Item {… is the same as: class Item extends Object { … Thus all classes are polymorphic All may use Object methods –Object has no properties

21. Copyright © 1999-2015 Curt Hill Object There are a variety of methods: –equals –getClass –hashcode –notify –toString –wait These are often overridden in descendent classes

22. Copyright © 1999-2015 Curt Hill The Object Hierarchy Since every class is a subclass of Object then Object obarr[]; can hold anything but a primitive –It can hold a wrapper such as Integer These can be determined at run-time Versions of Java prior to 5 had no generics –Polymorphism substituted

23. Copyright © 1999-2015 Curt Hill Containers and Polymorphism An array is not the only container Java also has a variety of container classes such as LinkList, Stack, TreeSet, Vector etc. These are dynamic data structures that may contain objects of certain types The types only have to be polymorphically related

24. Conclusion Inheritance requires the extends reserved word –Interfaces uses implements Java objects are handle based so always available –The handles are always the same size Object is the root of a single inheritance tree, so a container class could contain any object Copyright © 1999-2015 Curt Hill