COMP 110: Introduction to Programming Tyler Johnson Apr 8, 2009 MWF 11:00AM-12:15PM Sitterson 014

COMP 110: Spring Announcements Lab 8 is due tomorrow by midnight

COMP 110: Spring Questions?

COMP 110: Spring Today in COMP 110 Go over Program 4 More Inheritance Programming Demo

COMP 110: Spring Program 4

COMP 110: Spring Inheritance Section 8.3 in text

COMP 110: Spring Inheritance Define a general class Later, define specialized classes based on the general class These specialized classes inherit properties from the general class

COMP 110: Spring Inheritance Hierarchies Put these classes into an inheritance hierarchy AnimalReptileMammalHumanCrocodileWhale Animal ReptileMammal HumanCrocodileWhale

COMP 110: Spring The is-a relationship This inheritance relationship is known as an is-a relationship A Doctoral student is a Grad student A Grad student is a Student A Student is a Person Is a Person a Student? Not necessarily!

COMP 110: Spring When to Use Inheritance Use inheritance when an is-a relationship is present A Student is a Person, Student inherits from Person Do not confuse an is-a relationship with a has-a relationship A Student has a date of enrollment, so an object of class Date should be an instance variable of the Student class, and should not inherit from it

COMP 110: Spring The Keyword extends public class Derived_Class_Name extends Base_Class_Name { Declaration_of_Added_Instance_Variables Definitions_of_Added_And_Overridden_Methods } public class Student extends Person { // stuff goes here } A derived (child) class inherits the public instance variables and public methods of its base (parent) class

COMP 110: Spring Method Overriding public class Person { //... public void printInfo() { System.out.println("Name: " + name); } public class Student extends Person { //... public void printInfo() { System.out.println("Name: " + getName()); System.out.println("ID: " + getID()); } Overridden The printInfo method in the class Student overrides the printInfo method inherited from the class Person

COMP 110: Spring Overriding Methods Java handles this situation as follows: If a derived class defines a method with the same name, number and types of parameters, and return type as a method in the base class, the derived class method overrides the base class method The method definition in the derived class is the one that is used for objects of the derived class

COMP 110: Spring Overriding Methods Why is it useful? You often want derived classes to perform custom behavior public class Shape { public void draw(Graphics g) { } public class Circle extends Shape { public void draw(Graphics g) { g.drawOval(…arguments…); } public class Rectangle extends Shape { public void draw(Graphics g) { g.drawRect(…arguments…); }

COMP 110: Spring The Keyword super Last time we saw that the keyword super could be used to invoke the constructor of the base class public class Person { private String name; public Person() { name = "No name yet"; } public class Student extends Person { private int id; public Student() { super(); id = -1; } Student student = new Student(); //name is set to No name yet, and id is set to -1

COMP 110: Spring The Keyword super The use of super must always be the first action taken in the constructor Not required to be specified, but if it is, it must come first Java will automatically call the default constructor of the base class if super is not specified public Student() { super(); id = -1; } public Student() { id = -1; } public Student() { id = -1; super(); } OK ERROR Equivalent

COMP 110: Spring The Keyword super You cannot use repeated supers If specified, specify only once public Student() { super(); id = -1; } public Student() { super(); super("No name yet"); id = -1; } OKERROR

COMP 110: Spring The Keyword super What if the base class is a derived class? public class Person { private String name; public Person() { name = "No name yet"; } public class Student extends Person { private int id; public Student() { super(); id = -1; } public class Undergraduate extends Student { private int level; public Undergraduate() { super(); //calls constructor of Student, not Person level = -1; }

COMP 110: Spring Method Overriding public class Person { //... public void printInfo() { System.out.println("Name: " + name); } public class Student extends Person { //... public void printInfo() { System.out.println("Name: " + getName()); System.out.println("ID: " + getID()); } Overridden Is there a way to call the printInfo() method of the class Person from within the class Student? Yes

COMP 110: Spring Calling Overridden Methods public class Person { //... public void printInfo() { System.out.println("Name: " + name); } public class Student extends Person { //... public void printInfo() { super.printInfo(); System.out.println("ID: " + getID()); } } The keyword super can also be used to call overridden methods of a base class

COMP 110: Spring Inheritance and Overloading public class Person { private String name; public void setData(String newName) { name = newName; } public class Student extends Person { private int id; public void setData(String newName, int newID) { setData(newName); id = newID; } Overloaded Methods with a different number or type of parameters are overloaded, not overridden

COMP 110: Spring Type Compatibilities Given this inheritance heirarchy… Person Athlete HighJumper Skydiver ExtremeAthlete XGamesSkater

COMP 110: Spring Is This Code Legal? Person p = new Person(); Yes! Person Athlete HighJumper Skydiver ExtremeAthlete XGamesSkater

COMP 110: Spring Is This Code Legal? HighJumper h = new HighJumper(); Yes! Person Athlete HighJumper Skydiver ExtremeAthlete XGamesSkater

COMP 110: Spring Is This Code Legal? Person p = new Athlete(); Yes! An Athlete is a Person, so this is okay Person Athlete HighJumper Skydiver ExtremeAthlete XGamesSkater

COMP 110: Spring Is This Code Legal? Person p = new SkyDiver(); Yes! A SkyDiver is a Person, so this is okay Person Athlete HighJumper Skydiver ExtremeAthlete XGamesSkater

COMP 110: Spring Is This Code Legal? Skydiver s = new Person(); No! A Person is not necessarily a Skydiver, so this is illegal Person Athlete HighJumper Skydiver ExtremeAthlete XGamesSkater

COMP 110: Spring Is This Code Legal? Athlete ath = new Athlete(); XGamesSkater xgs = ath; No! An Athlete is not necessarily an XGamesSkater, so this is illegal Person Athlete HighJumper Skydiver ExtremeAthlete XGamesSkater

COMP 110: Spring Dynamic Binding Person person = new Person("John Smith"); person.printlnfo(); //calls printInfo of Person class Student student = new Student("John Smith", ); student.printlnfo(); //calls printInfo of Student class Person p = new Student(Tom Jones", ); p.printInfo(); //which printInfo method is called? The printInfo method of the Student class is called because p is really a student!

COMP 110: Spring The Operator instanceof We can test whether an object is of a certain class type using the instanceof operator: public void objectType(Person p) { if(p instanceof Student) { System.out.println("p is an instance of the class Student"); } else if(p instanceof GradStudent) { System.out.println("p is an instance of the class GradStudent"); } … } Syntax: object instanceof Class_Name

COMP 110: Spring The Class Object Every class in Java is derived from the class Object Every class in Java is an Object Animal ReptileMammal HumanCrocodileWhale Object Person StudentEmployee

COMP 110: Spring The Class Object The Java class Object provides methods that are inherited by every class For example equals, toString These methods should be overridden with methods appropriate for the classes you create

COMP 110: Spring Printing Objects to Screen The method println is overloaded to accept objects as an argument System.out.println(int i) System.out.println(double d) System.out.println(Object o) When printing an object with System.out.println(Object o), the objects toString() method is called and the result is printed public void println(Object o) { System.out.println(o.toString()); //dynamic binding }

COMP 110: Spring Overriding toString public class Person { private String name; public Person() { name = "No name yet"; } public void setName(String newName) { name = newName; } //converts a Person object to a String, overrides toString in the class Object public String toString() { return "Name: " + name + "\n"; } public static void main(String[] args) { Person person = new Person(); person.setName("John Smith"); System.out.println(person); //prints "Name: John Smith" to screen (without quotes) }

COMP 110: Spring Overriding toString public class Student extends Person { private int id; public Student() { super(); id = -1; } public void setID(String newName) { name = newName; } //converts a Person object to a String, overrides toString in the class Person public String toString() { return "Name: " + name + "\nID: " + id + "\n"; } public static void main(String[] args) { Student student = new Student(); student.setName("John Smith"); student.setID( ); System.out.println(student); //prints "Name: John Smith" // "ID: " to screen (without quotes) }

COMP 110: Spring Programming Demo Programming with inheritance Sports Define the following classes Person Athlete HighJumper Skydiver ExtremeAthlete XGamesSkater

COMP 110: Spring Sports Each class has a method called jump Simply prints a message to screen Person System.out.println("Whee!"); Athlete System.out.println("I jump well!"); ExtremeAthlete System.out.println("EXTREMEEEEEEEEEE JUMP!"); HighJumper System.out.println("I jump REALLY HIGH!"); ShotPutter System.out.println("I don't jump very much."); XGamesSkater System.out.println("360 nollie to frontside air");

COMP 110: Spring Sports Write a class called Sports that demonstrates Type compatibility Dynamic binding instanceof operator

COMP 110: Spring Friday No recitation, enjoy the Holidays