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

COMP 110:Introduction to Programming

Tyler JohnsonApr 8, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

Announcements

Lab 8 is due tomorrow by midnight


Questions?


Today in COMP 110

Go over Program 4

More Inheritance

Programming Demo


Program 4


Inheritance

Section 8.3 in text


Inheritance

Define a general class

Later, define specialized classes based on the general class

These specialized classes inherit properties from the general class


Inheritance Hierarchies

Put these classes into an inheritance hierarchy

AnimalReptile MammalHumanCrocodile Whale

Animal

Reptile Mammal

HumanCrocodile Whale


The is-a relationship

This inheritance relationship is known as an is-a relationship

A Doctoral student is a Grad studentA Grad student is a StudentA Student is a Person

Is a Person a Student?Not necessarily!


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


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


Method Overriding

public class Person {

// ...

public void printInfo() { System.out.println("Name: " +

name); }}

public class Student extends Person {

// ...


getName()); System.out.println("ID: " + getID()); }}

Overridden

The printInfo method in the class Student overrides the printInfo method inherited from the class Person


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’ methodThe method definition in the derived class is the one that is used for objects of the derived class


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…);

}}


The Keyword super

Last time we saw that the keyword super could be used to invoke the constructor of the base class


private String name;

public Person() {name = "No

name yet";}

}


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


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 firstJava 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 OK ERROR

Equivalent


The Keyword super

You cannot use repeated supersIf specified, specify only once

public Student() { super(); id = -1;}

public Student() { super(); super("No name yet"); id = -1;}

OK ERROR


The Keyword super

What if the base class is a derived class?



public Person() {name = "No

name yet";}

}


private int id;


}}public class Undergraduate extends Student {

private int level;

public Undergraduate() {super(); //calls constructor of Student, not Personlevel = -1;

}}


Method Overriding


// ...


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


Calling Overridden Methods


// ...


name); }}


// ...

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


Inheritance and Overloading



public void setData(String newName) {

name = newName; }}


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


Type Compatibilities

Given this inheritance heirarchy…

Person

Athlete

HighJumper

Skydiver

ExtremeAthlete

XGamesSkater


Is This Code Legal?

Person p = new Person();Yes!

Person

Athlete

HighJumper

Skydiver

ExtremeAthlete

XGamesSkater


Is This Code Legal?

HighJumper h = new HighJumper();Yes!

Person

Athlete

HighJumper

Skydiver

ExtremeAthlete

XGamesSkater


Is This Code Legal?

Person p = new Athlete();Yes! An Athlete is a Person, so this is okay

Person

Athlete

HighJumper

Skydiver

ExtremeAthlete

XGamesSkater


Is This Code Legal?

Person p = new SkyDiver();Yes! A SkyDiver is a Person, so this is okay

Person

Athlete

HighJumper

Skydiver

ExtremeAthlete

XGamesSkater


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


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


Dynamic Binding

Person person = new Person("John Smith");person.printlnfo(); //calls printInfo of Person class

Student student = new Student("John Smith", 42352352);student.printlnfo(); //calls printInfo of Student class

Person p = new Student(“Tom Jones", 733234242);p.printInfo(); //which printInfo method is called?

The printInfo method of the Student class is called because p is really a student!


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


The Class Object

Every class in Java is derived from the class Object

Every class in Java is an Object

Animal

Reptile Mammal

HumanCrocodile Whale

Object

Person

Student Employee


The Class Object

The Java class Object provides methods that are inherited by every class

For exampleequals, toString

These methods should be overridden with methods appropriate for the classes you create


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 object’s toString() method is called and the result is printed

public void println(Object o) {System.out.println(o.toString()); //dynamic binding

}


Overriding toString



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 Objectpublic 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)

}}


Overriding toString


private int id;


}

public void setID(String newName) {name = newName;

}

//converts a Person object to a String, overrides toString in the class Personpublic String toString() {

return "Name: " + name + "\nID: " + id + "\n";}

public static void main(String[] args) {Student student = new Student();student.setName("John Smith");student.setID(342652342);System.out.println(student); //prints "Name: John Smith"

// "ID: 342652342" to screen (without quotes)}

}


Programming Demo

Programming with inheritanceSports

Define the following classesPerson

Athlete

HighJumper

Skydiver

ExtremeAthlete

XGamesSkater


Sports

Each class has a method called jumpSimply prints a message to screen

PersonSystem.out.println("Whee!");

AthleteSystem.out.println("I jump well!");

ExtremeAthleteSystem.out.println("EXTREMEEEEEEEEEE JUMP!");

HighJumperSystem.out.println("I jump REALLY HIGH!");

ShotPutterSystem.out.println("I don't jump very much.");

XGamesSkaterSystem.out.println("360 nollie to frontside air");


Sports

Write a class called Sports that demonstrates

Type compatibilityDynamic bindinginstanceof operator


Friday

No recitation, enjoy the Holidays

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

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