Object Oriented Programming with Java

Object Orientated Programming with Java

Jussi PohjolainenTampere University of Applied Sciences

MOTIVATION

Job Opportunities?

• Little acronyms from job descriptions:– S60, C++, J2ME, Java, Java EE, SCRUM, JAX-WS 2.0,

EJB 3.0, Spring, Hybernate, Struts, SQL, XHTML, CSS, Symbian C++, Perl, PHP, Python, LDAP, MFC, XML, .NET, Visual Basic, AJAX, Objective-C, JSP, Servlet, GTK, Qt, JavaScript, Oracle, SQL Server, DB Design…

• Where is Object Orientated programming?

“Basic” programming skills“Basic” programming skills

OO skillsOO skills

Data Structures / DBData Structures / DB UI programmingUI programming

Symbian C++Symbian C++

S60S60

Developing S60 apps?


OO skillsOO skills

Data Structures / DBData Structures / DB UI programmingUI programming

Objective-CObjective-C

iPhoneiPhone

Developing iPhone apps?


OO skillsOO skills

Data Structures /DBData Structures /DB UI programmingUI programming

JavaJava

ServletServlet

Web-development with Java?

Www-techniques: Xhtml, CSS, XML

Www-techniques: Xhtml, CSS, XML

AppletsAppletsJSPJSP

OO CONCEPTS

Intro to OO• Object Orientated programming is a programming

paradigm.• One way of describing the structure of the application• Several paradigms available.

– Procedural, Functional, ...

• OO is nowdays one of the most popular.– C++– Java– C#– PHP 5– ...

Benefits

• Reusability– Once you written code, you can reuse it!

• Management– Application is programmed using classes and objects.

Pieces that communicate with each other.• Maintanence– When changing the code, it does not influence the whole

application.

Basic Concept: Object

• In real life, the world consists of objects:– cars, buildings, trees, ships, humans, flowers..

• Every object has actions (=methods!) that can incluence other objects– jack drives ferrari. – Object jack has a method drive that influences some

way to object ferrari.– jack.drive(ferrari);

• In OO, you should implement the app so that it consists of objects that influence each other!

Example about an Object

• Datsun 100A is an object• Datsun 100A has different actions or methods:

drive, brake, park...• Datsun 100A has information or attributes:

color, amount of gears, amount of doors...

Basic Concept: Class

• Class is a blueprint or template of an object• Class describes the state and behaviour to it's

objects.• Object is created from the class.

Example about an Class

• If Datsun 100A is an object, Car is a class.• If one wants to create Datsun 100A, you have

to have first the blueprints of the Datsun. • Blueprints of an object: Class• Class Car -> Object Datsun 100A

Examples: Class to Object

Class Object

Car datsun 100a

Human Jack Bauer

Color red

Laptop MacBook Pro

String "some string"

Array {1,2,3,2,4}

... ...

Class and Object

• Car - class

Datsun 100ADatsun 100A

Lamborghini DiabloLamborghini Diablo

Peugeot 406Peugeot 406

Car's Blueprint

• When building a Car's blueprint (class), you have to think that what is similar in all car's

• So what is similar in datsun, lamborghini and peugeot?

Objects datsun, lambo, peugeot• datsun:– brand: Datsun 100A , motor: 1.0, fuzzy

dices: yes, color: red• lambo– brand: Lamborghini Diablo, motor: 8.0,

fuzzy dices: no, color: punainen• peugeot– brand: Peugeot 406, motor: 2.2, fuzzy

dices: no, color: blue

Car's Blueprint (Class) in UML

Car

brandmotoramountOfDoorscolorhasFuzzyDices..

From Class to Object

datsun

Datsun 100A1.03redtrue

lambo

Lamborghini Diablo8.03redfalse

Car

brandmotoramountOfDoorscolorhasFuzzyDices..

Car-class, extension

Car

brandmotoramountOfDoorscolorhasFuzzyDices

driveparkbrake

Class

• Class is a template or blueprint to object• Class holds– Attributes (=variables)– Actions (=methods)

• Class instances are called objects

CLASSES AND OBJECTS IN JAVA

Person Class to Objects

george

GeorgeSmith40Teacher09-12345

Jack

JackPuupää60Toimistopäällikkö03-654321

Person

firstnamelastnameageprofessionphonenumber

eatsleepdrinkBeer

eatsleepdrinkBeer

Person

firstnamelastnameageprofessionphonenumber

eatsleepdrinkBeer

Person – class to Java

class Person { public String firstname;

public String lastname; public int age; public String profession; public int phonenumber;

public void eat() { System.out.println("Eating!"); }

public void sleep() { System.out.println("Sleeping!"); }

public void drinkBeer() { System.out.println("Drinking!"); }}

From Class to Object

• App always starts from the main-method• Let's test the Person – class• This creates a variable a which type is integer– int a;

• This creates a object jack which type is Person– Person jack;

From Class to Objectclass Person { ....}

class JustTesting { public static void main(String [] args) { // Declare the object Person jack; // Initialize the object jack = new Person(); jack.firstname = "Jack"; jack.lastname = "Smith"; jack.drinkBeer(); }}

Example: Car - class

class Car{ public String brand; public int amountOfGas; public void drive() { amountOfGas--; }}

Creating Objects From the Classclass Car { ....}

class JustTesting { public static void main(String [] args) { Car datsun = new Car(); datsun.amountOfGas = 100; datsun.drive(); System.out.println(datsun.amountOfGas);

Car ferrari = new Car(); ferrari.amountOfGas = 300; ferrari.drive(); System.out.println(ferrari.amountOfGas); }}

Basic Concept - Encapsulationprivate

public method

About Attributes

• Attributes are usually marked as private• The reason for this is that other objects

cannot change the values as they will• You don't for example want that every object

in the world can change person's weight to 500kg...

Example: Person - class

class Person { private String name; private int weight;}

class Person { private String name; private int weight;}

class JustTesting { public static void main(String [] args) { Person jack = new Person(); jack.name = "Jack Smith"; jack.weight = 500; }}

RESULT:TB308POHJUS-L-2:temp pohjus$ javac Person.java Person.java:9: name has private access in Person jack.name = "Jack Smith"; ^Person.java:10: weight has private access in Person jack.weight = 500; ^2 errors

class Person { private String name; private int weight;

public void setName(String n) { name = n; } public String getName() { return name; } public void setWeight(int w) { if(w > 0 && w <= 150) weight = w; } public int getWeight() { return weight; }}

class JustTesting { public static void main(String [] args) { Person jack = new Person(); jack.setName("Jack Smith"); jack.setWeight(200); System.out.println(jack.getName()); }}

Accessor and Mutator - methodsclass Person { private String name; private int weight;

// Mutator public void setName(String n) { name = n; } // Accessor public String getName() { return name; }

// Mutator public void setWeight(int w) { if(w > 0 && w <= 150) weight = w; } // Accessor public int getWeight() { return weight; }}

JAVA TYPES

Java Types

• Java has two type of types• 1) Primitive types– byte, short, int, long, double, float, char, boolean

• 2) Class types– String, Scanner, Array, JButton, JFrame ...

Differences

• Primitive types are spelled with lowercase:– int, double, float...

• Class types are spelled with uppercase– String, Scanner, Person, Cat, Car ...

• Primitive type declaring and initialization– int a = 5;

• Class type declaring and initialization with new– Dog spot = new Dog();

Differences

• Primitive type– int a = 5;

• Class type– int [] b= new int[5];

• b holds memory address• a holds value 5.

Memory Address?

int [] b = new int[2];b[0] = 1;b[1] = 2;// prints 0x01System.out.println(b);

RAMRAM

address value

0x01 1

0x02 2

address value

0x09 0x01

variable b

Memory Address?

int [] b = new int[2];b[0] = 1;b[1] = 2;int [] a = b;// prints 0x01System.out.println(b);// prints 0x01System.out.println(a);

RAMRAM

address value

0x01 1

0x02 2

address value

0x09 0x01

variable b

address value

0x19 0x01

variable a

Output?

int [] b = new int[2];b[0] = 1;b[1] = 2;int [] a = b;b[0] = 99;// Output?System.out.println(a[0]);

Differences Again


• Class type– int [] b= new int[5];

• b holds memory address• a holds value 5.

Differences Again


• Class type– Person jack = new Person()

• jack holds memory address• a holds value 5.

Output?

• Person jack = new Person();• jack.setName("Jack Smith");• Person james = jack;• james.setName("James Bond");• // output?• System.out.println(jack.getName());

Methods and Variables

public void method(int x) { x++;}public void main(String [] args) { int y = 3; method(y); // Output is 3! System.out.println(y);}

Methods and Variables

public void method(int [] x) { x[0] = 12;}public void main(String [] args) { int [] y = {1,2,3}; method(y); // Output is 12 since array is class type! System.out.println(y[0]);}

String

• String is an exception to the rules• String is a class type that acts like primitive

type• String is the only class type that can be

initialized without the new word.– String a = "hello";

• String is passed by value in methods, so String is copied when moving strings in methods.

String and Memory

• String variables are objects => holds memory address.

• Comparing contents– a.equals(b);

• Comparing memory addresses– a == b

CONSTRUCTOR

Constructors

• Constructor is a “init method” that is called when an object is created

• Java provides default constructor (= constructor with no parameters)

• Constructor has the same name than the class• Constructor does not return anything• Constructor usually initalizes class members

Exampleclass Car {

public Car() {

System.out.println("Constructor!");

}

}

class Test {

public static void main(String [] args) {

Car datsun = new Car();

}

}

> java TestConstructor!

class Car {

private String brand;

public Car(String b) {

brand = b;

}

public String getBrand() {

return brand;

}

}

class Test {


Car datsun = new Car("Datsun 100A");

System.out.println( datsun.getBrand() );

}

}

> java TestDatsun 100A

Multiple Constructorsclass Car {

public Car() {

// Do something

}

public Car(String brand) {

// Do something else

}

}

class Test {


Car datsun = new Car();

Car ferrari = new Car("Ferrari");

}

}

Problem?class Car {

String brand;


brand = brand;

}

}

class Test {


Car datsun = new Car("Datsun 100a");

}

}> java Testnull

Solutionclass Car {

String brand;


this.brand = brand;

}

}

class Test {


Car datsun = new Car("Datsun 100a");

}

}> java TestDatsun 100a

COMPOSITION

Composition

• Relatioship between objects, where one object owns, or has the other object

• Car has or owns Motor• When Car is build, it's motor is built also• When Car is destroyed it's motor is destroyed

UML notation

Java: Composition

// Composition

class Car

{

private Motor motor;

public Car() {

motor = new Motor();

}

}

One to Many?

Java: One to Manyclass Department{ private Professor [] members; private int numberOfMembers;

public Department(Professor prof) { members = new Professor[20]; members[0] = prof; numberOfMembers = 1; }

public void addProfessor(Professor prof) { members[numberOfMembers] = prof; numberOfMembers++; }}

INHERITANCE

Introduction to Inheritance

• Inheritance is a relationship between two or more classes where derived class inherites behaviour and attributes of pre-existing (base) classes

• Intended to help reuse of existing code with little or no modification

Inheritance

• Inheritance can be continous– Derived class can inherit another class, which

inherits another class and so on– When changing the base class all the derived

classes changes also• Example: – Mammal <– Human <– Worker <- Programmer

• Could mammal be a derived class? If so, what would be the base class?

Picture about Inheritanceab

Class A

features: a,b

c

Class B

Features: a,b,cde

Class C

Features: a,b,d,e

f

Class D

Features: a,b,d,e,f

Multiple Inheritance

• In multiple inheritance a derived class has multiple base classes

• C++ supports multiple base classes, Java don't

Driver- license- Year of approval

Conductor- Account number

Taxi Driver- area

House Boat

Houseboat

Inheritance and Capsulation

• private– Is accessible only via the base class

• public– Is accessible everywhere (base class, derived class,

othe classes)

• protected– Is accessible by the base class and derived classes

Basic example

• What are Programmer's attributes and methods?

Human

string name

void sleep()void drink()void eat()

Programmer

int salary

void implementApps()void beNerd()

Overriding?

• What about now?

Human

string name

void sleep()void drink()void eat()

Programmer

int salary

void implementApps()void beNerd()void drink()void eat()

Overriding

• Since programmer eats and drinks differently than humans (only Coke and Pizza) the eat and drink methods are overriden in Programmer!

Abstract Class

• Abstract class is a class which you cannot instantiate (create objects)

• You can inherit abstract class and create objects from the inherited class, if it is concrete one

• Abstract class in C++ has abstract methods, that do not have implementations

• These methods forces derived classes to implement those methods

Example<<abstract>>Mammal

string name

void makesound() {abstract}

Elephant

int trunkLength

makesound()

Example<<abstract>>Figure

int x, y

double calculateArea() {abstract}

Circle

double radius

double calculateArea()

Rect

double length, height

double calculateArea()

INHERITANCE IN JAVA

Example: Basic Inheritanceclass Human { public void sleep() { System.out.println("Human sleeps"); }}class Programmer extends Human {

}class Test { public static void main(String [] args) { Programmer jussi = new Programmer(); jussi.sleep(); // "Human sleeps" }}

Example: Overridingclass Human { public void sleep() { System.out.println("Human sleeps"); }}class Programmer extends Human { public void sleep() { System.out.println("Programmer sleeps"); }}class Test { public static void main(String [] args) { Programmer jussi = new Programmer(); jussi.sleep(); // "Programmer sleeps" }}

Example: superclass Human { public void sleep() { System.out.println("Human sleeps"); }}class Programmer extends Human { public void sleep() { super.sleep(); System.out.println("Programmer sleeps"); }}class Test { public static void main(String [] args) { Programmer jussi = new Programmer(); jussi.sleep(); }}

> java TestHuman sleepsProgrammer sleeps

Constructors and Inheritanceclass Human {

public Human() {

System.out.println("Human");

}

}

class Programmer extends Human {

public Programmer() {

System.out.println("Programmer");

}

}

class Test {


Programmer jussi = new Programmer();

}

}

> java TestHumanProgrammer

Constructors and Inheritance

• Constructor allways calls the base classes constructor!

• When creating a constructorvoid Human() {

}

• Java adds super() – call to it:void Human() {

super(); // calls base classes constructor

}

class Human {

public Human() {


}

}




}

}

class Test {



}

}


class Human {

public Human() {

super(); // Java adds this! Calls base classes contructor


}

}



super(); // Java adds this! Calls base classes contructor


}

}

class Test {



}

}


What the...? What base class?class Human {

public Human() {

// Java adds this! Calls base classes contructor

super();


}

}

Object

• Every class derives from a class called Object.

// Java adds the extends Object too!

class Human extends Object {

public Human() {

super();


}

}

Object

clone()equals()finalize()toString()...

Human

String name

...

http://java.sun.com/javase/6/docs/api/java/lang/Object.html

class Human {

public Human(int a) {


}

}




}

}

class Test {



}

}

> javac Test.java> DOES NOT COMPILE!!! Why?

class Human {



}

}



super(); // Java adds this and it calls constructor

// Human() that does not exist..


}

}

class Test {



}

}

class Human {



}

}



super(5); // Now it works: Human(int a) exists.


}

}

class Test {



}

}

class Human {

private String name

public Human(String name) {

this.name = name;

}

}


private int salary;

public Programmer(String name, int salary) {

super(name);

this.salary = salary;

}

}

class Test {


Programmer jussi = new Programmer("Jussi", 5000);

}

}

Abstract Class

• From abstract class you cannot create objects!

• Abstract class is usually used with inheritance• Abstract class may contain abstract methods.• Abstract method forces derived classes to

implement the abstract method.

Abstract Class: example

abstract class Mammal { abstract void makeSound();}class Dog extends Mammal { // You have to implement this! public void makeSound() { System.out.println("Bark!"); }}

Abstract Class: example

// Does NOT work, since Mammal is

// abstract class

Mammal object = new Mammal();

// Does work

Dog spot = new Dog();

Java: Abstract class and Interface

• Abstract class can hold "normal" methods and abstract methods.

• Interface holds only abstract methods• Abstract class:– class A extends someAbstractClass

• Interface– class A implements someInterface

Abstract class to Interface

abstract class Movable { abstract public void start(); abstract public void stop();}interface Movable { public void start(); public void stop();}

Implementing the Interfaceinterface Movable { public void start(); public void stop();}class Car implements Movable { // You have to implement these public void start() { // Do something } public void stop() { // Do something } }

Abstract class vs Interface

• Abstract class can hold normal methods and abstract methods

• Interface can hold only abstract methods• Class can inherite only one base class• Class can implement several interfaces!

class Car extends Vehicle implements Movable, RunsOnGasoline {

// You have to implement these

public void start() {

// Do something

}

public void stop() {

// Do something

}

public void reduceGasoline() {

// Do something

}

public void addGasoline() {

// Do something

}

}

POLYMORPHISM

int as parameterclass Exercise13 {


int x = 4;

myMethod(x);

}

public static void myMethod(int a) {

}

}

Human parameterclass Human {

}

class Exercise13 {


Human jack = new Human();

myMethod(jack);

}

public static void myMethod(Human a) {

}

}

Mammal parameterclass Mammal {}class Human extends Mammal {}class Dog extends Mammal {}

class Exercise13 { public static void main(String [] args) { Human jack = new Human(); Dog spot = new Dog(); Mammal mammal = new Mammal(); // these work! You can pass mammals, dogs and humans to the method! myMethod(jack); myMethod(dog); myMethod(mammal); } public static void myMethod(Mammal a) { }}

Object parameter...

class Exercise13 { public static void main(String [] args) { Human jack = new Human(); Dog spot = new Dog(); Mammal mammal = new Mammal(); // these work! You can pass every object to the method myMethod(jack); myMethod(dog); myMethod(mammal); myMethod("hello"); // String } public static void myMethod(Object a) { }}

Calling methods from Mammalclass Mammal {}class Human extends Mammal { public void bark() { System.out.println("Bark!"); };}class Dog extends Mammal {}

class Exercise13 { public static void main(String [] args) { Human jack = new Human(); Dog spot = new Dog(); Mammal mammal = new Mammal(); myMethod(jack); myMethod(dog); myMethod(mammal); } public static void myMethod(Mammal a) { a.bark(); // Why this does not work? }}

Solutionclass Exercise13 { public static void main(String [] args) { Human jack = new Human(); Dog spot = new Dog(); Mammal mammal = new Mammal(); myMethod(jack); myMethod(dog); myMethod(mammal); } public static void myMethod(Mammal a) { // Now it works if(a instanceof Dog) { Dog spot = (Dog) a; spot.bark(); } }}

This works, why?class Mammal { public void giveBirth() { System.out.println("Giving birth"); };}class Human extends Mammal {}class Dog extends Mammal {}

class Exercise13 { public static void main(String [] args) { Human jack = new Human(); Dog spot = new Dog(); Mammal mammal = new Mammal(); myMethod(jack); myMethod(dog); myMethod(mammal); } public static void myMethod(Mammal a) { a.giveBirth(); // Why this works? }}

class Movable { public void start(); public void stop();}class Vehicle {

}class Car extends Vehicle implements Movable { public void start() { // Do something } public void stop() { // Do something }}

class Exercise13 { public static void main(String [] args) { Car c = new Car(); myMethod(c); } // You can pass every object that implements the Movable! public static void myMethod(Movable a) { a.start(); }}

Technology

Object Oriented Programming with Java