Object Oriented Programming with Java

Object Orientated Programming with Java Jussi Pohjolainen Tampere University of Applied Sciences

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?

Developing S60 apps? “ Basic” programming skills OO skills Data Structures / DB UI programming Symbian C++ S60

Developing iPhone apps? “ Basic” programming skills OO skills Data Structures / DB UI programming Objective-C iPhone

Web-development with Java? “ Basic” programming skills OO skills Data Structures /DB UI programming Java Servlet Www-techniques: Xhtml, CSS, XML Applets JSP

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 100A Lamborghini Diablo Peugeot 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 brand motor amountOfDoors color hasFuzzyDices . .

From Class to Object datsun Datsun 100A 1.0 3 red true lambo Lamborghini Diablo 8.0 3 red false Car brand motor amountOfDoors color hasFuzzyDices . .

Car-class, extension Car brand motor amountOfDoors color hasFuzzyDices drive park brake

Class Class is a template or blueprint to object Class holds Attributes (=variables) Actions (=methods) Class instances are called objects

Person Class to Objects george George Smith 40 Teacher 09-12345 Jack Jack Puupää 60 Toimistopäällikkö 03-654321 eat sleep drinkBeer Person firstname lastname age profession phonenumber eat sleep drinkBeer

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!"); } } Person firstname lastname age profession phonenumber eat sleep drinkBeer

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 Object class 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 Class class 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 - Encapsulation private 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 - methods class 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 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 0x01 System.out.println(b); RAM variable b address value 0x01 1 0x02 2 address value 0x09 0x01

Memory Address? int [] b = new int[2]; b[0] = 1; b[1] = 2; int [] a = b; // prints 0x01 System.out.println(b); // prints 0x01 System.out.println(a); RAM variable b variable a address value 0x01 1 0x02 2 address value 0x09 0x01 address value 0x19 0x01

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 Primitive type int a = 5; Class type int [] b= new int[5]; b holds memory address a holds value 5.

Differences Again Primitive type int a = 5; 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

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

Example class Car { public Car() { System.out.println("Constructor!"); } } class Test { public static void main(String [] args) { Car datsun = new Car(); } } > java Test Constructor!

class Car { private String brand; public Car(String b) { brand = b; } public String getBrand() { return brand; } } class Test { public static void main(String [] args) { Car datsun = new Car("Datsun 100A"); System.out.println( datsun.getBrand() ); } } > java Test Datsun 100A

Multiple Constructors class Car { public Car() { // Do something } public Car(String brand) { // Do something else } } class Test { public static void main(String [] args) { Car datsun = new Car(); Car ferrari = new Car("Ferrari"); } }

Problem? class Car { String brand; public Car(String brand) { brand = brand; } } class Test { public static void main(String [] args) { Car datsun = new Car("Datsun 100a"); } } > java Test null

Solution class Car { String brand; public Car(String brand) { this.brand = brand; } } class Test { public static void main(String [] args) { Car datsun = new Car("Datsun 100a"); } } > java Test Datsun 100a

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

Java: Composition // Composition class Car { private Motor motor; public Car() { motor = new Motor(); } }

Java: One to Many class 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++; } }

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 Inheritance C lass B F eatures: a,b,c C lass D F eatures: a,b,d,e,f a b C lass A features: a,b c d e C lass C F eatures: 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 - Y ear of approval Conductor - A ccount 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()

Example: Basic Inheritance class 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: Overriding class 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: super class 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 Test Human sleeps Programmer sleeps

Constructors and Inheritance class Human { public Human() { System.out.println("Human"); } } class Programmer extends Human { public Programmer() { System.out.println("Programmer"); } } class Test { public static void main(String [] args) { Programmer jussi = new Programmer(); } } > java Test Human Programmer

Constructors and Inheritance Constructor allways calls the base classes constructor! When creating a constructor void Human() { } Java adds super() – call to it: void Human() { super(); // calls base classes constructor }

class Human { public Human() { System.out.println("Human"); } } class Programmer extends Human { public Programmer() { System.out.println("Programmer"); } } class Test { public static void main(String [] args) { Programmer jussi = new Programmer(); } } > java Test Human Programmer

class Human { public Human() { super(); // Java adds this! Calls base classes contructor System.out.println("Human"); } } class Programmer extends Human { public Programmer() { super(); // Java adds this! Calls base classes contructor System.out.println("Programmer"); } } class Test { public static void main(String [] args) { Programmer jussi = new Programmer(); } } > java Test Human Programmer

What the...? What base class? class Human { public Human() { // Java adds this! Calls base classes contructor super(); System.out.println("Human"); } }

Object Every class derives from a class called Object. // Java adds the extends Object too! class Human extends Object { public Human() { super(); System.out.println("Human"); } }

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 ) { System.out.println("Human"); } } class Programmer extends Human { public Programmer() { System.out.println("Programmer"); } } class Test { public static void main(String [] args) { Programmer jussi = new Programmer(); } } > javac Test.java > DOES NOT COMPILE!!! Why?

class Human { public Human( int a ) { System.out.println("Human"); } } class Programmer extends Human { public Programmer() { super(); // Java adds this and it calls constructor // Human() that does not exist.. System.out.println("Programmer"); } } class Test { public static void main(String [] args) { Programmer jussi = new Programmer(); } }

class Human { public Human( int a ) { System.out.println("Human"); } } class Programmer extends Human { public Programmer() { super(5); // Now it works: Human(int a) exists. System.out.println("Programmer"); } } class Test { public static void main(String [] args) { Programmer jussi = new Programmer(); } }

class Human { private String name public Human( String name ) { this.name = name; } } class Programmer extends Human { private int salary; public Programmer(String name, int salary) { super(name); this.salary = salary; } } class Test { public static void main(String [] args) { 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 Interface interface 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 } }

int as parameter class Exercise13 { public static void main(String [] args) { int x = 4; myMethod(x); } public static void myMethod(int a) { } }

Human parameter class Human { } class Exercise13 { public static void main(String [] args) { Human jack = new Human(); myMethod(jack); } public static void myMethod(Human a) { } }

Mammal parameter class 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 Mammal class 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? } }

Solution 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) { // 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(); } }

Object Oriented Programming with Java

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