Object Orientated Programming with Java Jussi Pohjolainen Tampere University of Applied Sciences
Java OO Revisited
Nov 11, 2014
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Object Orientated Programming with Java
Jussi Pohjolainen Tampere University of Applied Sciences
Object Orientated Concepts
• Class • Object • Inheritance • Constructors • Abstract class • Interface • Polymorphism
Class class Student {
private String name;
public Student(String name) {
this.name = name;
}
public void setName(String name) {
this.name = name
}
public void getName() {
return name;
}
}
CreaAng objects
Student jack = new Student(“Jack”);
Student bill = new Student(“Bill”);
System.out.println( jack.getName() );
System.out.println( bill.getName() );
Reference?
Student jack = new Student(“Jack”);
Student bill = jack;
jack.setName(“Lisa”);
// What is the output?
System.out.println(bill.getName());
Reference
• In Java, every object is passed by reference • If you want to clone a object, you use special techniques (later on the material)
Inheritance
class Person {
private String name;
...
}
class Student extends Person {
private int id;
...
}
Constructors class Person {
private String name;
public Person() {
System.out.println(“Person”);
}
}
class Student extends Person {
private int id;
public Student() {
System.out.println(“Student”);
}
}
// What is the output?
Student s = new Student();
Default Constructor
• If programmer does not define a constructor, Java creates a default constructor: class Person {
}
=>
class Person {
public Person() {
super();
}
}
Default Constructor
class Person {
private String name;
public Person() {
System.out.println(“Person”);
}
}
class Student extends Person {
private int id;
}
// What is the output?
Student s = new Student();
Default Constructor Problem
class Person {
private String name;
public Person(String name) {
System.out.println(“Person”);
}
}
class Student extends Person {
private int id;
}
// What is the output?
Student s = new Student();
Abstract Class
• You cannot create a object from abstract class • Abstract class may contain abstract method
• Abstract method is a method declaraAon which must be implemented in inherited classes
Abstract Class
abstract class Graphic {
abstract double calculateSurfaceArea();
}
class Circle extends Graphic {
private int radius;
double calculateSurfaceArea() {
...
}
}
Abstract Class
abstract class A {
abstract void m();
}
abstract class B extends A {
// What is the implementation of the class B?
}
Interface
• Interface is a abstract class that contain only abstract methods
• Interface can contain also public staAc final variables
• Class can inherit only one class, but it can implement many interfaces
Interface
interface class A {
public void m();
}
class B implements A {
public void m() {
...
}
}
Polymorphism
• Declaring a object: – Graphic c;!
• IniAalizing the object: – c = new Graphic();!
• This is also possible: – c = new Circle();!– c = new Rect();!
• If Circle and Rect are inherited from Graphic!
Polymorphism
class Polymorphism {
public static void main(String [] args) {
// What are the possible objects to be passed?
method(??)
}
public static void method(Graphic c) {
...
}
}
Polymorphism
interface R {
...
}
class Polymorphism {
public static void main(String [] args) {
// What are the possible objects to be passed?
method(??)
}
public static void method(R r) {
...
}
}
Cloning
class Person implements Cloneable {
private name;
public Person(String name) {
this.name = name;
}
public Object clone() {
return new Person(this.name);
}
}
Person a = new Person(“jack”);
Person b = a.clone();
Equals
// What happens here?
Student jack1 = new Student(“Jack”);
Student jack2 = new Student(“Jack”);
System.out.println( jack1 == jack2 ); // true or false?
System.out.println( jack1.equals(jack2) ); // ?
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