Java OO Revisited
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Here are the key points about object equality in Java: - The == operator checks if the objects are the same instance - The equals() method checks if the objects have the same value - By default, equals() only returns true if comparing the same instance - Classes should override equals() to check for value equality instead of reference equality So in this case: - jack1 == jack2 will print false as they are different instances - jack1.equals(jack2) will print false by default, as equals() isn't overridden
![Polymorphism
class Polymorphism {
public static void main(String [] args) {
// What are the possible objects to be passed?
method(??)
}
public static void method(Graphic c) {
...
}
}](https://image.slidesharecdn.com/00oorevisited-1218053794137089-8/85/Java-OO-Revisited-18-320.jpg)
![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) {
...
}
}](https://image.slidesharecdn.com/00oorevisited-1218053794137089-8/85/Java-OO-Revisited-19-320.jpg)
Java OO Revisited
- 1. Object Orientated Programming with Java Jussi Pohjolainen Tampere University of Applied Sciences
- 2. Object Orientated Concepts • Class • Object • Inheritance • Constructors • Abstract class • Interface • Polymorphism
- 3. 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; } }
- 4. CreaAng objects Student jack = new Student(“Jack”); Student bill = new Student(“Bill”); System.out.println( jack.getName() ); System.out.println( bill.getName() );
- 5. Reference? Student jack = new Student(“Jack”); Student bill = jack; jack.setName(“Lisa”); // What is the output? System.out.println(bill.getName());
- 6. Reference • In Java, every object is passed by reference • If you want to clone a object, you use special techniques (later on the material)
- 7. Inheritance class Person { private String name; ... } class Student extends Person { private int id; ... }
- 8. 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();
- 9. Default Constructor • If programmer does not define a constructor, Java creates a default constructor: class Person { } => class Person { public Person() { super(); } }
- 10. 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();
- 11. 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();
- 12. 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
- 13. Abstract Class abstract class Graphic { abstract double calculateSurfaceArea(); } class Circle extends Graphic { private int radius; double calculateSurfaceArea() { ... } }
- 14. Abstract Class abstract class A { abstract void m(); } abstract class B extends A { // What is the implementation of the class B? }
- 15. 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
- 16. Interface interface class A { public void m(); } class B implements A { public void m() { ... } }
- 17. 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!
- 18. Polymorphism class Polymorphism { public static void main(String [] args) { // What are the possible objects to be passed? method(??) } public static void method(Graphic c) { ... } }
- 19. 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) { ... } }
- 20. 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();
- 21. 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) ); // ?