Lecture 3

1. Lecture 3 Casting Abstract Classes and Methods Interfaces

2. Object Bicycle MountainBike RacingBike Casting A class hierarchy Q: Mountainbike is a ? of bicycle? Q: what java keyword is used to do this hierarchy? Q:Does the Bicycle class use this keyword ?

3. Casting and object types • When you create an object, its type is the class from which is was instantiated..e.g. .. • myBike is a MountainBike (And it’s also is a Bicycle and an Object) • (Why an object?) • But, a Bicycle may be a MountainBike but not necessarily… • (could be a RacingBike) MountainBike myBike = new MountainBike();

4. Object Bicycle MountainBike RacingBike Casting and object types • Casting allows the use of an object of one type in place of another type (down the hierarchy) Bicycle yourBike = new MountainBike(); myBike= (MountainBike) yourBike; **OK** Cast

5. You can check the type of an object.. if (yourBike instanceof MountainBike) { // Cast MountainBike newBike = (MountainBike)yourBike; }

6. Casting • Why are we looking at casting/object types? • Because it’s used in GUI programming • Used to check which GUI element was clicked etc… More later!

7. Abstract Class • An abstract class represents a generic concepte.g. Shape, Food, Fruit, … • it encapsulates the features that all kinds of elements of the concept have, e.g. Shapes have area, circumference • these features are implemented as abstract methods e.g. area(), circumference() • abstract methods cannot have an implementation meaningless at the conceptual, generic level

8. Abstract Classes • An abstract class or method is defined by the keyword abstract • abstract class Shape { … abstract double area(); abstract double cicumference(); …}; • Any class with an abstract method is automatically abstract and must be declared as such Note: no body

9. Abstract Class • An abstract class cannot be instantiated • A subclass of an abstract class can be instantiated only if it overrides each of the abstract methods of its superclass and provides an implementation for all of them • this subclass is known as a concrete class • If a subclass of an abstract class does not implement all the abstract methods it inherits, the subclass is itself abstract • (Note: static, final and private methods cannot be abstract as they cannot be overridden)

10. Abstract Class • An abstract class effectively defines a complete programming interface – providing its subclasses with the method declarations for all of the methods necessary to implement that programming interface. • i.e. It defines what methods the subclass is going to have to implement… • Note: an abstract method in Java is like a pure virtual function in C++

11. Abstract Class Example See CODE • public abstract class Shape • { public abstract double area(); // note no body}

12. Implementing the abstract class • __-__________________________________________-__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ • Public class Circle extends Shape • { • // put in attributes needed here... • public Circle (double radius) • { • // constructor code.. } public double getRadius() • { // code... } public double area() • { //implement abstract method // Code for area calculation... }}

13. Implementing the abstract class • class Rectangle extends Shape{ • // put in attributes needed here... • // constructor public Rectangle (double width,double height) • { // constructor code.. • } public double area() • { • //implement area abstract method... } // other methods ..etc • }

14. Using the Shape Objects Shape[] shapes = new Shape[3]; //setup array of shapes // then.. Instantiate the various shapes to be used..circle, square etc. // then.. Calculate the total area of the shapes.. double totalArea=0; // compute total area for (inti=0; i<shapes.length; i++){ totalArea+=shapes[i].area(); • Dynamic method lookup is used • the area of a circle is computed using the method defined by Circle class • the area of a rectangle is computed using the method defined by the Rectangle class

15. Interfaces in java

16. Interfaces • Consider another type of shape, one where the centre point of the shape is known • We want CenteredCircle to support area() and circumference() methods already defined without re-implementing the methods  Problem, no multiple inheritance in java abstract class CenteredShape { // instance fields double x; double y;} class CenteredCircle extends CenteredShape {…} etc…

17. Interfaces • Solution  an interface • An Interface is a simple a set of methods in java • Use the type Interface instead of class in the java source code… public interfacemyInterface { //method declarations ...}

18. Interface • An interface defines a protocol of behaviour that can be implemented by any class anywhere in the class hierarchy. • It defines a set of methods but does not implement them. • A class that implements the interface agrees to implement all the methods defined in the interface, thereby agreeing to certain behaviour.

19. Some Interface Rules • an interface contains no implementation.. Just the method names/signatures • the methods of an interface are implicitly abstract (but abstract qualifier not needed) • the only fields allowed in an interface are constants (i.e. declared static final) • all methods of an interface are implicitly public • an interface cannot be instantiated  no constructor

20. Interface Example • The interface Centered is defined as • It defines the methods that a Shape subclass should implement if it knows the x,y coords of its center point public interface Centered { public void setCenter(double x,double y); public double getCenterX(); public double getCenterY();}

21. Implementing an Interface • A class uses the keyword implements to implement one or more interfaces public class className implements interfaceName { // class field definitions ... // class method definitions ... // interface method implementations ...} public class class1 implements interface1,interface2{...}

22. Implementing an Interface public class CenteredCircleextends Circle implementsCentered { // field defns double cx; //center x coord double cy; //center y coord // constructor public CenteredCircle(double cx, double cy, double radius){ super(radius); this.cx=cx; this.cy=cy; } Since the class CenteredCircle implements the circle interface, what extra methods must the CentredCircle class include?

23. Implementing an Interface // implementations for the interface methods public void setCenter(double x, double y){ cx=x; cy=y; } public double getCenterX(){ return cx; } public double getCenterY(){ return cy; }

24. Interfaces.. A bit more… • Before writing a class definition, determine the public interface • the set of services offered to users of the class • Java allows us to take this one stage further, by formally recording the interface as a Java interface • a java interface is just a collection of abstract methods (i.e. we state the signatures, but not the bodies)

25. Using an Interface • Where a class implements an interface instances of the class can be treated as objects of the interface type as well as objects of their own class type • E.g. instances of CenteredCircle and CenteredRectangle can also be treated as - instances of Shape as they extend Shape - instances of Centered as they implement Centered

26. Quiz…what did you absorb? • Explain in your own words what an interface is? Why is it needed in Java? • Can an interface implement its methods? Can an abstract class? • Can an interface be implemented by classes from different hierarchies? (e.g. a Bicycle class, and a Circle class? • Can a class have more than one superclass? • Can a class implement more than one interface? Stopped here

27. Abstract Classes vs Interfaces • Differences between abstract classes and interfaces • An interface cannot have any implementation, whereas an abstract class can. • An interface is not part of the class hierarchy. Unrelated classes can implement the same interface. • A class can implement many interfaces but can have only one superclass.