Object-Oriented Programming in Java

04/22/23 Peter Cappello [email protected]

Object-Oriented Programming Object-Oriented Programming in Javain Java

mailto:[email protected]

04/22/23 [email protected]

Object-Oriented ProgrammingObject-Oriented ProgrammingOutlineOutline

IntroductionSuperclass & subclassprotected membersRelationship between superclass objects &

subclass objectsConstructors & Finalizers in subclassesComposition versus Inheritance



Object-Oriented ProgrammingObject-Oriented ProgrammingOutline ...Outline ...

Dynamic Binding & Polymorphismfinal Methods & ClassesAbstract classes & Concrete subclassesPolymorphism examplesCreating & Using InterfacesType-Wrapper Classes for Primitive Types



IntroductionIntroduction

Essential features of object-oriented programming (OOP) versus just using objects are:– inheritance - enables reuse of classes– polymorphism - enables flexible use of

classes



Superclasses & SubclassesSuperclasses & Subclasses

Reuse a class via inheritance when a new class is a refinement of it.

Examples:– Saturn is a Heavenlybody– Square is a Rectangle– Rectangle is a quadralateral– Quadralateral is a Polygon– Polygon is a shape



Human Being

HonestHuman Being

Criminal

OrganizedCriminal

Free-lanceCriminal

Burglar

Rapist

MurdererMafiaAgent

GovernmentAgent

Senator

Congressperson

IRS

BATF

DEA



protected membersprotected members

Protected members (data & methods) are accessible to all members in:– its class– its subclasses– classes in its package



Human Being

HonestHuman Being

Criminal

OrganizedCriminal

Free-lanceCriminal

Burglar

Rapist

MurdererMafiaAgent

GovernmentAgent

Senator

Congressperson

IRS

BATF

DEA

Bribe Source

Protected data



Relationship between Superclass Relationship between Superclass & Subclass Objects& Subclass Objects

An object can be referred to as a the type of its superclass.

Example: Zapem


http://www.cs.ucsb.edu/~cappello/10/lectures.html


Using Constructors & Finalizers Using Constructors & Finalizers in Subclassesin Subclasses

When constructing an object, you can invoke the superclass constructor to initialize its instance variables: – Invoke the superclass constructor, using

super([args]); as the first statement of the constructor.

Example: Saturn.java If you do not invoke super(…), Java run-time invokes

the no-argument superclass constructor to initialize its variables.



Composition versus InheritanceComposition versus Inheritance

Composition: has a relationInheritance: is a relationOddly, the Point, Circle example in the

text seems precisely off the point (a Circle is not a Point):– The attributes of a Circle are its center and

its radius: it has a: • center (a Point)• radius (a double)



Point & CirclePoint & Circle

public class Point {// Instance variablesprivate double x, y; // coordinates of the Point// Methodspublic Point(double X, double Y) { setPoint(X, Y); }public void setPoint(double X, double Y) { x = X; y = Y; }public double getX() { return x; }public double getY() { return y; }public String toString() { return “[“ + x + “, “ + y + “]”; }

}



Point & Circle ...Point & Circle ...public class Circle {

// Instance variables: a circle has a ...private Point center; // center of the Circleprivate double radius; // radius of the Circle// Methodspublic Circle(double x, double y, double r) { center = new Point(x, y);radius = r;}public double getRadius() { return radius; }public double area() { return Math.PI*radius*radius; }

}



Dynamic Binding & PolymorphismDynamic Binding & Polymorphism

Consider the statement:objectname.methodname([args])

When this statement executes, it: – detects the type of object that objectname

actually refers to;– invokes the appropriate version of the

methodname method.Look at the draw method in the Zapem

example.



final Methods & Classesfinal Methods & ClassesIf a method is declared final, it cannot

be overridden in a subclass.A private method is implicitly final.A static method is implicitly final.If a class is declared final: – it cannot be subclassed (i.e., extended);– all its methods are implicitly final.



Abstract Classes & Concrete Abstract Classes & Concrete SubclassesSubclasses

An abstract class is a class that is: – designed to be extended, – not designed to construct objects.

An example of such a class is our Roach class: An actual roach is either a Rectangle or an Oval.

You cannot construct objects of an abstract class.



Abstract Classes & Concrete Abstract Classes & Concrete Subclasses ...Subclasses ...

You declare a class abstract by using the abstract keyword.

The purpose of doing so is to provide: – interface: method names & signatures– [implementation]: implement some methods

for its extensions.Please see modified Zapem example.



Polymorphism examplesPolymorphism examples

ZapemSolarSystem - Please see use of Heavenly

body class, and draw & move methods.We can add new Roach subclasses &

HeavenlyBody classes without recompiling the zapem & SolarSystem Applets.



Creating & Using InterfacesCreating & Using InterfacesInterface isn’t just an idea, it’s a keyword!An interface is used instead of a[n abstract]

class when: – there is no implementation to inherit– the class already extends another class.

A modified Zapem uses an interface.You also have seem their use in

ActionListener and MouseListener



Type-Wrapper Classes for Type-Wrapper Classes for Primitive TypesPrimitive Types

Each primitive data type has a corresponding wrapper class that has the same name but with the 1st letter capitalized

Integer, Double, Boolean, …These classes allow you to treat their

objects as objects:– refer to them polymorphically– pass them call-by-reference to methods.



Type-Wrapper Classes for Type-Wrapper Classes for Primitive TypesPrimitive Types

Each defines some methods. E.g.:– int Integer.parseInt(String)– Double valueOf(String)– String toString()

Look in the java.lang package of the Java Core API Specification to find out more about these methods.


Documents

Object-Oriented Programming in Java