OOP 02

CLASSES, OBJECTS, VARIABLES, AND METHODS IN JAVAclass followed by a class name and braces surrounding the declaration and the data of objects of the class... CLASSES, OBJECTS, VARIABLES,

CHAPTER 2

Fundamentals of Object Orientation

In this chapter, we review:

(OO) programming in order to understand

how such programming is different from

nonobject-oriented programming

applied to elegant software design

inheritance

Unified Modeling Language (UML) class

diagrams

options (e.g., public and private) of data,

methods, and classes, as they apply to Java

OOP VS NON–OOP

Non OOP:

• A program is usually process-oriented or data-oriented

In such programs, there are typically data globally

available and procedures globally available

• The main program, or its subprograms, are in control and manipulate the data.

• The main program, through its subprograms, has all the

“intelligence” or behavior in the program and the data has no intelligence

• The main program and its subprograms are responsible

OOP VS NON–OOP

OOP:

• A program is partitioned into a set of

communicating objects Each object encapsulates all the behavior and knowledge relating to one

concept

• When an object needs something from another object, it sends it a message to the other object, which then performs some action and possibly

OOP VS NON–OOP

typically creates a few objects and start them communicating with each other

Ex: Windows, menus, buttons are objects

objects share the work and the

responsibilities, should seem familiar in that

it is the way humans typically interact with

each other

OOP VS NON–OOP

Object-Oriented Languages

• We call a programming language object-oriented

if the language supports classes, objects,

messages, inheritance, and (subtype)

polymorphism.

• In an OO programming language, classes can be viewed as templates for objects that describe a certain type of behavior or a certain set of

OOP VS NON–OOP

Object-Oriented Languages

of a class An object’s associated class defines the type of data the object maintains and its behavior or responsibilities toward that data

(their own state) to maintain

OOP VS NON–OOP

Object-Oriented Languages

• The way objects communicate and get each other

to perform some action is by sending messages

to each other

• By sending a message to another object, the first object causes the second object to execute some code That code is actually a procedure—which in object-oriented languages is called a method —

OOP VS NON–OOP

Advantages of OO Programming

objects Therefore, it is easier to keep things in

small manageable units and to understand

how the units affect each other

OOP VS NON–OOP

Advantages of OO Programming

structure of the global data in a non-OO

that access that data

little global data and instead stores the data in

OOP VS NON–OOP

Advantages of OO Programming

• If a programmer decides that a particular object

is working too inefficiently, the programmer can redesign the object’s behavior to be more

efficient without affecting the rest of the system, thus supporting the maintainability of the

software

• Since each object has typically one small

well-defined role and carries the data it needs with it,

it is usually easier to reuse these objects in other situations.

OOP VS NON–OOP

Advantages of OO Programming

readability, reusability, and maintainability of the software

CLASSES, OBJECTS, VARIABLES, AND METHODS IN JAVA

class followed by a class name and braces

surrounding the declaration and

the data of objects of the class

CLASSES, OBJECTS, VARIABLES, AND METHODS IN JAVApublic class Person

{

private String name;

private Date birthdate;

public Person(String name, Date birthdate)

{

this.name = name;

this.birthdate = birthdate;

}

CLASSES, OBJECTS, VARIABLES, AND METHODS IN JAVApublic String getName()

CLASSES, OBJECTS, VARIABLES, AND METHODS IN JAVA

construct a Person object using the

constructor, as in

Person firstPerson =

new Person(“Adam”, new Date(0));

and then send the object a message, such as

String firstPersonName =

CLASSES, OBJECTS, VARIABLES, AND METHODS IN JAVA

• In order to execute their methods properly, most objects need to store data This data is stored in

instance variables in Java.

• Instance variables differ from local variables They exist and store data during the whole life of the object.

• In Java, only variables of a primitive type actually store their data in the variable For all variables of

an object type, the variables store a reference to the data.

CLASSES, OBJECTS, VARIABLES, AND METHODS IN JAVA

most objects need to store data This data is

of the object

CLASSES, OBJECTS, VARIABLES, AND METHODS IN JAVA

actually store their data in the variable For all variables of an object type, the variables store

a reference to the data

CLASSES, OBJECTS, VARIABLES, AND METHODS IN JAVA

• The methods of a class correspond to

messages that objects can send to an object of this class These methods correspond to the

behavior of objects of this class They can also

this class can provide for other objects

CLASS METHODS AND VARIABLES IN JAVA

methods

an object of the class and class variables can

CLASS METHODS AND VARIABLES IN JAVA

constants In Java, a constant is indicated by

public final static int C = 299792458;

CLASS METHODS AND VARIABLES IN JAVA

• Class methods can be thought of as methods that are not a form of message passing to objects of that class and instead can be invoked

independently of any objects of the class

• In general, class methods are useful when objects

of that class are stateless (i.e., have no instance variables) or when some of the methods do not use the state of the objects and instead merely manipulate the data passed in as parameters

CLASS METHODS AND VARIABLES IN JAVA

Math class

double y = Math.sin(x);

defining a Set class There are (at least) two

public Set intersect(Set otherSet)

CLASS METHODS AND VARIABLES IN JAVA

• Usages:

Set intersection = s1.intersect(s2);

or

Set intersection = Set.intersect(s1, s2);

CLASS METHODS AND VARIABLES IN JAVA

• Class methods can be thought of as methods that are not a form of message passing to objects of that class and instead can be invoked

independently of any objects of the class

• In general, class methods are useful when objects

of that class are stateless (i.e., have no instance variables) or when some of the methods do not use the state of the objects and instead merely

INTRODUCTION TO UML CLASS

DIAGRAMS

be very helpful

called the Unified Modeling Language for such diagrams

UML 2.0, among which are class diagrams,

state diagrams, and sequence diagrams

INTRODUCTION TO UML CLASS

DIAGRAMS

modeling language For ex:

INTRODUCTION TO UML CLASS

DIAGRAMS

interfaces and the relationships between

them

classes and relationships rather than a

dynamic view of the interactions among

objects of those classes

three sections

INTRODUCTION TO UML CLASS

DIAGRAMS

• A class is represented by a box divided into three sections

– The top section gives the name of the class

– The middle section gives the attributes or properties held by objects of the class These properties are

abstractions of the data or state of an object and so are usually implemented using instance variables

– The bottom section gives the operations of a class,

CLASS METHODS AND VARIABLES IN JAVA

IMPLEMENTATION INHERITANCE

programming is “implementation inheritance”

or “subclassing,” which greatly increases the

reusability of classes and also minimizes the duplication of code

specialization and generalization

IMPLEMENTATION INHERITANCE

Specialization

Creating a drawing program in which

rectangles can grow, shrink, or move around

on a panel under the control of the user

stores the relevant information about the

rectangle, such as its size and position

IMPLEMENTATION INHERITANCE

Specialization

satisfying our needs

setCenter(int x, int y) method and Rectangle

minimal effort?

IMPLEMENTATION INHERITANCE

Specialization

1 We could modify the class (add methods)

2 We could copy the Rectangle class code and

insert it into a new class EnhancedRectangleand then add the new code

to take, but not in either of these ways Both

of these approaches have inelegant aspects

IMPLEMENTATION INHERITANCE

Specialization

works if only the compiled code, and not the source code, for the Rectangle class is

available

inheritance can be used

IMPLEMENTATION INHERITANCE

Specialization

• Java (and other OO languages) allows the

programmer to define one class as a subclass of another class (which makes the second class a

superclass of the first class)

• A subclass inherits all the features (all the

variables and methods, but not the constructors)

of the superclass, which means all those features are automatically included in the subclass and, if not declared private, are accessible in the

subclass

IMPLEMENTATION INHERITANCE

Specialization

subclass to the superclass is

IMPLEMENTATION INHERITANCE

public class EnhancedRectangle extends Rectangle {

public EnhancedRectangle(int x, int y, int w, int h)

IMPLEMENTATION INHERITANCE

Specialization

• This declaration makes EnhancedRectangle a

subclass of Rectangle and makes Rectangle a

superclass of EnhancedRectangle Because it is a subclass, the new EnhancedRectangle class

inherits all the methods and all the data in the

Rectangle class

• Note that since constructors are not inherited ,

IMPLEMENTATION INHERITANCE

Specialization

IMPLEMENTATION INHERITANCE

Specialization

class can use it as follows:

EnhancedRectangle rectangle = new

EnhancedRectangle(1, 2, 50, 60);

rectangle.setLocation(10, 10);

//inherited method

IMPLEMENTATION INHERITANCE

Specialization

• Note that EnhancedRectangle objects behave as

if all methods inherited from the Rectangle class

had been defined in their class.

• In this way, subclassing provides a way to reuse the code and data of an existing class to create a

new class that is identical except that it has more features (data and/or behavior)

• This process of extending an existing class by

adding new features is called using inheritance for specialization.

IMPLEMENTATION INHERITANCE

The Object Superclass in Java

• All Java classes that do not explicitly extend

another class implicitly extend the Object class

• Therefore, all Java classes extend the Object class

either directly or indirectly via one or more

intermediate classes in an inheritance chain

• This means that your classes will automatically inherit the methods in the Object class: clone,

IMPLEMENTATION INHERITANCE

Specialization

program using the Java Swing package

Suppose that the program allows the user to

borders and stores all drawn ovals in a

collection of Oval objects

how to draw themselves:

return new Point(x,y); } // other methods

}

IMPLEMENTATION INHERITANCE

to enhance the program to allow the user to

IMPLEMENTATION INHERITANCE

Specialization

job done in an elegant way?

(How many ways? Which way is best?)

IMPLEMENTATION INHERITANCE

Specialization

1 She could ignore the Oval class and design her own

FilledOval class that she implements from scratch.

2 She could realize that the Oval is exactly what she

needs except that Ovals do not have black interiors when drawn, and so she could copy and paste the

Oval class source code into a new FilledOval class and then make small changes to it to change the way it

draws itself.

3 She could use implementation inheritance and

IMPLEMENTATION INHERITANCE

Specialization - overriding

methods and data in the Oval class

FilledOval to inherit all the behavior of Oval She wants her class to have a slightly different implementation of the same draw method

implementation of draw

IMPLEMENTATION INHERITANCE

public class FilledOval extends Oval {

public FilledOval(int x, int y, int w, int h) {

super(x, y, w, h);

} public void draw(Graphics g) {

g.fillOval(x, y, w, h);

}

IMPLEMENTATION INHERITANCE

Specialization

didn’t want to enhance the Oval class by

adding new attributes or behavior, but instead

slightly differently

IMPLEMENTATION INHERITANCE

Generalization

keeps track of all its current customers

customers and business customers, the

designers of the billing system created two

unrelated classes, BusinessCustomer and

HumanCustomer

IMPLEMENTATION INHERITANCE

Generalization

keeps track of all its current customers,

including human customers and business

customers

IMPLEMENTATION INHERITANCE

Generalization

becauseboth of these classes might have

some similar data and some identical

methods

introduce a new Customer class that is a

superclass of both the BusinessCustomer and HumanCustomer classes

IMPLEMENTATION INHERITANCE

Generalization

IMPLEMENTATION INHERITANCE

Generalization

want the user to create any Customer objects, since that class has been created merely as a place to put common features to avoid

duplication One way to prevent creation of

Customer objects is to declare the class

“abstract.”

String humanName = human.getName();

String businessName = business.getName();

IMPLEMENTATION INHERITANCE

Generalization

be used to avoid code and data duplication

among several classes by creating a superclass

to those classes and moving the duplicate

code and data up into that superclass, which is

commonly abstract

generalization

IMPLEMENTATION INHERITANCE

Single Inheritance in Java

IMPLEMENTATION INHERITANCE

Multiple Inheritance might cause ambiguity

TYPES, SUBTYPES, AND INTERFACE INHERITANCE

Type

operations that can be performed on them

– Primitive types.

– Classes.

– Interfaces.

TYPES, SUBTYPES, AND INTERFACE INHERITANCE

Interface

of operations All objects whose classes

set of objects of that type Ex:

public interface Runnable

{

public void run();

}

TYPES, SUBTYPES, AND INTERFACE INHERITANCEInterface

• The set of objects of type Runnable consists of all objects of all classes that implement Runnable.

public class SimpleRunner implements Runnable

{

public void run() {

System.out.println(“I’m running.” ); }

TYPES, SUBTYPES, AND INTERFACE INHERITANCEInterface

• The class

SimpleRunner defines

and implements a

method run() of the form

required by the Runnable

interface and the class

explicitly declares that it

“implements Runnable.”

TYPES, SUBTYPES, AND INTERFACE INHERITANCE

Interface

inherit from them similar to the way

inheritance works with classes

public interface Movable extends Runnable {

public void walk();

TYPES, SUBTYPES, AND INTERFACE INHERITANCE

Polymorphism

a variable of one type to store an object of a subtype

Runnable r = new SimpleRunner();

legally used wherever an object of a supertype

TYPES, SUBTYPES, AND INTERFACE INHERITANCE

important concepts in object orientation and

is one of the main things that makes OO

languages so useful

extensively In fact, the Java 1.5 API includes over 1000 interfaces Everywhere one of these Java interfaces is used, polymorphism is also

The Value of Polymorphism

program

java.util.LinkedList class extensively as the

collection class in a large software system

uses of the LinkedList class with a different

collection class such as the ArrayList class

The Value of Polymorphism

require significant changes to the system

with such a replacement in mind, you would have to make a lot of changes

work, but this whole process might need to be repeated again (if it is later found that