CS111: Programming Language II · Every object can access a reference to itself with keyword this. Implicitly: refer to the object’s instance variables and other methods inside

CS111: PROGRAMMING

LANGUAGE II

Lecture 1: Introduction to classes

Lecture Contents

Dr. Amal Khalifa, 2014

What is a class?

Encapsulation

Class basics:

Data

Methods

Objects

Defining and using a class

2

In Java

Java is an object-oriented programming language

As the term implies, an object is a fundamental entity in a Java program

Everything must be in a class (Even Main method).

3


What is a class?

Class:

Defines a data type (blueprint)

Combines data and operations in

one place:

data / fields/ properties / state

variables

Operations/ behavior / methods that

modify state


4

What is a data type?

Any data type includes

Data (range of values)

Operations (that can be performed on data)

Example: int data type has: Data: +-32,767 Operations: +, - , * , / , %

Same with classes

Collection of data values together with set of basic operations to be allowed for the values

6-5 Dr. Amal Khalifa, 2014

Class = Blueprint

One blueprint to create several similar, but different,

houses:

Copyright © 2012 Pearson Education,

Inc.

• The bundling of data and procedures(methods) into a single unit(called class).

• Class : various data elements and member functions are wrapped up together.

• main feature of object oriented programming

Encapsulation 7


http://www.defit.org/2012/01/data.html

http://www.defit.org/2012/02/class.html

http://www.defit.org/2012/02/class.html

http://www.defit.org/2012/02/object-oriented-programming-oop.html

Objects First with Java - A

Practical Introduction using BlueJ,

© David J. Barnes, Michael Kölling

Objects & Classes

objects

represent ‘things’ from the real world, or from some

problem domain (example: “the red car down there in

the car park”)

classes

represent all objects of a kind (example: “car”)

Objects & Classes

variables of class type are objects

Object:

an instance / realization of a class

same as relationship between a dataType and its

variables

9


How to define a Class??


10

Type definition

Object declaration

Member access

Objects First with Java - A

Practical Introduction using BlueJ,

© David J. Barnes, Michael Kölling

Basic class structure

public class ClassName

{

Fields

Constructors

Methods

}

The contents of a class

Defining Classes


12

Identifying Class Attributes Classes have attributes (data) and operations (behaviors).

Class attributes are implemented in Java programs as fields, and class

operations are implemented as methods.

the Unified Modeling Language (UML)


13

can be defined as a modeling language to capture

the architectural, behavioral and structural aspects

of a system.

The UML has an important role in object oriented

analysis and design.

Objects are the key to this object oriented world

The basic requirement of is to identify the object

efficiently.

UML Class diagrams


14

Class diagrams describes the objects in a system

and their relationships, so it is widely used by the

developer community.

The UML representation of a class is a rectangle

containing three compartments stacked vertically.

The top compartment shows the class's name.

The middle compartment lists the class's attributes.

The bottom compartment lists the class's operations.

What about a student??

Examples 15


The student class - Exercise


16

Draw UML class diagram for class

named Student where data is:

name, UnivID, date of birth,

department, level, GPA

Methods are:

PrintData()

A class in Java


17

.java file-

name extension

The class

keyword

Public

keyword

access modifier

class is

“available to

the public

No main

can’t execute

// Student.java

public class Student {

// data..

//methods...

//not static!!

}

Student Test


18

Another class.

Why not

Import

student.java??

Using new

object

instantiation

Dot separator

// StudentTest.java

public class Students {

public static void main (String[] args)

{

//define object(s)

//call a method

}

Access modifiers

Controlling Access to Members

Computer Science Department

19

Access modifiers

Public Methods

Client’s view of the services provided by the class

Private data

not accessible outside the class

protected.

Public and Private Members

Data in class almost always designated private in

definition!

Upholds principles of OOP (Data Hiding)

Hide data from user

Allow manipulation only via operations

Which are member methods

6-20 Dr. Amal Khalifa, 2014

Marks for UML-supported visibility types 21


Mark Visibility type

+ Public

# Protected

- Private

~ Package

Example


22

. Modify class

efinition

// Student.java


// data private

//methods public

}

Student Test


23

Try to access

private

members!!

What about

the public

ones ?

// StudentTest.java

public class StudentTest {


{

//define object(s)

//access different members

}

Set/get methods

Computer Science Department

24

• Object needs to “have access" to its data

• accessor methods

• Allow object to read data

• Also called "get methods"

• Simple retrieval of member data

• Mutator methods

• Allow object to change data

• Also called “set methods"

• Manipulated based on application

Example: set/get methods


25

. Class

Methods have

direct access

to class

members

(data or

methods)

// Student.java


// data

//methods

//set methods

//get methods

}

Student Test


26

The set

method can

be used to

modify a data

field

the get

method can

be used to

display the

updated

value

// StudentTest.java



{

//define object(s)

//modify data!!

}

Constructors


27

A constructor can be used to initialize an object of a

class when the object is created.

Example: constructors


28

. Constructor

must be public

Same name

as class

no return type

Parameters??

// Student.java


// data

//an empty constructor

public Student()

{

}

//set/get methods

//other methods

}

Student Test


29

Keyword new

calls the

class’s

constructor to

perform the

initialization.

// StudentTest.java



{

//define object(s)using empty

//constructor

//print data

}

The default Constructor

By default, the compiler provides a default

constructor with no parameters in any class that

does not explicitly include a constructor.

If you declare any constructors for a class, the Java

compiler will not create a default constructor for that class

With the default constructor, its instance variables

are initialized to their default values.


30

Overloaded Constructors


31

Overloaded constructors

enable objects of a class to be initialized in different

ways.

multiple constructor declarations with different

signatures.

the compiler differentiates signatures by :

the number of parameters,

the types of the parameters and

the order of the parameter types in each signature.



32

. Define more

Constructors

Parameters??

// Student.java


// data

//constructors

public Student()

{

}

//set/get methods

//other methods

}

Student Test


33

More objects

Different

initialization

Same

methods,

different data

// StudentTest.java



{

//define object(s)using different

//constructors

//print data

}

Using this Reference


34

Every object can access a reference to itself with keyword this.

Implicitly:

refer to the object’s instance variables and other methods inside member methods.

Enables the class’s code to know which object should be manipulated.

Explicitly:

Can also use keyword this in a non-static method’s body.



35

. Use this

reference to

access class

members

// Student.java


// data

//constructors

public Student()

{

}

//set/get methods

//other methods

}

Important hint 36


Be careful!! 37


Text Book:

Chapter 3

That’s all for today…..


38

Documents

CS111: Programming Language II · Every object can access a reference to itself with keyword this. Implicitly: refer to the object’s instance variables and other methods inside