Mpl 9 oop

What is Object Oriented Programming?
• Object-oriented programming (OOP) involves programming using
classes and objects
• OOP using two mechanisms:
• Classes
• Objects
• How OOP is different from Procedural Programming?

What is Class?
• Class can be defined in the following ways:
• A class is a template for an object
• A class is creating a user defined type
• A class is a blueprint
• A class is a template or blueprint that defines what an object’s data
fields and methods will be
• Class contain:(class members)
• Properties
• Methods

Properties:
• Properties are the attributes of the objects of a class. These are used
to differentiate one class of objects from another class.
• For example a class that represents a pen could be created with the
following attributes:
• Color of the pen
• Size of the pen
• Type of the pen

Methods:
• Methods represent behavior of the class and consist of codes that
operate on data.
• A method is a set of code which is referred to by name and can be
called (invoked) at any point in a program simply by utilizing the
method's name. Think of a method as a subprogram that acts on
data and often returns a value.
• How to define a method, lets understand with the help of simple
example.

How to define a Class in Java?
General Syntax:
class classname
{
type instance-variable1;
type instance-variable2;
type methodname1(parameter-list)
{
statements;
}
}
A Java class uses variables to
define data fields and methods
to define actions.

What is Object?
• Object is an instance of the class, you can create many instances of a
class.

A Simple Class
class Box {
double width;
double height;
double depth;
}
9

Creating Objects
There are two steps to create an object:
Method 1:
1st create an object reference variable:
Box mybox;
2nd physically create an object and assign it to the reference variable
mybox = new Box( );
Method 2:
Box mybox = new Box( );
• After executing this statement, an object of class Box will be created with
the name mybox.
10

Object Oriented Program Features
• Encapsulation:
• Combining data and methods into a single unit called class and the process is
known as Encapsulation.
• Data encapsulation is important feature of a class that provide security.
• The insulation of the data from direct access by the program is called data
hiding or information hiding.

• Inheritance:
• The process by which object of one class acquire the properties or features of
objects of another class.
• The concept of inheritance provide the idea of reusability means we can add
additional features to an existing class without Modifying it.
• This is possible by deriving a new class from the existing one. The new class
will have the combined features of both the classes.
• Polymorphism
• A Greek term means ability to take more than one form.
• Function name will be the same, but parameter list will be different
• Java provides two ways to implement polymorphism.
• Static Polymorphism (compile time polymorphism/ Method overloading)
• Dynamic Polymorphism (run time polymorphism/ Method Overriding)

Access Modifiers
• A Java access modifier specifies which classes can access a given class
and its fields, constructors and methods.
• Classes, fields, constructors and methods can have one of four
different Java access modifiers:
• private
• default (package)
• protected
• public

• private Access Modifier
• If a method or variable is marked as private then only code inside
the same class can access the variable, or call the method.
• Code inside subclasses cannot access the variable or method, nor can
code from any external class.
• Private access modifier is not allowed for classes
• default (package) Access Modifier
• The default Java access modifier is declared by not writing any access
modifier at all.
• The default access modifier means that code inside the class itself as
well as code inside classes in the same package as this class, can
access the class, field, constructor or method which the default access
modifier is assigned to.

• protected Access Modifier
• The protected access modifier provides the same access as the default access
modifier, with the addition that subclasses can access protected methods and
member variables (fields) of the superclass.
• This is true even if the subclass is not located in the same package as the
superclass.
• public Access Modifier
• The Java access modifier public means that all code can access the class, field,
constructor or method, regardless of where the accessing code is located.
• The accessing code can be in a different class and different package.

Program 1
class Box {
double width;
double height;
double depth;
}
class BoxDemo {
public static void main(String args[]) {
Box mybox = new Box();
double vol;
mybox.width = 10;
18

Program1…
mybox.height = 20;
mybox.depth = 15;
vol = mybox.width * mybox.height * mybox.depth;
System.out.println("Volume is " + vol);
}
}
19

Example 2
class Box {
double width;
double height;
double depth;
}
class BoxDemo2 {
Box mybox1 = new Box();
20

Example 2…
double vol;
mybox1.width = 10;
mybox1.height = 20;
mybox1.depth = 15;
mybox2.width = 3;
mybox2.height = 6;
mybox2.depth = 9;
21

Example 2…
// compute volume of first box
vol = mybox1.width * mybox1.height *mybox1.depth;
// compute volume of second box
vol = mybox2.width * mybox2.height * mybox2.depth;
}
}
22

Output of Example 2…
The output produced by this program is shown here:
Volume is 3000.0
Volume is 162.0
23

Adding A Method to The Box Class
Since the volume of a box is dependent upon the size of the box, it
makes sense to have the Box class compute it. To do this, we must add a
method to Box.
24

Adding A Method to The Box Class
class Box {
double width;
double height;
double depth;
void volume() {
System.out.print("Volume is ");
System.out.println(width * height * depth);
}
}
class BoxDemo3 {
25

Continue…
mybox1.width = 10;
mybox1.height = 20;
mybox1.depth = 15;
mybox2.width = 3;
mybox2.height = 6;
mybox2.depth = 9;
mybox1.volume();
mybox2.volume();
}
}
26

Returning A Value
• While the implementation of volume( ) does move the computation of a box’s
volume inside the Box class where it belongs, it is not the best way to do it.
• For example, what if another part of your program wanted to know the
volume of a box, but not display its value? A better way to implement volume(
) is to have it compute the volume of the box and return the result to the
caller.
• The following example, does just that.
27

Returning Value Program…
// Now, volume() returns the volume of a box.
class Box {
double width;
double height;
double depth;
// compute and return volume
double volume() {
return width * height * depth;
}
}
class BoxDemo4 {
double vol;
28

Returning Value Program…
// assign values to mybox1's instance variables
mybox1.width = 10;
mybox1.height = 20;
mybox1.depth = 15;
/* assign different values to mybox2's
instance variables */
mybox2.width = 3;
mybox2.height = 6;
mybox2.depth = 9;
// get volume of first box
vol = mybox1.volume();
// get volume of second box
}
} 29

Method That Takes Parameter
• Parameters allow a method to be generalized. That is, a parameterized
method can operate on a variety of data and be used in a number of slightly
different situations. To show this point, let’s use a very simple example.
Here is a method that returns the square of the number 10:
int square()
{
return 10 * 10;
}
30

Method That Takes Parameter
• While the previous method does, indeed, return the value of 10 squared, its
use is very limited.
• However, if we modify the method so that it takes a parameter, as shown next,
then we can make square( ) much more useful.
int square(int i)
{
return i * i;
}
31

Box Program
• Thus, a better approach to setting the dimensions of a box is to create a
method that takes the dimension of a box in its parameters and sets each
instance variable appropriately.
• This concept is implemented by the following program:
32

Box Program…
// This program uses a parameterized method.
class Box {
double width;
double height;
double depth;
double volume() {
}
33

Box Program…
// sets dimensions of box
void setDim(double w, double h, double d) {
width = w;
height = h;
depth = d;
}
}
34

Box Program…
class BoxDemo5 {
double vol;
// initialize each box
mybox1.setDim(10, 20, 15);
mybox2.setDim(3, 6, 9);
35

Box Program…
}
}
36

Constructor
• A constructor is a method that executes automatically whenever an object
is created.
• It has the same name as the class in which it is created.
• Constructors have no return type, not even void. This is because the
implicit return type of a class’ constructor is the class type itself.
37

Example
class Box {
double width;
double height;
double depth;
// This is the constructor for Box.
Box() {
System.out.println("Constructing Box");
width = 10;
height = 10;
depth = 10;
}
double volume() {
}
} 38

Example…
class BoxDemo6 {
// declare, allocate, and initialize Box objects
double vol;
}
}
39

Parameterized Constructors
• While the Box( ) constructor in the previous example does
initialize a Box object, but it is not very useful because all boxes
have the same dimensions.
• Actually we need a way to construct Box objects of various
dimensions. The easy solution is to add parameters to the
constructor.
40

Example Program
class Box {
double width;
double height;
double depth;
// This is the constructor for Box.
Box(double w, double h, double d) {
width = w;
height = h;
depth = d;
}
double volume() {
}
} 41

Example Program…
class BoxDemo7 {
// declare, allocate, and initialize Box objects
Box mybox1 = new Box(10, 20, 15);
Box mybox2 = new Box(3, 6, 9);
double vol;
}
} 42

Output
The output from this program is shown here:
Volume is 3000.0
Volume is 162.0
43

Mpl 9 oop

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Mpl 9 oop