Java sem i

Prof. Priyanka V. Barhate
T.Y.Bsc (computer Science )

 Introduce to classes and objects in Java.
 Understand how some of the OO concepts learnt so far
are supported in Java.
 Understand important features in Java classes.

 Java is a true OO language and therefore the underlying
structure of all Java programs is classes.
 Anything we wish to represent in Java must be encapsulated
in a class that defines the “state” and “behaviour” of the
basic program components known as objects.
 Classes create objects and objects use methods to
communicate between them. They provide a convenient
method for packaging a group of logically related data items
and functions that work on them.
 A class essentially serves as a template for an object and
behaves like a basic data type “int”. It is therefore important
to understand how the fields and methods are defined in a
class and how they are used to build a Java program that
incorporates the basic OO concepts such as encapsulation,
inheritance, and polymorphism.

 A class is a collection of fields (data) and methods
(procedure or function) that operate on that data.
Circle
centre
radius
circumference()
area()

 A class is a collection of fields (data) and methods (procedure
or function) that operate on that data.
 The basic syntax for a class definition:
 Bare bone class – no fields, no methods
public class Circle {
// my circle class
}
class ClassName [extends
SuperClassName]
{
[fields declaration]
[methods declaration]
}


Add fields

The fields (data) are also called the instance varaibles.
public double x, y; // centre coordinate
public double r; // radius of the circle
}

 A class with only data fields has no life. Objects
created by such a class cannot respond to any
messages.
 Methods are declared inside the body of the class
but immediately after the declaration of data fields.
 The general form of a method declaration is:
type MethodName (parameter-list)
{
Method-body;
}

public double x, y; // centre of the circle
public double r; // radius of circle
//Methods to return circumference and area
public double circumference() {
return 2*3.14*r;
}
public double area() {
return 3.14 * r * r;
}
}
Method Body

 Declare the Circle class, have created a new data type –
Data Abstraction
 Can define variables (objects) of that type:
Circle aCircle;
Circle bCircle;

 aCircle, bCircle simply refers to a Circle object, not an
object itself.
aCircle
Points to nothing (Null Reference)
bCircle
Points to nothing (Null Reference)
null null


Objects are created dynamically using the new keyword.

aCircle and bCircle refer to Circle objects
bCircle = new Circle() ;aCircle = new Circle() ;

Creating objects of a class
aCircle = new Circle();
bCircle = new Circle() ;
bCircle = aCircle;

Creating objects of a class
aCircle = new Circle();
bCircle = new Circle() ;
bCircle = aCircle;
P
aCircle
Q
bCircle
Before Assignment
P
aCircle
Q
bCircle
Before Assignment

 The object does not have a reference and cannot
be used in future.
 The object becomes a candidate for automatic garbage
collection.
 Java automatically collects garbage periodically and
releases the memory used to be used in the future.
Q

 Similar to C syntax for accessing data defined in a
structure.
Circle aCircle = new Circle();
aCircle.x = 2.0 // initialize center and radius
aCircle.y = 2.0
aCircle.r = 1.0
ObjectName.VariableName
ObjectName.MethodName(parameter-list)

 Using Object Methods:
Circle aCircle = new Circle();
double area;
aCircle.r = 1.0;
area = aCircle.area();
sent ‘message’ to aCircle

// Circle.java: Contains both Circle class and its user class
//Add Circle class code here
class MyMain
{
public static void main(String args[])
{
Circle aCircle; // creating reference
aCircle = new Circle(); // creating object
aCircle.x = 10; // assigning value to data field
aCircle.y = 20;
aCircle.r = 5;
double area = aCircle.area(); // invoking method
double circumf = aCircle.circumference();
System.out.println("Radius="+aCircle.r+" Area="+area);
System.out.println("Radius="+aCircle.r+" Circumference ="+circumf);
}
} [raj@mundroo]%: java MyMain
Radius=5.0 Area=78.5
Radius=5.0 Circumference =31.400000000000002

Java sem i

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Java sem i