The document discusses key concepts in Java including classes, objects, methods, and command line arguments. A class defines common properties and behaviors for objects through fields and methods. Objects are instantiated from classes and can access fields and methods using dot notation. Command line arguments allow passing data into a Java application and are accessed through the args parameter in the main method.

1. JAVA
P.Prathibha

2. Topics for Today’s Session
About JAVA
Creating Classes and Objects
Command line arguments
How to Create class and Object

3. Java
 Java is a computing platform for application
development and an object-oriented,
 Java is Class-based and Concurrent programming
language
 It means the code can be executed by multiple
processes at the same time.
 Java can run on all platforms and free to access.
 Java is Simple, Secure, Robust, Complete Object
oriented and Platform Independent High level
Language
 It is Portable and Multi-thread technology gives
High Performance.

4. Objects and Classes in Java
 An object in Java is the physical as well as a logical entity, whereas, a class in Java is a
logical entity only.
 An entity that has state and behavior is known as an object e.g., chair, bike, pen, table,
car, etc.
 It can be physical or logical (tangible and intangible). The example of an intangible
object is the banking system.
 An object consists of :
 State : It is represented by attributes of an object. It also reflects the properties
of an object.
 Behavior : It is represented by methods of an object. It also reflects the
response of an object with other objects.
 Identity : An object identity is typically implemented via a unique ID. The value of the ID is
not visible to the external user. However, it is used internally by the JVM to identify each
object uniquely.
For Example, Pen is an object. Its name is Reynolds; color is Blue, known as its state. It is used to
write, so writing is its behavior.
Identity State / Attributes
Breed
Color
Age
Behaviours
Bark
Eat
Sleep
Bite

5.  An object is an instance of a class. A class is a template or blueprint from
which objects are created. So, an object is the instance(result) of a class.
Object Definitions:
 An object is a real-world entity.
 An object is a runtime entity.
 The object is an entity which has state and behavior.
 The object is an instance of a class.

6. Class
 A class is a group of objects which have common properties.
 It is a template or blueprint from which objects are created.
 It is a logical entity. It can't be physical.


 A class in Java can contain:
 Fields
 Methods
 Constructors
 Blocks
 Nested class and interface

7. Create a Class






 Example: Create a class named "MyClass" with a variable a:
Class classname [extends superclassname]
{
[variable/field declaration];
[methods declaration];
}
class MyClass
{
int a = 5;
}
Here
Everything inside the brackets is optional.
classname and superclassname are any valid java identifiers.
The keyword extends indicate that the properties of the superclass name are
extended to the classname class.
This concept is known as Inheritance.

8. Fields declaration:
 Data is encapsulated in a class by placing data fields in the
class. These variables are called “instance variables”.
 These are declared just as local variables;
Methods Declaration:
 A class with only data fields has no life. Such classes cannot
respond to any messages.
 We must add methods that are useful for manipulating the data
of the class.
 Methods for a class are declared inside the body of the class
but immediately after the declaration of the variables.
Type methodname (parameter list)
{
method_body;
}
Method declaration having four parts:
1. The name of the method ( method name) (It is any
valid identifier)
2. The type of the value of the method returns (type)
(it is any valid data type, it is void when it does not
return any value.)
3. A list of parameters (parameter list) (This list
contains variable names and types of all the values
we want to send as input, when no input data are
required, we must use empty parentheses)
4. The body of the method. It describes the operations
to be performed on the data.
Example: : int sum (int a, int b)
{
c=a+b;
return c;
Method in Java
 In Java, a method is like a
function which is used to
expose the behavior of an
object.
 Advantage of Method
 Code Reusability
 Code Optimization

9. Local variables
Instance variable




Class variables
.

10. Example
class Dog {
String breed;
int age;
String color;
void barking() {
}
void hungry() {
}
void sleeping() {
}
}
Creating Objects:
 An object is a block of memory that contains space to store all the
instance variables.
 An object is a software entity (unit) that combines a set of data with
set of operations to manipulate that data.
 A class defines a type of object. i.e., each object belongs to some class
object is also called as an instance.
 Java has an operator new to create objects. Creating an object is also
referred to as instantiating an object.
 Objects in java can be created by using the syntax.
Syntax:
Classname objname;
Objname = new classname();
 The first statement declares a variable to hold
the object reference and second one actually
assigns the object reference to the variable.
 Ex
triangle tri1; // declaring the object)
Tri1 = new triangle //instantiating the object)
The above two statements are combined into a
single statement as
Triangle tri1 = new triangle();

11. Accessing class Members: every object contains its own set of variables.
 We should assign values to these variables in order to use them in our
program.
 When we are outside the class we cannot access the instance variables and
methods directly. For this we use dot operator.
objectname.varaiblename = value;
objectname.methodname (parameter list);
Example: tri1.length = 15;
tri2.length = 20;
tri1.getData( 20,30)
public class MyClass {
int a = 5;
public static void main(String[]
args) {
MyClass myObj = new
MyClass();
System.out.println(myObj.a);
}
}

12. Ex: A java program to demonstrate application ofclasses, objects and methods.(Pra.Prog. 17)
class triangle {
int length, width;
void getData(int x, int y)
{
length = x;
width= y;
}
int triarea( )
{
int area = (length * width)/2;
return(area);
}
}
class triarea {
public static void main (String args[ ])
{
int area1, area2;
triangle tri1 = new triangle();
triangle tri2 = new triangle();
tri1.length= 20;
tri1.width = 30;
area1 =( tri1.length * tri1.width)/2;
tri2.getData( 10, 15);
area2 = tri2.triarea();
System.out.println( "area1 = " + area1);
System.out.println( "area2 = " + area2);
}
}
Instance variables declared in a single
line
getData is a method, it does not return any value
so its type is void, we are passing two integer
values and they are assigned to length and width
triarea is another method, it returns a value of
data type int. We are not passing values, so
the parameter list is empty
class with main method
Creating objects
Accessing variables
Accessing Methods

13. Command line Arguments
 The java command-line argument is an argument i.e. passed at the time of running the java
program.
 A Java application can accept any number of arguments from the command line.
 The String array stores all the arguments passed through the command line.
 Arguments are always stored as strings and always separated by white-space.
// Example using Commandline arguments
class Cmdargs {
public static void main(String[] args) {
System.out.println("Command-Line arguments
are");
// loop through all arguments
for(String str: args) {
System.out.println(str);
}
} }
1. To compile the code
javac Cmdargs.java
2. To run the code
java Cmdargs
Now suppose we want to pass some arguments
while running the program,
we can pass the arguments after the class name. For
example,
java Main apple ball cat
output.
Command-Line arguments are
Apple
Ball
Cat

14. // Program to check for command line arguments
class Hello {
public static void main(String[] args) {
// check if length of args array is greater than 0
if (args.length > 0) {
System.out.println("The command line"+ " arguments are:");
// iterating the args array and printing the command line arguments
for (String s:args)
System.out.println(s);
}
else
System.out.println("No command line "+ "arguments found.");
} }
javac Hello.java
java Hello Lion Dog Cat Elephant
Output:
The Command-Line arguments are
Lion
Dog
Cat
Elephant
javac Hello.java
java Hello
Output
No Command-Line arguments
foun

15. 



