Lect 1-java object-classes

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JAVA - OBJECT & CLASSESJAVA - OBJECT & CLASSES
Java is an Object-Oriented Language. As a language that has the Object Oriented feature, Java
supports the following fundamental concepts:
Polymorphism
Inheritance
Encapsulation
Abstraction
Classes
Objects
Instance
Method
Message Parsing
In this chapter, we will look into the concepts Classes and Objects.
Object - Objects have states and behaviors. Example: A dog has states - color, name, breed
as well as behaviors -wagging, barking, eating. An object is an instance of a class.
Class - A class can be defined as a template/blue print that describes the behaviors/states
that object of its type support.
Objects in Java:
Let us now look deep into what are objects. If we consider the real-world we can find many objects
around us, Cars, Dogs, Humans, etc. All these objects have a state and behavior.
If we consider a dog, then its state is - name, breed, color, and the behavior is - barking, wagging,
running
If you compare the software object with a real world object, they have very similar characteristics.
Software objects also have a state and behavior. A software object's state is stored in fields and
behavior is shown via methods.
So in software development, methods operate on the internal state of an object and the object-to-
object communication is done via methods.
Classes in Java:
A class is a blue print from which individual objects are created.
A sample of a class is given below:
public class Dog{
String breed;
int ageC
String color;
void barking(){
}
void hungry(){
}
void sleeping(){
}

}
A class can contain any of the following variable types.
Local variables: Variables defined inside methods, constructors or blocks are called local
variables. The variable will be declared and initialized within the method and the variable will
be destroyed when the method has completed.
Instance variables: Instance variables are variables within a class but outside any method.
These variables are initialized when the class is instantiated. Instance variables can be
accessed from inside any method, constructor or blocks of that particular class.
Class variables: Class variables are variables declared with in a class, outside any method,
with the static keyword.
A class can have any number of methods to access the value of various kinds of methods. In the
above example, barking, hungry and sleeping are methods.
Below mentioned are some of the important topics that need to be discussed when looking into
classes of the Java Language.
Constructors:
When discussing about classes, one of the most important sub topic would be constructors. Every
class has a constructor. If we do not explicitly write a constructor for a class the Java compiler
builds a default constructor for that class.
Each time a new object is created, at least one constructor will be invoked. The main rule of
constructors is that they should have the same name as the class. A class can have more than one
constructor.
Example of a constructor is given below:
public class Puppy{
public Puppy(){
}
public Puppy(String name){
// This constructor has one parameter, name.
}
}
Java also supports Singleton Classes where you would be able to create only one instance of a
class.
Note: We have two different types of constructors we are going to discuss constructors in detail in
coming chapters.
Creating an Object:
As mentioned previously, a class provides the blueprints for objects. So basically an object is
created from a class. In Java, the new key word is used to create new objects.
There are three steps when creating an object from a class:
Declaration: A variable declaration with a variable name with an object type.
Instantiation: The 'new' key word is used to create the object.
Initialization: The 'new' keyword is followed by a call to a constructor. This call initializes
the new object.
Example of creating an object is given below:
public class Puppy{

System.out.println("Passed Name is :" + name );
}
public static void main(String []args){
// Following statement would create an object myPuppy
Puppy myPuppy = new Puppy( "tommy" );
}
}
If we compile and run the above program, then it would produce the following result:
Passed Name is :tommy
Accessing Instance Variables and Methods:
Instance variables and methods are accessed via created objects. To access an instance variable
the fully qualified path should be as follows:
/* First create an object */
ObjectReference = new Constructor();
/* Now call a variable as follows */
ObjectReference.variableName;
/* Now you can call a class method as follows */
ObjectReference.MethodName();
Example:
This example explains how to access instance variables and methods of a class:
public class Puppy{
int puppyAge;
System.out.println("Name chosen is :" + name );
}
public void setAge( int age ){
puppyAge = age;
}
public int getAge( ){
System.out.println("Puppy's age is :" + puppyAge );
return puppyAge;
}
public static void main(String []args){
/* Object creation */
Puppy myPuppy = new Puppy( "tommy" );
/* Call class method to set puppy's age */
myPuppy.setAge( 2 );
/* Call another class method to get puppy's age */
myPuppy.getAge( );
/* You can access instance variable as follows as well */
System.out.println("Variable Value :" + myPuppy.puppyAge );
}
}

If we compile and run the above program, then it would produce the following result:
Name chosen is :tommy
Puppy's age is :2
Variable Value :2
Source file declaration rules:
As the last part of this section let's now look into the source file declaration rules. These rules are
essential when declaring classes, import statements and package statements in a source file.
There can be only one public class per source file.
A source file can have multiple non public classes.
The public class name should be the name of the source file as well which should be
appended by .java at the end. For example: the class name is public class Employee{} then
the source file should be as Employee.java.
If the class is defined inside a package, then the package statement should be the first
statement in the source file.
If import statements are present then they must be written between the package statement
and the class declaration. If there are no package statements then the import statement
should be the first line in the source file.
Import and package statements will imply to all the classes present in the source file. It is not
possible to declare different import and/or package statements to different classes in the
source file.
Classes have several access levels and there are different types of classes; abstract classes, final
classes, etc. we will be explaining about all these in the access modifiers chapter.
Apart from the above mentioned types of classes, Java also has some special classes called Inner
classes and Anonymous classes.
Java Package:
In simple, it is a way of categorizing the classes and interfaces. When developing applications in
Java, hundreds of classes and interfaces will be written, therefore categorizing these classes is a
must as well as makes life much easier.
Import statements:
In Java if a fully qualified name, which includes the package and the class name, is given then the
compiler can easily locate the source code or classes. Import statement is a way of giving the
proper location for the compiler to find that particular class.
For example, the following line would ask compiler to load all the classes available in directory
java_installation/java/io :
import java.io.*;
A Simple Case Study
For our case study, we will be creating two classes. They are Employee and EmployeeTest.
First open notepad and add the following code. Remember this is the Employee class and the class
is a public class. Now, save this source file with the name Employee.java.
The Employee class has four instance variables name, age, designation and salary. The class has
one explicitly defined constructor, which takes a parameter.
import java.io.*;

public class Employee{
String name;
int age;
String designation;
double salary;
// This is the constructor of the class Employee
public Employee(String name){
this.name = name;
}
// Assign the age of the Employee to the variable age.
public void empAge(int empAge){
age = empAge;
}
/* Assign the designation to the variable designation.*/
public void empDesignation(String empDesig){
designation = empDesig;
}
/* Assign the salary to the variable salary.*/
public void empSalary(double empSalary){
salary = empSalary;
}
/* Print the Employee details */
public void printEmployee(){
System.out.println("Name:"+ name );
System.out.println("Age:" + age );
System.out.println("Designation:" + designation );
System.out.println("Salary:" + salary);
}
}
As mentioned previously in this tutorial, processing starts from the main method. Therefore in-
order for us to run this Employee class there should be main method and objects should be
created. We will be creating a separate class for these tasks.
Given below is the EmployeeTest class, which creates two instances of the class Employee and
invokes the methods for each object to assign values for each variable.
Save the following code in EmployeeTest.java file
import java.io.*;
public class EmployeeTest{
public static void main(String args[]){
/* Create two objects using constructor */
Employee empOne = new Employee("James Smith");
Employee empTwo = new Employee("Mary Anne");
// Invoking methods for each object created
empOne.empAge(26);
empOne.empDesignation("Senior Software Engineer");
empOne.empSalary(1000);
empOne.printEmployee();
empTwo.empAge(21);
empTwo.empDesignation("Software Engineer");
empTwo.empSalary(500);
empTwo.printEmployee();
}
}
Now, compile both the classes and then run EmployeeTest to see the result as follows:
C:> javac Employee.java
C:> javac EmployeeTest.java
C:> java EmployeeTest

Name:James Smith
Age:26
Designation:Senior Software Engineer
Salary:1000.0
Name:Mary Anne
Age:21
Designation:Software Engineer
Salary:500.0
What is Next?
Next session will discuss basic data types in Java and how they can be used when developing Java
applications.
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Lect 1-java object-classes

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