Java Classes and Objects Introduction

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 of Classes and Objects.

Object – Objects have states and behaviors. Example: A Cat has states – color, name, breed, and behaviors – walking, eating, etc. and an object is an instance of a class.

Class – A class can be defined as a template that describes the behaviors and states that object of its type support.

Objects in Java

If we consider the real world we can find many objects around us, Cats, Cars, Humans, etc. All these objects have a state and behavior. 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

public class Dog{
 String breed;
 int age;
 String color;
void barking(){
 }
 void eating(){
 }
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 instantiated when the class is loaded. 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(), eating() 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 Dog{
public Dog(){
}

public Dog(String name){
// This constructor has one parameter called name.
}
}

Java also supports Singleton Classes where you would be able to create only one instance of a class in a JVM.

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 keyword 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’ keyword 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 Dog{

public Dog(String name){
System.out.println("Passed Name is :" + name );
}
public static void main(String []args){
// Below statement would create an object myPuppy
Dog myPuppy = new Dog( "rocky" );
}
}

If we compile and run the above program, then it would produce the following result:

Passed Name is :rocky

Accessing instance variable 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 */Dog dog = new Dog();

/* Now call a variable as follows */dog.variableName;

/* Now you can call a class method as follows */dog.MethodName();

Example:

This example explains how to access instance variables and methods of a class:

public class Dog{

int dogAge;

public Dog(String name){
System.out.println("Passed Name is :" + name );
}
public void setAge( int age ){
dogAge = age;
}

public int getAge( ){
return dogAge;
}
public static void main(String []args){
/* Object creation */Dog dog = new Dog( "rocky" );

/* Call class method to set dog's age */dog.setAge( 2 );

/* Call another class method to get dog's age */dog.getAge( );

/* You can access instance variable as follows as well */System.out.println("Variable Value :" + dog.dogAge );
}
}

If we compile and run the above program, then it would produce the following result:

Passed Name is :rocky
Dog'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. I 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 the compiler to load all the classes available in the directory java_installation/java/io:

A Simple Case Study: For our case study, we will be creating two classes. They are Employee and EmployeeTest. First open the 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("Rahul Verma");
Employee empTwo = new Employee("Rohit Verma");

// 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:

test@radium:~$ javac Employee.java
test@radium:~$ javac  EmployeeTest.java
test@radium:~$ java EmployeeTest

Name:Rahul Verma
Age:26
Designation:Senior Software Engineer
Salary:1000.0
Name:Rohit Verma
Age:21
Designation:Software Engineer
Salary:500.0