Object and Class in Java

In this page, we will learn about java objects and classes. In object-oriented programming technique, we design a program using objects and classes.

Object is the physical as well as logical entity whereas class is the logical entity only.

Object in Java

An entity that has state and behavior is known as an object e.g. chair, bike, marker, pen, table, car etc. It can be physical or logical (tangible and intangible). The example of intangible object is banking system.

An object has three characteristics:

state: represents data (value) of an object.
behavior: represents the behavior (functionality) of an object such as deposit, withdraw etc.
identity: Object identity is typically implemented via a unique ID. The value of the ID is not visible to the external user. But, it is used internally by the JVM to identify each object uniquely.

For Example: Pen is an object. Its name is Reynolds, color is white etc. known as its state. It is used to write, so writing is its behavior.

Object is an instance of a class. Class is a template or blueprint from which objects are created. So object is the instance(result) of a class.

Object Definitions:

Object is a real world entity.
Object is a run time entity.
Object is an entity which has state and behavior.
Object is an instance of a class.

Class in Java

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

Syntax to declare a class:

class <class_name>{
field;
method;
}

Instance variable in Java

A variable which is created inside the class but outside the method, is known as instance variable. Instance variable doesn't get memory at compile time. It gets memory at run time when object(instance) is created. That is why, it is known as instance variable.

Method in Java

In java, a method is like function i.e. used to expose behavior of an object.

Advantage of Method

Code Reusability
Code Optimization

new keyword in Java

The new keyword is used to allocate memory at run time. All objects get memory in Heap memory area.

Object and Class Example: main within class

In this example, we have created a Student class that have two data members id and name. We are creating the object of the Student class by new keyword and printing the objects value.

Here, we are creating main() method inside the class.

File: Student.java

class Student{
int id;//field or data member or instance variable
String name;
public static void main(String args[]){
Student s1=new Student();//creating an object of Student
System.out.println(s1.id);//accessing member through reference variable
System.out.println(s1.name);
}
}

Output:

0 
null

Object and Class Example: main outside class

In real time development, we create classes and use it from another class. It is a better approach than previous one. Let's see a simple example, where we are having main() method in another class.

We can have multiple classes in different java files or single java file. If you define multiple classes in a single java source file, it is a good idea to save the file name with the class name which has main() method.

File: TestStudent1.java

class Student{
int id;
String name;
}
class TestStudent1{
public static void main(String args[]){
Student s1=new Student();
System.out.println(s1.id);
System.out.println(s1.name);
}
}

Output:

0 
null

3 Ways to initialize object

There are 3 ways to initialize object in java.

By reference variable
By method
By constructor

1) Object and Class Example: Initialization through reference

Initializing object simply means storing data into object. Let's see a simple example where we are going to initialize object through reference variable.

File: TestStudent2.java

class Student{
int id;
String name;
}
class TestStudent2{
public static void main(String args[]){
Student s1=new Student();
s1.id=101;
s1.name="Sonoo";
System.out.println(s1.id+" "+s1.name);//printing members with a white space
}
}

Output:

101 Sonoo

We can also create multiple objects and store information in it through reference variable.

File: TestStudent3.java

class Student{
int id;
String name;
}
class TestStudent3{
public static void main(String args[]){
//Creating objects
Student s1=new Student();
Student s2=new Student();
//Initializing objects
s1.id=101;
s1.name="Sonoo";
s2.id=102;
s2.name="Amit";
//Printing data
System.out.println(s1.id+" "+s1.name);
System.out.println(s2.id+" "+s2.name);
}
}

Output:

101 Sonoo
102 Amit

2) Object and Class Example: Initialization through method

In this example, we are creating the two objects of Student class and initializing the value to these objects by invoking the insertRecord method. Here, we are displaying the state (data) of the objects by invoking the displayInformation() method.

File: TestStudent4.java

class Student{
int rollno;
String name;
void insertRecord(int r, String n){
rollno=r;
name=n;
}
void displayInformation(){System.out.println(rollno+" "+name);}
}
class TestStudent4{
public static void main(String args[]){
Student s1=new Student();
Student s2=new Student();
s1.insertRecord(111,"Karan");
s2.insertRecord(222,"Aryan");
s1.displayInformation();
s2.displayInformation();
}
}

Output:

111 Karan
222 Aryan

As you can see in the above figure, object gets the memory in heap memory area. The reference variable refers to the object allocated in the heap memory area. Here, s1 and s2 both are reference variables that refer to the objects allocated in memory.

3) Object and Class Example: Initialization through constructor

We will learn about constructors in java later.

Object and Class Example: Employee

Let's see an example where we are maintaining records of employees.

File: TestEmployee.java

class Employee{
int id;
String name;
float salary;
void insert(int i, String n, float s) {
id=i;
name=n;
salary=s;
}
void display(){System.out.println(id+" "+name+" "+salary);}
}
public class TestEmployee {
public static void main(String[] args) {
Employee e1=new Employee();
Employee e2=new Employee();
Employee e3=new Employee();
e1.insert(101,"ajeet",45000);
e2.insert(102,"irfan",25000);
e3.insert(103,"nakul",55000);
e1.display();
e2.display();
e3.display();
}
}

Output:

101 ajeet 45000.0
102 irfan 25000.0
103 nakul 55000.0

Object and Class Example: Rectangle

There is given another example that maintains the records of Rectangle class.

File: TestRectangle1.java

class Rectangle{
int length;
int width;
void insert(int l, int w){
length=l;
width=w;
}
void calculateArea(){System.out.println(length*width);}
}
class TestRectangle1{
public static void main(String args[]){
Rectangle r1=new Rectangle();
Rectangle r2=new Rectangle();
r1.insert(11,5);
r2.insert(3,15);
r1.calculateArea();
r2.calculateArea();
}
}

Output:

55 
45

What are the different ways to create an object in Java?

There are many ways to create an object in java. They are:

By new keyword
By newInstance() method
By clone() method
By deserialization
By factory method etc.

We will learn these ways to create object later.

Anonymous object

Anonymous simply means nameless. An object which has no reference is known as anonymous object. It can be used at the time of object creation only.

If you have to use an object only once, anonymous object is a good approach. For example:

new Calculation();//anonymous object

Calling method through reference:

Calculation c=new Calculation();
c.fact(5);

Calling method through anonymous object

new Calculation().fact(5);

Let's see the full example of anonymous object in java.

class Calculation{
void fact(int n){
int fact=1;
for(int i=1;i<=n;i++){
fact=fact*i;
}
System.out.println("factorial is "+fact);
}
public static void main(String args[]){
new Calculation().fact(5);//calling method with anonymous object
}
}

Output:

Factorial is 120

Creating multiple objects by one type only

We can create multiple objects by one type only as we do in case of primitives.

Initialization of primitive variables:

int a=10, b=20;

Initialization of refernce variables:

Rectangle r1=new Rectangle(), r2=new Rectangle();//creating two objects

Let's see the example:

class Rectangle{
int length;
int width;
void insert(int l,int w){
length=l;
width=w;
}
void calculateArea(){System.out.println(length*width);}
}
class TestRectangle2{
public static void main(String args[]){
Rectangle r1=new Rectangle(),r2=new Rectangle();//creating two objects
r1.insert(11,5);
r2.insert(3,15);
r1.calculateArea();
r2.calculateArea();
}
}

Output:

55 
45

Real World Example: Account

File: TestAccount.java

class Account{
int acc_no;
String name;
float amount;
void insert(int a,String n,float amt){
acc_no=a;
name=n;
amount=amt;
}
void deposit(float amt){
amount=amount+amt;
System.out.println(amt+" deposited");
}
void withdraw(float amt){
if(amount<amt){
System.out.println("Insufficient Balance");
}else{
amount=amount-amt;
System.out.println(amt+" withdrawn");
}
}
void checkBalance(){System.out.println("Balance is: "+amount);}
void display(){System.out.println(acc_no+" "+name+" "+amount);}
}
class TestAccount{
public static void main(String[] args){
Account a1=new Account();
a1.insert(832345,"Ankit",1000);
a1.display();
a1.checkBalance();
a1.deposit(40000);
a1.checkBalance();
a1.withdraw(15000);
a1.checkBalance();
}}

Output:

832345 Ankit 1000.0
Balance is: 1000.0
40000.0 deposited
Balance is: 41000.0
15000.0 withdrawn
Balance is: 26000.0

FOUND JAVA

Thursday 13 July 2017