Object and Class in Java
- Object in Java
- Class in Java
- Instace Variable
in Java
- Method in Java
- Example of
Object and class that maintains the records of student
- Annonymous
Object
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 (tengible and intengible). The example of
integible 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.
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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.
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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.
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Class in
Java
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A
class is a group of similar objects that has common properties. It is a
template or blueprint from which objects are created.
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A class
in java can contain:
- data member
- method
- constructor
- block
- class and interface
Syntax to declare a
class:
1.
class <class_name>{
2. data member;
3. method;
4. }
Simple Example of Object and 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.
1.
class Student1{
2. int id;//data member (also instance variable)
3. String name;//data member(also instance variable)
4.
5. public static void main(String args[]){
6. Student1 s1=new Student1();//creating an object of Student
7. System.out.println(s1.id);
8. System.out.println(s1.name);
9. }
10. }
Output:0 null
Instance variable in Java
|
A
variable that 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 runtime when object(instance) is created.That is why, it is
known as instance variable.
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Method in Java
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In
java, a method is like function i.e. used to expose behaviour of an object.
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Advantage of Method
- Code Reusability
- Code Optimization
new keyword
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The
new keyword is used to allocate memory at runtime.
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Example
of Object and class that maintains the records of students
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In
this example, we are creating the two objects of Student class and
initializing the value to these objects by invoking the insertRecord method
on it. Here, we are displaying the state (data) of the objects by invoking
the displayInformation method.
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1.
class Student2{
2. int rollno;
3. String name;
4.
5. void insertRecord(int r, String n){ //method
6. rollno=r;
7. name=n;
8. }
9.
10. void displayInformation(){System.out.println(rollno+" "+name);}//method
11.
12. public static void main(String args[]){
13. Student2 s1=new Student2();
14. Student2 s2=new Student2();
15.
16. s1.insertRecord(111,"Karan");
17. s2.insertRecord(222,"Aryan");
18.
19. s1.displayInformation();
20. s2.displayInformation();
21.
22. }
23. }
Output:111 Karan
222 Aryan
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As
you see in the above figure, object gets the memory in Heap area and
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.
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Another
Example of Object and Class
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There
is given another example that maintains the records of Rectangle class. Its
exaplanation is same as in the above Student class example.
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1. class Rectangle{
2. int length;
3. int width;
4.
5. void insert(int l,int w){
6. length=l;
7. width=w;
8. }
9.
10. void calculateArea(){System.out.println(length*width);}
11.
12. public static void main(String args[]){
13. Rectangle r1=new Rectangle();
14. Rectangle r2=new Rectangle();
15.
16. r1.insert(11,5);
17. r2.insert(3,15);
18.
19. r1.calculateArea();
20. r2.calculateArea();
21. }
22. }
Output:55
45
What are the different ways to create an object in Java?
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There
are many ways to create an object in java. They are:
·
By new keyword
·
By newInstance() method
·
By clone() method
·
By factory method etc.
We
will learn, these ways to create the object later.
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Annonymous
object
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Annonymous
simply means nameless.An object that have no reference is known as annonymous
object.
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If
you have to use an object only once, annonymous object is a good approach.
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1. class Calculation{
2.
3. void fact(int n){
4. int fact=1;
5. for(int i=1;i<=n;i++){
6. fact=fact*i;
7. }
8. System.out.println("factorial is "+fact);
9. }
10.
11. public static void main(String args[]){
12. new Calculation().fact(5);//calling method with annonymous object
13. }
14. }
Output:Factorial is 120
Creating
multiple objects by one type only
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We
can create multiple objects by one type only as we do in case of primitives.
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1. Rectangle r1=new Rectangle(),r2=new Rectangle();//creating two objects
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Let's
see the example:
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1. class Rectangle{
2. int length;
3. int width;
4.
5. void insert(int l,int w){
6. length=l;
7. width=w;
8. }
9.
10. void calculateArea(){System.out.println(length*width);}
11.
12. public static void main(String args[]){
13. Rectangle r1=new Rectangle(),r2=new Rectangle();//creating two objects
14.
15. r1.insert(11,5);
16. r2.insert(3,15);
17.
18. r1.calculateArea();
19. r2.calculateArea();
20. }
21. }
Output:55
45
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