Constructor in Java
Constructor in java is a special type of
method that is used to
initialize the object.
Java
constructor is invoked at the time
of object creation.
It constructs the values i.e. provides data for the object that is why it is
known as constructor.
Rules for creating java constructor
There
are basically two rules defined for the constructor.
- Constructor
name must be same as its class name
- Constructor
must have no explicit return type
Types of java constructors
There
are two types of constructors:
- Default
constructor (no-arg constructor)
- Parameterized
constructor
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Java Default Constructor
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A constructor that have
no parameter is known as default constructor.
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Syntax of default constructor:
1. <class_name>(){}
Example of default constructor
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In this example, we are
creating the no-arg constructor in the Bike class. It will be invoked at the
time of object creation.
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1. class Bike1{
2.
Bike1(){System.out.println("Bike is created");}
3. public static void main(String args[]){
4.
Bike1 b=new Bike1();
5.
}
6.
}
Output:
Bike is created
Rule: If there is no constructor in a class, compiler automatically creates a default constructor.
Q) What is the purpose of default constructor?
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Default constructor
provides the default values to the object like 0, null etc. depending on the
type.
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Example of default constructor that displays the default
values
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1.
class Student3{
2.
int id;
3.
String name;
4.
5.
void display(){System.out.println(id+" "+name);}
6.
7.
public static void main(String args[]){
8.
Student3 s1=new Student3();
9.
Student3 s2=new Student3();
10.
s1.display();
11.
s2.display();
12.
}
13.
}
Output:
0 null
0 null
Explanation:In the above
class,you are not creating any constructor so compiler provides you a default
constructor.Here 0 and null values are provided by default constructor.
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Java parameterized constructor
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A constructor that have parameters is
known as parameterized constructor.
|
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Why use parameterized constructor?
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Parameterized constructor is used to
provide different values to the distinct objects.
|
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Example of parameterized constructor
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In this example, we have created the
constructor of Student class that have two parameters. We can have any
number of parameters in the constructor.
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1.
class Student4{
2.
int id;
3.
String name;
4.
5.
Student4(int i,String n){
6.
id = i;
7.
name = n;
8.
}
9.
void display(){System.out.println(id+" "+name);}
10.
11.
public static void main(String args[]){
12.
Student4 s1 = new Student4(111,"Karan");
13.
Student4 s2 = new Student4(222,"Aryan");
14.
s1.display();
15.
s2.display();
16.
}
17.
}
Output:
111 Karan
222 Aryan
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Constructor Overloading in Java
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Constructor overloading is a technique
in Java in which a class can have any number of constructors that differ in
parameter lists.The compiler differentiates these constructors by taking
into account the number of parameters in the list and their type.
|
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Example of Constructor Overloading
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1.
class Student5{
2.
int id;
3.
String name;
4.
int age;
5.
Student5(int i,String n){
6.
id = i;
7.
name = n;
8.
}
9.
Student5(int i,String n,int a){
10.
id = i;
11.
name = n;
12.
age=a;
13.
}
14.
void display(){System.out.println(id+" "+name+" "+age);}
15.
16.
public static void main(String args[]){
17.
Student5 s1 = new Student5(111,"Karan");
18.
Student5 s2 = new Student5(222,"Aryan",25);
19.
s1.display();
20.
s2.display();
21.
}
22.
}
Output:
111 Karan 0
222 Aryan 25
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Difference between constructor and method
in java
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There are many
differences between constructors and methods. They are given below.
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Java Constructor
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Java Method
|
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Constructor is used to initialize the
state of an object.
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Method is used to expose behaviour of an
object.
|
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Constructor must not have return type.
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Method must have return type.
|
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Constructor is invoked implicitly.
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Method is invoked explicitly.
|
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The java compiler provides a default
constructor if you don't have any constructor.
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Method is not provided by compiler in
any case.
|
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Constructor name must be same as the
class name.
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Method name may or may not be same as
class name.
|
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Java Copy Constructor
|
There is no copy
constructor in java. But, we can copy the values of one object to another
like copy constructor in C++.
There are many ways
to copy the values of one object into another in java. They are:
o By constructor
o By assigning the values of one object into another
o By clone() method of Object class
In this example, we
are going to copy the values of one object into another using java
constructor.
|
1.
class Student6{
2. int id;
3. String name;
4. Student6(int i,String n){
5. id = i;
6. name = n;
7. }
8.
9. Student6(Student6 s){
10. id = s.id;
11. name =s.name;
12. }
13. void display(){System.out.println(id+" "+name);}
14.
15. public static void main(String args[]){
16. Student6 s1 = new Student6(111,"Karan");
17. Student6 s2 = new Student6(s1);
18. s1.display();
19. s2.display();
20. }
21. }
Output:
111 Karan
111 Karan
Copying values without constructor
We
can copy the values of one object into another by assigning the objects values
to another object. In this case, there is no need to create the constructor.
1.
class Student7{
2. int id;
3. String name;
4. Student7(int i,String n){
5. id = i;
6. name = n;
7. }
8. Student7(){}
9. void display(){System.out.println(id+" "+name);}
10.
11. public static void main(String args[]){
12. Student7 s1 = new Student7(111,"Karan");
13. Student7 s2 = new Student7();
14. s2.id=s1.id;
15. s2.name=s1.name;
16. s1.display();
17. s2.display();
18. }
19. }
Output:
111 Karan
111 Karan
Q) Does constructor return any value?
Ans:yes, that is current
class instance (You cannot use return type yet it returns a value).
Can constructor perform other tasks instead of
initialization?
Yes,
like object creation, starting a thread, calling method etc. You can perform
any operation in the constructor as you perform in the method.
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