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 constructorThere 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 constructorsThere are two types of constructors:
- Default constructor (no-arg constructor)
- Parameterized constructor
Java Default Constructor
A constructor that have no parameter is known as default constructor.
Example of default constructorIn this example, we are creating the no-arg constructor in the Bike class. It will be invoked at the time of object creation.
class Bike1
{
Bike1()
{
System.out.println("Bike is created")
;}
public static void main(String args[])
{
Bike1 b=new Bike1();
}
}
Output:
Bike is created
Q)What is the purpose of default constructor?
Default constructor provides the default values to the object like 0, null etc. depending on the type.
Java parameterized constructor
A constructor that have parameters is known as parameterized constructor.
Why use parameterized constructor?
Parameterized constructor is used to provide different values to the distinct objects.
Example of parameterized constructorIn this example, we have created the constructor of Student class that have two parameters. We can have any number of parameters in the constructor.
class Student4{
int id;
String name;
Student4(int i,String n){
id = i;
name = n;
}
void display()
{
System.out.println(id+" "+name);
}
public static void main(String args[]){
Student4 s1 = new Student4(111,"Karan");
Student4 s2 = new Student4(222,"Aryan");
s1.display();
s2.display();
}
}
111 Karan
222 Aryan
Constructor Overloading in Java
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.
Example of Constructor Overloading
class Student5{
int id;
String name;
int age;
Student5(int i,String n){
id = i;
name = n;
}
Student5(int i,String n,int a){
id = i;
name = n;
age=a;
}
void display(){System.out.println(id+" "+name+" "+age);}
public static void main(String args[]){
Student5 s1 = new Student5(111,"Karan");
Student5 s2 = new Student5(222,"Aryan",25);
s1.display();
s2.display();
}
}
111 Karan 0
222 Aryan 25
Difference between constructor and method in java
There are many differences between constructors and methods. They are given below.
Java Constructor Java Method
Constructor is used to initialize the state of an object. Method is used to expose behaviour of an object.
Constructor must not have return type. Method must have return type.
Constructor is invoked implicitly. Method is invoked explicitly.
The java compiler provides a default constructor if you don't have any constructor . Method is not provided by compiler in any case.
Constructor name must be same as the class name. Method name may or may not be same as class name.
Q) Does constructor return any value?
yes, that is current class instance (You cannot use return type yet it returns a value).
Q)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.