// java program to create and call a default constructor
class Bike1 {
// creating a default constructor
Bike1() {System.out.println("Bike is created");}
// main method
public static void main(String args[]) {
// calling a default constructor
Bike1 b = new Bike1();
}
}

output

Rule: if there is no constructor in a class, compiler automatically creates a default constructor.

Q) What is the purpose of a default constructor ?
The default constructor is used to provide the default values to the object like 0, null, etc., depending on the type.

// Let us see another example of default constructor
// which displays the default values
class Student3 {
int id;
String name;
// method to display the value of id and name
void display(){System.out.println(id+" "+name);}

public static void main(String args[]){
// creating object
Student3 s1=new Student3();
Student3 s2=new Student3();
// displaying values of the object
s1.display();
s2.display();
}
}

output

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.

Java Parameterized Constructor
A constructor which has a specific number of parameters is called a parameterized constructor.

Why use the parameterized constructor ?
The parameterized constructor is used to provide different values to distinct objects. However, you can provide the same values also.

Example of parameterized constructor
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.

// java program to demonstrate the use of the parameterized constructor
class Student4 {
int id;
String name;
// creating a parameterized constructor
Student4(int i, String n) {
id = i;
name = n;
}
// method to display the values
void display(){System.out.println(id+" " + name);}

public static void main(String args[]) {
// creating objects and passing values
Student4 s1= new Student4(111, "Karan");
Student4 s2 = new Student4(222, "Aryan");
// calling method to display the values of object
s1.display();
s2.display();
}
}

output

Constructor Overloading in Java
In Java, a constructor is just like a method but without return type. It can also be overloaded like Java methods.

Constructor overloading in Java is a technique of having more than one constructor with different parameter lists. They are arranged in a way that each constructor performs a different task. They are differentiated by the compiler by the number of parameters in the list and their types.

// java program to overload constructor
class Student5{
int id;
String name;
int age;
// creating two arg constructor
Student5(int i, String n) {
id = i;
name = n;
}
// creating three arg constructor
Student5(int i, String n) {
id = i;
name = n;
}

// creating three arg constructor
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[]) {
Student s1 = new Student5(111, "Karan");
Student s2 = new Student5(222, "Aryan", 25);
s1.display();
s2.display();
}
}

output

Difference between constructor and method in java
There are many differences between constructors and methods. They are given below.

Difference between constructor and method in Java

Java Copy Constructor
There is no copy constructor in Java. However, we can copy the values from 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:

By constructor
By assigning the values of one object into another
By clone() method of Object class

In this example, we are going to copy the values of one object into another using Java constructor.

// Java program to initialize the values from one object to another object
class Student6 {
int id;
String name;
// constructor to initialize integer and string
Student6(int i, String n) {
id = i;
name = n;
}
// constructor to initialize another object
Student6(Student6 s) {
id = s.id;
name = s.name;
}
void display() {System.out.println(id + " " + name);}

public static void main(String args[]) {
Student6 s1 = new Student6(111, "Karan");
Student6 s2 = new Student6(s1);
s1.display();
s2.display();
}
}

Output

Copying values without constructor
We can copy the values of one object into another by assigning the object values to another object. In this case, there is no need to create the constructor.

class Student7 {
int id;
String name;
Student7(int i, String n) {
id = i;
name = n;
}
Student7(){}
void display() {System.out.println(id + " " + name);}

public static void main(String args[]) {
Student7 s1 = new Student7(111, "Karan");
Student7 s2 = new Student7();
s2.id=s1.id;
s2.name=s1name;
s1.display();
s2.display();
}
}