Java Constructors with Examples

Constructor In Java

Constructor is the special member of a class it is automatically invoked whenever an object of that class is created.

It has the same name as the class name in which it is declared it has no return type, not even void.

It is used to initialize the data members of a class.

Note -

Explicitly constructor can not return any value, but implicitly it returns the current instance of a class.
A constructor can take parameters.
A class can have more than one constructor, but signatures are different.
Constructors follow the access rule of a class.
Constructor can not be inherited.
Constructor can not be final.
It is used to initialize the data members of a class.

The difference between constructor, and method -

Constructor is used to initialize the state of an object whereas method is used to expose the behavior of data member of a class.
Constructors have no return type not even void, whereas methods must have a return type.
It is invoked implicitly, the method is invoked explicitly.
Constructor name same as the class name, method name may, or may not same as the class name.
Java compiler automatically created default constructor if your class have no explicit constructor, No method definition is provided by compiler.

Example 1-

package javaLearnings;
import java.util.Scanner;
//Example 1
class Sample{
   void show(){ // method
       System.out.println("show method here");
   }
}

public class Main {
   public static void main(String[] args) {
// write your code here
       Sample ob = new Sample();
       ob.show();
      }

   }

Example 2 -

package javaLearnings;

//Example 2
class Sample{
   void sample(){ // method
       System.out.println("Sample method here");
   }
}

public class Main {
   public static void main(String[] args) {
// write your code here
       Sample ob = new Sample();
       ob.sample();
      }

   }

Example 3-

package javaLearnings;

//Example 3
class Sample{
    Sample(){ // Constructor
       System.out.println("Sample method here");
   }
}

public class Main {
   public static void main(String[] args) {
// write your code here
       Sample ob = new Sample();
      }

   }

Example 4-

class Sample{
    Sample(){ // C.T. error
       System.out.println("Sample method here");
   }
}

Types of constructor

There are three types of constructor in java given as follows-

Default constructor,
Parametrized constructor
Copy Constructor

Default Constructor

The constructor with no parameter is called default constructor.
If your class has no constructor then the compiler automatically created a default constructor in a class.
We need default constructor to initialize the value of data members with there default value

Example -

Output :

Parameterized Constructor

A constructor with parameter is called parameterised constructor you must pass the value at the time of object creation if your class have parameterised constructor.

package javaLearnings;

class Sample<r> {
   String n;
   int m,r;
    Sample(String a, int b,int c){
n =a;
m = b;
r = c;
    }
void show() {
     System.out.println("Name = "+n);
        System.out.println("Marks = "+m);
     System.out.println("roll No "+r);
}

}

public class Main {
   public static void main(String[] args) {
// write your code here
       Sample ob = new Sample("Ram",90,400);
       ob.show();;
   }

   }

Output -