In Java, a constructor is a block of codes similar to the method. It is called when an instance of the class is created. At the time of calling constructor, memory for the object is allocated in the memory.
It is a special type of method which is used to initialize the object.
Every time an object is created using the new() keyword, at least one constructor is called.
It calls a default constructor if there is no constructor available in the class. In such case, Java compiler provides a default constructor by default.
There are two types of constructors in Java: no-arg constructor, and parameterized constructor.
Note: It is called constructor because it constructs the values at the time of object creation. It is not necessary to write a constructor for a class. It is because java compiler creates a default constructor if your class doesn't have any.
There are two rules defined for the constructor.
There are two types of constructors in Java:
A constructor is called "Default Constructor" when it doesn't have any parameter.
<class_name>(){}Example of default constructor
| In this example, we are creating the no-arg constructor in the Bike class. It will be invoked at the time of object creation. |
//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();
}
}
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 objects
Student3 s1=new Student3();
Student3 s2=new Student3();
//displaying values of the object
s1.display();
s2.display();
}
}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.
A constructor which has a specific number of parameters is called a parameterized constructor.
The parameterized constructor is used to provide different values to distinct objects. However, you can provide the same values also.
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();
}
}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 constructors
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,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();
}
}There are many differences between constructors and methods. They are given below.
| Java Constructor | Java Method |
|---|---|
| A constructor is used to initialize the state of an object. | A method is used to expose the behavior of an object. |
| A constructor must not have a return type. | A method must have a return type. |
| The constructor is invoked implicitly. | The method is invoked explicitly. |
| The Java compiler provides a default constructor if you don't have any constructor in a class. | The method is not provided by the compiler in any case. |
| The constructor name must be same as the class name. | The method name may or may not be same as the class name. |
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:
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();
}
}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.
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=s1.name;
s1.display();
s2.display();
}
}Yes, it is the current class instance (You cannot use return type yet it returns a value).
Yes, like object creation, starting a thread, calling a method, etc. You can perform any operation in the constructor as you perform in the method.
Yes.
Java provides a Constructor class which can be used to get the internal information of a constructor in the class. It is found in the java.lang.reflect package.
