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.

Table of Contents

Rules for creating Java constructor

There are two rules defined for the constructor.
1. Constructor name must be the same as its class name
2. A Constructor must have no explicit return type
3. A Java constructor cannot be abstract, static, final, and synchronized
Note: We can use access modifiers while declaring a constructor. It controls the object creation. In other words, we can have private, protected, public or default constructor in Java.

Types of Java constructors

There are two types of constructors in Java:
1. Default constructor (no-arg constructor)
2. Parameterized constructor

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ava Default Constructor
A constructor is called “Default Constructor” when it doesn’t have any parameter.
Syntax of default constructor:
1. <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.
1. //Java Program to create and call a default constructor
2. class Bike1{
3. //creating a default constructor
4. Bike1(){System.out.println(“Bike is created”);}
5. //main method
6. public static void main(String args[]){
7. //calling a default constructor
8. Bike1 b=new Bike1();
9. }
10. }

Output:
Bike is created
Rule: If there is no constructor in a class, compiler automatically creates a default constructor.

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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.

Example of default constructor that displays the default values

1. //Let us see another example of default constructor
2. //which displays the default values
3. class Student3{
4. int id;
5. String name;
6. //method to display the value of id and name
7. void display(){System.out.println(id+” “+name);}
8.
9. public static void main(String args[]){
10. //creating objects
11. Student3 s1=new Student3();
12. Student3 s2=new Student3();
13. //displaying values of the object
14. s1.display();
15. s2.display();
16. }
17. }

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.

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.
1. //Java Program to demonstrate the use of the parameterized constructor.
2. class Student4{
3. int id;
4. String name;
5. //creating a parameterized constructor
6. Student4(int i,String n){
7. id = i;
8. name = n;
9. }
10. //method to display the values
11. void display(){System.out.println(id+” “+name);}
12.
13. public static void main(String args[]){
14. //creating objects and passing values
15. Student4 s1 = new Student4(111,”Karan”);
16. Student4 s2 = new Student4(222,”Aryan”);
17. //calling method to display the values of object
18. s1.display();
19. s2.display();
20. }
21. }

Output:
111 Karan
222 Aryan

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.
Example of Constructor Overloading
1. //Java program to overload constructors
2. class Student5{
3. int id;
4. String name;
5. int age;
6. //creating two arg constructor
7. Student5(int i,String n){
8. id = i;
9. name = n;
10. }
11. //creating three arg constructor
12. Student5(int i,String n,int a){
13. id = i;
14. name = n;
15. age=a;
16. }
17. void display(){System.out.println(id+” “+name+” “+age);}
18.
19. public static void main(String args[]){
20. Student5 s1 = new Student5(111,”Karan”);
21. Student5 s2 = new Student5(222,”Aryan”,25);
22. s1.display();
23. s2.display();
24. }
25. }

Output:
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
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.

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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.
1. //Java program to initialize the values from one object to another object.
2. class Student6{
3. int id;
4. String name;
5. //constructor to initialize integer and string
6. Student6(int i,String n){
7. id = i;
8. name = n;
9. }
10. //constructor to initialize another object
11. Student6(Student6 s){
12. id = s.id;
13. name =s.name;
14. }
15. void display(){System.out.println(id+” “+name);}
16.
17. public static void main(String args[]){
18. Student6 s1 = new Student6(111,”Karan”);
19. Student6 s2 = new Student6(s1);
20. s1.display();
21. s2.display();
22. }
23. }

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?

Yes, it is the 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 a method, etc. You can perform any operation in the constructor as you perform in the method.

Is there Constructor class in Java?

Yes.

What is the purpose of Constructor class?

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.

this keyword in java

There can be a lot of usage of java this keyword. In java, this is a reference variable that refers to the current object.

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Usage of java this keyword

Here is given the 6 usage of java this keyword.
1. this can be used to refer current class instance variable.
2. this can be used to invoke current class method (implicitly)
3. this() can be used to invoke current class constructor.
4. this can be passed as an argument in the method call.
5. this can be passed as argument in the constructor call.
6. this can be used to return the current class instance from the method.
Suggestion: If you are beginner to java, lookup only three usage of this keyword.

java 18

1) this: to refer current class instance variable

The this keyword can be used to refer current class instance variable. If there is ambiguity between the instance variables and parameters, this keyword resolves the problem of ambiguity.
Understanding the problem without this keyword
Let’s understand the problem if we don’t use this keyword by the example given below:
1. class Student{
2. int rollno;
3. String name;
4. float fee;
5. Student(int rollno,String name,float fee){
6. rollno=rollno;
7. name=name;
8. fee=fee;
9. }
10. void display(){System.out.println(rollno+” “+name+” “+fee);}
11. }
12. class TestThis1{
13. public static void main(String args[]){
14. Student s1=new Student(111,”ankit”,5000f);
15. Student s2=new Student(112,”sumit”,6000f);
16. s1.display();
17. s2.display();
18. }}

Output:
0 null 0.0
0 null 0.0
In the above example, parameters (formal arguments) and instance variables are same. So, we are using this keyword to distinguish local variable and instance variable.
Solution of the above problem by this keyword
1. class Student{
2. int rollno;
3. String name;
4. float fee;
5. Student(int rollno,String name,float fee){
6. this.rollno=rollno;
7. this.name=name;
8. this.fee=fee;
9. }
10. void display(){System.out.println(rollno+” “+name+” “+fee);}
11. }
12.
13. class TestThis2{
14. public static void main(String args[]){
15. Student s1=new Student(111,”ankit”,5000f);
16. Student s2=new Student(112,”sumit”,6000f);
17. s1.display();
18. s2.display();
19. }}

Output:
111 ankit 5000
112 sumit 6000
If local variables(formal arguments) and instance variables are different, there is no need to use this keyword like in the following program:
Program where this keyword is not required
1. class Student{
2. int rollno;
3. String name;
4. float fee;
5. Student(int r,String n,float f){
6. rollno=r;
7. name=n;
8. fee=f;
9. }
10. void display(){System.out.println(rollno+” “+name+” “+fee);}
11. }
12.
13. class TestThis3{
14. public static void main(String args[]){
15. Student s1=new Student(111,”ankit”,5000f);
16. Student s2=new Student(112,”sumit”,6000f);
17. s1.display();
18. s2.display();
19. }}

Output:
111 ankit 5000
112 sumit 6000
It is better approach to use meaningful names for variables. So we use same name for instance variables and parameters in real time, and always use this keyword.

2) this: to invoke current class method

You may invoke the method of the current class by using the this keyword. If you don’t use the this keyword, compiler automatically adds this keyword while invoking the method. Let’s see the example

java 19

1. class A{
2. void m(){System.out.println(“hello m”);}
3. void n(){
4. System.out.println(“hello n”);
5. //m();//same as this.m()
6. this.m();
7. }
8. }
9. class TestThis4{
10. public static void main(String args[]){
11. A a=new A();
12. a.n();
13. }}
Output:
hello n
hello m

3) this() : to invoke current class constructor

The this() constructor call can be used to invoke the current class constructor. It is used to reuse the constructor. In other words, it is used for constructor chaining.

Calling default constructor from parameterized constructor:

1. class A{
2. A(){System.out.println(“hello a”);}
3. A(int x){
4. this();
5. System.out.println(x);
6. }
7. }
8. class TestThis5{
9. public static void main(String args[]){
10. A a=new A(10);
11. }}
Output:
hello a
10

Calling parameterized constructor from default constructor:

1. class A{

2. A(){
3. this(5);
4. System.out.println(“hello a”);
5. }
6. A(int x){
7. System.out.println(x);
8. }
9. }
10. class TestThis6{
11. public static void main(String args[]){
12. A a=new A();
13. }}
Output:
5
hello a

Real usage of this() constructor call

The this() constructor call should be used to reuse the constructor from the constructor. It maintains the chain between the constructors i.e. it is used for constructor chaining. Let’s see the example given below that displays the actual use of this keyword.
1. class Student{
2. int rollno;
3. String name,course;
4. float fee;
5. Student(int rollno,String name,String course){
6. this.rollno=rollno;
7. this.name=name;
8. this.course=course;
9. }
10. Student(int rollno,String name,String course,float fee){
11. this(rollno,name,course);//reusing constructor
12. this.fee=fee;
13. }
14. void display(){System.out.println(rollno+” “+name+” “+course+” “+fee);}
15. }
16. class TestThis7{
17. public static void main(String args[]){
18. Student s1=new Student(111,”ankit”,”java”);
19. Student s2=new Student(112,”sumit”,”java”,6000f);
20. s1.display();
21. s2.display();
22. }}

Output:
111 ankit java null
112 sumit java 6000
Rule: Call to this() must be the first statement in constructor.
1. class Student{
2. int rollno;
3. String name,course;
4. float fee;
5. Student(int rollno,String name,String course){
6. this.rollno=rollno;
7. this.name=name;
8. this.course=course;
9. }
10. Student(int rollno,String name,String course,float fee){
11. this.fee=fee;
12. this(rollno,name,course);//C.T.Error
13. }
14. void display(){System.out.println(rollno+” “+name+” “+course+” “+fee);}
15. }
16. class TestThis8{
17. public static void main(String args[]){
18. Student s1=new Student(111,”ankit”,”java”);
19. Student s2=new Student(112,”sumit”,”java”,6000f);
20. s1.display();
21. s2.display();
22. }}

Compile Time Error: Call to this must be first statement in constructor

4) this: to pass as an argument in the method

The this keyword can also be passed as an argument in the method. It is mainly used in the event handling. Let’s see the example:
1. class S2{
2. void m(S2 obj){
3. System.out.println(“method is invoked”);
4. }
5. void p(){
6. m(this);
7. }
8. public static void main(String args[]){
9. S2 s1 = new S2();
10. s1.p();
11. }
12. }

Output:
method is invoked

Application of this that can be passed as an argument:

In event handling (or) in a situation where we have to provide reference of a class to another one. It is used to reuse one object in many methods.

5) this: to pass as argument in the constructor call

We can pass the this keyword in the constructor also. It is useful if we have to use one object in multiple classes. Let’s see the example:
1. class B{
2. A4 obj;
3. B(A4 obj){
4. this.obj=obj;
5. }
6. void display(){
7. System.out.println(obj.data);//using data member of A4 class
8. }
9. }
10.
11. class A4{
12. int data=10;
13. A4(){
14. B b=new B(this);
15. b.display();
16. }
17. public static void main(String args[]){
18. A4 a=new A4();
19. }
20. }

Output:10

6) this keyword can be used to return current class instance

We can return this keyword as an statement from the method. In such case, return type of the method must be the class type (non-primitive). Let’s see the example:

Syntax of this that can be returned as a statement

1. return_type method_name(){
2. return this;
3. }

Example of this keyword that you return as a statement from the method

1. class A{
2. A getA(){
3. return this;
4. }
5. void msg(){System.out.println(“Hello java”);}
6. }
7. class Test1{
8. public static void main(String args[]){
9. new A().getA().msg();
10. }
11. }

Output:
Hello java

Proving this keyword

Let’s prove that this keyword refers to the current class instance variable. In this program, we are printing the reference variable and this, output of both variables are same.
1. class A5{
2. void m(){
3. System.out.println(this);//prints same reference ID
4. }
5. public static void main(String args[]){
6. A5 obj=new A5();
7. System.out.println(obj);//prints the reference ID
8. obj.m();
9. }
10. }

Output:
A5@22b3ea59
A5@22b3ea59

So, this brings us to the end of blog. This Tecklearn ‘Constructors in Java’ blog helps you with commonly asked questions if you are looking out for a job in Java Programming. If you wish to learn Java and build a career Java Programming domain, then check out our interactive, Java and JEE Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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