Constructors in Java

Constructor is a block of code that initializes the newly created object. A constructor resembles an instance method in java but it's not a method as it doesn't have a return type. In short constructor and method are different(More on this at the end of this guide). People often refer constructor as special type of method in Java.

Constructor has same name as the class and looks like this in a java code.

public class MyClass{
   //This is the constructor
   MyClass(){
   }
   ..
}

Note that the constructor name matches with the class name and it doesn't have a return type.

How does a constructor work

To understand the working of constructor, lets take an example. lets say we have a class MyClass. When we create the object of MyClass like this:

The new keyword here creates the object of class MyClass and invokes the constructor to initialize this newly created object.

You may get a little lost here as I have not shown you any initialization example, lets have a look at the code below:

A simple constructor program in java

Here we have created an object obj of class Hello and then we displayed the instance variable name of the object. As you can see that the output is noidatut.com which is what we have passed to the name during initialization in constructor. This shows that when we created the object obj the constructor got invoked. In this example we have used this keyword, which refers to the current object, object obj in this example. We will cover this keyword in detail in the next tutorial.

public class Hello {
   String name;
   //Constructor
   Hello(){
      this.name = "noidatut.com";
   }
   public static void main(String[] args) {
      Hello obj = new Hello();
      System.out.println(obj.name);
   }
}

Types of Constructors

There are three types of constructors: Default, No-arg constructor and Parameterized.

Default constructor

If you do not implement any constructor in your class, Java compiler inserts a default constructor into your code on your behalf. This constructor is known as default constructor. You would not find it in your source code(the java file) as it would be inserted into the code during compilation and exists in .class file. This process is shown in the diagram below:

no-arg constructor:

Constructor with no arguments is known as no-arg constructor. The signature is same as default constructor, however body can have any code unlike default constructor where the body of the constructor is empty.

Example: no-arg constructor

Case 2

In this example, we have break statement after each Case block, this is because if we don't have it then the subsequent case block would also execute. The output of the same program without break would be Case 2 Case 3 Default.

class Demo
{
     public Demo()
     {
         System.out.println("This is a no argument constructor");
     }
     public static void main(String args[]) {
    	 new Demo();
     }
}

Output:

This is a no argument constructor

Parameterized constructor

Constructor with arguments(or you can say parameters) is known as Parameterized constructor.

Example: parameterized constructor

In this example we have a parameterized constructor with two parameters id and name. While creating the objects obj1 and obj2 I have passed two arguments so that this constructor gets invoked after creation of obj1 and obj2.

public class Employee {

   int empId;  
   String empName;  
	    
   //parameterized constructor with two parameters
   Employee(int id, String name){  
       this.empId = id;  
       this.empName = name;  
   }  
   void info(){
        System.out.println("Id: "+empId+" Name: "+empName);
   }  
	   
   public static void main(String args[]){  
	Employee obj1 = new Employee(10245,"Noidatut");  
	Employee obj2 = new Employee(92232,"Proverb");  
	obj1.info();  
	obj2.info();  
   }  
}

Output:

Id: 10245 Name: Noidatut

Id: 92232 Name: Proverb

Example2: parameterized constructor

In this example, we have two constructors, a default constructor and a parameterized constructor. When we do not pass any parameter while creating the object using new keyword then default constructor is invoked, however when you pass a parameter then parameterized constructor that matches with the passed parameters list gets invoked.

class Example2
{
      private int var;
      //default constructor
      public Example2()
      {
             this.var = 10;
      }
      //parameterized constructor
      public Example2(int num)
      {
             this.var = num;
      }
      public int getValue()
      {
              return var;
      }
      public static void main(String args[])
      {
              Example2 obj = new Example2();
              Example2 obj2 = new Example2(100);
              System.out.println("var is: "+obj.getValue());
              System.out.println("var is: "+obj2.getValue());
      }
}

Output:

var is: 10

var is: 100

What if you implement only parameterized constructor in class

class Example3
{
      private int var;
      public Example3(int num)
      {
             var=num;
      }
      public int getValue()
      {
              return var;
      }
      public static void main(String args[])
      {
              Example3 myobj = new Example3();
              System.out.println("value of var is: "+myobj.getValue());
      }
}

Output: It will throw a compilation error. The reason is, the statement Example3 myobj = new Example3() is invoking a default constructor which we don't have in our program. when you don't implement any constructor in your class, compiler inserts the default constructor into your code, however when you implement any constructor (in above example I have implemented parameterized constructor with int parameter), then you don't receive the default constructor by compiler into your code.

If we remove the parameterized constructor from the above code then the program would run fine, because then compiler would insert the default constructor into your code.

Constructor Chaining

When A constructor calls another constructor of same class then this is called constructor chaining.

Super()

Whenever a child class constructor gets invoked it implicitly invokes the constructor of parent class. You can also say that the compiler inserts a super(); statement at the beginning of child class constructor.

class MyParentClass {
   MyParentClass(){
	System.out.println("MyParentClass Constructor");
   }
}
class MyChildClass extends MyParentClass{
   MyChildClass() {
	System.out.println("MyChildClass Constructor");
   }
   public static void main(String args[]) {
	new MyChildClass();
   }
}

Constructor Overloading

Constructor overloading is a concept of having more than one constructor with different parameters list, in such a way so that each constructor performs a different task.

Copy Constructor

A copy constructor is used for copying the values of one object to another object.

class JavaExample{  
   String web; 
   JavaExample(String w){  
	web = w;
   }  

   /* This is the Copy Constructor, it 
    * copies the values of one object
    * to the another object (the object
    * that invokes this constructor)
    */
   JavaExample(JavaExample je){  
	web = je.web; 
   }  
   void disp(){
	System.out.println("Website: "+web);
   }  

   public static void main(String args[]){  
	JavaExample obj1 = new JavaExample("Javatutorial");  
		
	/* Passing the object as an argument to the constructor
	 * This will invoke the copy constructor
	 */
	JavaExample obj2 = new JavaExample(obj1);  
	obj1.disp();  
	obj2.disp();  
   }  
}

Difference between Constructor and Method

The purpose of constructor is to initialize the object of a class while the purpose of a method is to perform a task by executing java code.

Constructors cannot be abstract, final, static and synchronised while methods can be.

Constructors do not have return types while methods do.

Quick Recap

Every class has a constructor whether it's a normal class or a abstract class.

Constructors are not methods and they don't have any return type.

Constructor name should match with class name .

Constructor can use any access specifier, they can be declared as private also. Private constructors are possible in java but there scope is within the class only.

Like constructors method can also have name same as class name, but still they have return type, though which we can identify them that they are methods not constructors.

If you don't implement any constructor within the class, compiler will do it for.

this() and super() should be the first statement in the constructor code. If you don't mention them, compiler does it for you accordingly.

Constructor overloading is possible but overriding is not possible. Which means we can have overloaded constructor in our class but we can't override a constructor.

Constructors can not be inherited.

If Super class doesn't have a no-arg(default) constructor then compiler would not insert a default constructor in child class as it does in normal scenario.

Interfaces do not have constructors.

Abstract class can have constructor and it gets invoked when a class, which implements interface, is instantiated. (i.e. object creation of concrete class).

A constructor can also invoke another constructor of the same class - By using this(). If you want to invoke a parameterized constructor then do it like this: this(parameter list).