Java OOPS:Constructors in Java – A complete study!!

 

Constructors in Java – A complete study!!

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:

MyClass obj = new MyClass()

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

Output:

BeginnersBook.com

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:

If you implement any constructor then you no longer receive a default constructor from Java compiler.

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.

Although you may see some people claim that that default and no-arg constructor is same but in fact they are not, even if you write public Demo() { } in your class Demo it cannot be called default constructor since you have written the code of it.

Example: no-arg constructor

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,"Chaitanya");  
	Employee obj2 = new Employee(92232,"Negan");  
	obj1.info();  
	obj2.info();  
   }  
}

Output:

Id: 10245 Name: Chaitanya
Id: 92232 Name: Negan

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();
   }
}

Output:

MyParentClass Constructor
MyChildClass Constructor

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.

Java 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("BeginnersBook");  
		
	/* Passing the object as an argument to the constructor
	 * This will invoke the copy constructor
	 */
	JavaExample obj2 = new JavaExample(obj1);  
	obj1.disp();  
	obj2.disp();  
   }  
}

Output:

Website: BeginnersBook
Website: BeginnersBook

Quick Recap

1. Every class has a constructor whether it’s a normal class or a abstract class.
2. Constructors are not methods and they don’t have any return type.
3. Constructor name should match with class name .
4. 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.
5. 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.
6. If you don’t implement any constructor within the class, compiler will do it for.
7. this() and super() should be the first statement in the constructor code. If you don’t mention them, compiler does it for you accordingly.
8. 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.
9. Constructors can not be inherited.
10. 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.
11. Interfaces do not have constructors.
12. Abstract class can have constructor and it gets invoked when a class, which implements interface, is instantiated. (i.e. object creation of concrete class).
13. 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).

Java – private constructor example

The use of private constructor is to serve singleton classes. A singleton class is one which limits the number of objects creation to one. Using private constructor we can ensure that no more than one object can be created at a time. By providing a private constructor you prevent class instances from being created in any place other than this very class. We will see in the below example how to use private constructor for limiting the number of objects for a singleton class.

Example of Private Constructor

public class SingleTonClass {
   //Static Class Reference
   private static SingleTonClass obj=null;
   private SingleTonClass(){
      /*Private Constructor will prevent 
       * the instantiation of this class directly*/
   }
   public static SingleTonClass objectCreationMethod(){
	/*This logic will ensure that no more than
	 * one object can be created at a time */
	if(obj==null){
	    obj= new SingleTonClass();
	}
        return obj;
   }
   public void display(){
	System.out.println("Singleton class Example");
   }
   public static void main(String args[]){
	//Object cannot be created directly due to private constructor 
        //This way it is forced to create object via our method where
        //we have logic for only one object creation
	SingleTonClass myobject= SingleTonClass.objectCreationMethod();
	myobject.display();
   }
}

Output:

Singleton class Example

Java static constructor – Is it really Possible to have them in Java?

Have you heard of static constructor in Java? I guess yes but the fact is that they are not allowed in Java. A constructor can not be marked as static in Java.  Before I explain the reason let’s have a look at the following piece of code:

public class StaticTest
{
     /* See below - I have marked the constructor as static */
     public static StaticTest()
     {
         System.out.println("Static Constructor of the class");
     }
     public static void main(String args[])
     {
         /*Below: I'm trying to create an object of the class
          *that should invoke the constructor
          */
         StaticTest obj = new StaticTest();
     }
}

Output: You would get the following error message when you try to run the above java code.
“modifier static not allowed here”

Why java doesn’t support static constructor?

It’s actually pretty simple to understand – Everything that is marked static belongs to the class only, for example static method cannot be inherited in the sub class because they belong to the class in which they have been declared. Refer static keyword.
Lets back to the point, since each constructor is being called by its subclass during creation of the object of its subclass, so if you mark constructor as static the subclass will not be able to access the constructor of its parent class because it is marked static and thus belong to the class only. This will violate the whole purpose of inheritance concept and that is reason why a constructor cannot be static.

Let’s understand this with the help of an example –

public class StaticDemo
{
     public StaticDemo()
     {
         /*Constructor of this class*/
         System.out.println("StaticDemo");
     }
}
public class StaticDemoChild extends StaticDemo
{
     public StaticDemoChild()
     {
          /*By default super() is hidden here */
          System.out.println("StaticDemoChild");
     }
     public void display()
     {
          System.out.println("Just a method of child class");
     }
     public static void main(String args[])
     {
          StaticDemoChild obj = new StaticDemoChild();
          obj.display();
     }
}

Output:
StaticDemo
StaticDemoChild
Just a method of child class

Did you notice? When we created the object of child class, it first invoked the constructor of parent class and then the constructor of it’s own class. It happened because the new keyword creates the object and then invokes the constructor for initialization, since every child class constructor by default has super() as first statement which calls it’s parent class’s constructor. The statement super() is used to call the parent class(base class) constructor.

This is the reason why constructor cannot be static – Because if we make them static they cannot be called from child class thus object of child class cannot be created.

Another good point mentioned by Prashanth in the comment section: Constructor definition should not be static because constructor will be called each and every time when object is created. If you made constructor as static then the constructor will be called before object creation same like main method.

Static Constructor Alternative – Static Blocks

Java has static blocks which can be treated as static constructor. Let’s consider the below program –

public class StaticDemo{
    static{
        System.out.println("static block of parent class");
    }
}
public class StaticDemoChild extends StaticDemo{
    static{
        System.out.println("static block of child class");
    }
    public void display()
    {
        System.out.println("Just a method of child class");
    }
    public static void main(String args[])
    {
        StaticDemoChild obj = new StaticDemoChild();
        obj.display();
    }
}

Output:
static block of parent class
static block of child class
Just a method of child class

In the above example we have used static blocks in both the classes which worked perfectly. We cannot use static constructor so it’s a good alternative if we want to perform a static task during object creation.

Difference between Constructor and Method

I know I should have mentioned it at the beginning of this guide but I wanted to cover everything in a flow. Hope you don’t mind :)

1. 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.
2. Constructors cannot be abstract, final, static and synchronised while methods can be.
3. Constructors do not have return types while methods do.