Constructors In Java- An Overview for Beginners

A constructor in Java is a special method that is used to initialize objects. The constructor is called when an object of a class is created. It can be used to set initial values for object attributes.

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 the 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.

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How Constructors are Different From Methods in Java? 

• Constructors must have the same name as the class within which it is defined while it is not necessary for the method in Java.
• Constructors do not return any type while method(s) have the return type or voidif does not return any value.
• Constructors are called only once at the time of Object creation while method(s) can be called any number of times.

Need of Constructor

Think of a Box. If we talk about a box class then it will have some class variables (say length, breadth, and height). But when it comes to creating its object(i.e Box will now exist in the computer’s memory), then can a box be there with no value defined for its dimensions. The answer is no.
So constructors are used to assigning values to the class variables at the time of object creation, either explicitly done by the programmer or by Java itself (default constructor).

When is a Constructor called?

Each time an object is created using a new() keyword, at least one constructor (it could be the default constructor) is invoked to assign initial values to the data members of the same class.

The rules for writing constructors 

• Constructor(s) of a class must have the same name as the class name in which it resides.
• A constructor in Java can not be abstract, final, static, or Synchronized.
• Access modifiers can be used in constructor declaration to control its access i.e which other class can call the constructor.

Types of Constructors in Java

Now is the correct time to discuss types of the constructor, so primarily there are two types of constructors in java:

• No-argument constructor
• Parameterized Constructor

• No-argument constructor

A constructor that has no parameter is known as the default constructor. If we don’t define a constructor in a class, then the compiler creates a default constructor(with no arguments) for the class. And if we write a constructor with arguments or no-arguments then the compiler does not create a default constructor.

• Parameterized Constructor

A constructor that has parameters is known as parameterized constructor. If we want to initialize fields of the class with our own values, then use a parameterized constructor.

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Default Constructor

The default constructor in Java initializes the data members of the class to their default values such as 0 for int, 0.0 for double etc. This constructor is implemented by default by the Java compiler if there is no explicit constructor implemented by the user for the class.

If you observe the following example, we are not providing any constructor to it.

public class Sample {

int num;

public static void main(String args[]){

System.out.println(new Sample().num);

}

}

If you compile and run the above program the default constructor initializes the integer variable num with 0 and, you will get 0 as result.

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