Constructors in Java

Constructors are used to initialize the object’s state. Like methods, a constructor also contains collection of statements(i.e. instructions) that are executed at time of Object creation.

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

Constructor is invoked at the time of object or instance creation. For Example:

class Geek
{   
  .......

  // A Constructor
  new Geek() {}

  .......
}

// We can create an object of above class
// using below statement. This statement
// calls above constructor.
Geek obj = new Geek(); 

Rules for writing Constructor:

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

Types of constructor

There are two type of constructor in Java:

1. No-argument constructor: A constructor that has no parameter is known as default constructor. If we don’t define a constructor in a class, then compiler creates default constructor(with no arguments) for the class. And if we write a constructor with arguments or no-argument then compiler does not create default constructor.
   Default constructor provides the default values to the object like 0, null etc. depending on the type.

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   // Java Program to illustrate calling a // no-argument constructor import java.io.*;    class Geek {     int num;     String name;        // this would be invoked while object     // of that class created.     Geek()     {         System.out.println("Constructor called");     } }    class GFG {     public static void main (String[] args)     {         // this would invoke default constructor.         Geek geek1 = new Geek();            // Default constructor provides the default         // values to the object like 0, null         System.out.println(geek1.name);         System.out.println(geek1.num);     } }

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   Output :

   Constructor called
   null
   0
   

2. Parameterized Constructor: A constructor that has parameters is known as parameterized constructor. If we want to initialize fields of the class with your own values, then use parameterized constructor.
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   // Java Program to illustrate calling of // parameterized constructor. import java.io.*;    class Geek {     // data members of the class.     String name;     int id;        // contructor would initialized data members     // with the values of passed arguments while     // object of that class created.     Geek(String name, int id)     {         this.name = name;         this.id = id;     } }    class GFG {     public static void main (String[] args)     {         // this would invoke parameterized constructor.         Geek geek1 = new Geek("adam", 1);         System.out.println("GeekName :" + geek1.name +                            " and GeekId :" + geek1.id);     } }

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   Output:

   GeekName :adam and GeekId :1
   

Does constructor return any value?

There are no “return value” statements in constructor, but constructor returns current class instance. We can write ‘return’ inside a constructor.

Constructor Overloading

Like methods, we can overload constructors for creating objects in different ways. Compiler differentiates constructors on the basis of numbers of parameters, types of the parameters and order of the parameters.

Output:

Constructor with one argument - String : Shikhar
Constructor with two arguments - String and Integer : Dharmesh 26
Constructor with one argument - Long : 325614567

How constructors are different from methods in Java?

• Constructor(s) must have the same name as the class within which it defined while it is not necessary for the method in java.
• Constructor(s) do not any return type while method(s) have the return type or void if does not return any value.
• Constructor is called only once at the time of Object creation while method(s) can be called any numbers of time.

Related Articles:

• Constructor Chaining
• Copy constructor

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