Constructor in java

by May 10, 2017 Java

Let’s learn constructor in java.

Constructor in java

Constructor is a block of code used to initialize objects.

constructor in java

NOTE:

  • constructor does not have any return type.
  • constructor name is same as class name.
  • constructor is called when an object of a classs is created.
  • access modifiers applicable for constructors are public, private, protected and default.
  • constructor gets called when we write “new()” keyword.

Syntax:

class DemoClass
{
   // constructor name is same as class name
   DemoClass()
   {
      ....
   }
}
// calls DemoClass() constructor
DemoClass obj = new DemoClass();

Constructor example:

// create ConstructorDemo class
public class ConstructorDemo
{
   int a; // class attribute
   // create constructor for ConstructorDemo class
   ConstructorDemo()
   {
      a = 26; // initial value for class attribute 'a'
   }
   public static void main(String[] args)
   {
      // creating object for ConstructorDemo class
      // here we call constructor ConstructorDemo()
      ConstructorDemo obj = new ConstructorDemo();
      System.out.println(obj.a);
   }
}

Output:

26

Types of constructor

  • No-arg constructor
  • Parameterised constructor

No-argument (no-arg constructor)

No-argument (no-arg constructor) is a constructor with no arguments.

If a class do not have constructor then compiler creates default constructor with no arguments.

Let’s see an example on no-arg constructor.

class NoArgExample
{ 
   int number; 
   String name; 
   NoArgExample() 
   { 
      System.out.println("Constructor called"); 
   } 
}
public class NoArgConstructorDemo 
{
   public static void main(String[] args) 
   {
      // this will invoke default constructor. 
      NoArgExample obj = new NoArgExample();
      // default constructor provides default 
      // values to object like 0, null 
      System.out.println(obj.name); 
      System.out.println(obj.number);
   }
}

Output:

Constructor called
null
0

NOTE: In a java program if there is no constructor in a class then compiler automatically creates default constructor.

Here’s how it looks in .class file,

// Employee.java
class Employee
{
   public static void main(String args[])
   {
      // invoking default constructor
      Employee emp = new Employee();
   }
}

// Employee.class
class Employee
{
   Employee()
   {

   }
   public static void main(String args[])
   {
      Employee emp = new Employee();
   }
}

Parameterized constructor

Parameterized constructor is a constructor which has fixed number of parameters.

Parameterized constructor main purpose is to provide different values to objects. Let’s see an example on parameterized constructor.

class Employee
{
   int empID;
   String empName;
   // parameterized constructor
   Employee(int ID, String name)
   {
      empID = ID;
      empName = name;
   }
   void print()
   {
      System.out.println(empID + " " + empName);
   }
   public static void main(String args[])
   {
      Employee obj1 = new Employee(232, "dhoni");
      Employee obj2 = new Employee(233, "virat");
      obj1.print();
      obj2.print();
   }
}

Output:

232 dhoni
233 virat

Constructor overloading in java

Constructor overloading is an approach of having more than one constructors in a class with different arguments performing different tasks.

Constructors can be overloaded like methods. Let’s see a java program to overload constructors.

class Employee
{
   int empID;  
   String empName;  
   int empAge;  
   // two argument constructor  
   Employee(int id, String name)
   {  
      empID = id;  
      empName = name;  
   }  
   // three argument constructor  
   Employee(int id, String name, int age)
   {  
      empID = id;  
      empName = name;  
      empAge = age;  
   }  
   void print()
   {
      System.out.println(empID + " " + empName + " " + empAge);
   }
   public static void main(String args[])
   {  
      Employee obj1 = new Employee(232, "dhoni");  
      Employee obj2 = new Employee(233, "virat", 26);  
      obj1.print();
      obj2.print();
   }
}

Output:

232 dhoni 0
233 virat 26