Constructor In Java : Top Most Conclusion

Constructors in Java

Constructor in java is a special type of method that is used to initialize the object.

Java constructor is invoked at the time of object creation. It constructs the values i.e. provides data for the object that is why it is known as constructor.

Rules for creating java constructor:

Constructor name must be same as its class name.

Constructor must not have any explicit return type.

Constructrs can not be abstract, static, final or synchronized.

Example:

class Car
{
String name ;
String model;
Car( )     //Constructor
{
  name ="";
  model="";
}
}
Two types of Constructor:

1.Default constructor

(no-arg constructor) : Default constructor provides the default values to the object like 0, null etc.

2.Parameterized constructor

Default constructor provides the default values to the object like 0, null etc.
Each time a new object is created at least one constructor will be invoked.

Car c = new Car() //Default constructor invoked
Car c = new Car(name); //Parameterized constructor invoked
NOTE : If there is no constructor in a class, compiler automatically creates a default constructor.

Constructors Overloading

Like methods, a constructor can also be overloaded. Overloaded constructors are differentiated on the basis of their type of parameters or number of parameters.

Why do we Overload constructors ?

Constuctor overloading is done to construct object in different ways.

Example:

class Cricketer
{
String name;
String team;
int age;
Cricketer ()   //default constructor.
{
  name ="";
  team ="";
  age = 0;
}
Cricketer(String n, String t, int a)   //constructor overloaded
{
  name = n;
  team = t;
  age = a;
}
Cricketer (Cricketer ckt)     //constructor similar to copy constructor of c++
{
  name = ckt.name;
  team = ckt.team;
  age = ckt.age;
}
public String toString()
{
  return "This is " + name + " of "+team;
}
}

public class test
{
public static void main (String[] args)
{
  Cricketer c1 = new Cricketer();
  Cricketer c2 = new Cricketer("Sachin", "India", 43);
  Cricketer c3 = new Cricketer(c2 );

  c1.name = "Virat";
  c1.team= "India";
  c1.age = 27;

  System.out.println(c1);
  System.out.println(c2);
  System.out.println(c3);
}
}
Output:
This is Virat of India
This is Sachin of India
This is Sachin of India

Difference between constructor and method in java

Constructor Method
Constructor is used to initialize the state of an object. Method is used to expose behaviour of an object.
Constructor must not have return type. Method must have return type.
Object is a physical entity. Class is a logical entity.
Constructor is invoked implicitly. Method is invoked explicitly.
Object is created many times as per requirement. Class is declared once.
The java compiler provides a default constructor if you don't have any constructor. Method is not provided by compiler in any case.
Constructor name must be same as the class name. Method name may or may not be same as class name.

Q. What is constructor chaining in Java??

NOTE : Constructor chaining is a phenomena of calling one constructor from another constructor of same class. Since constructor can only be called from another constructor in Java, constructor chaining is used for this purpose.

class Test
{
Test()
{
  this(10);
}
Test(int x)
{
  System.out.println("x="+x);
}
}
Output:
x=10

Q. Does constructor return any value??

Ans:Yes, Constructor return current class instance (You cannot use return type yet it returns a value).

2.this keyword

this keyword is used to refer current class object.

this() can be used to invoke current class constructor.

this keyword can be used to invoke current class method(implicitly).

this can be passed as an argument in the method or constructor call.

this keyword can also be used to return the current class instance.

Example:

class Box
{
  Double width, weight, dept;
  Box (double w, double h, double d)
  {
   this.width = w;
   this.height = h;
   this.depth = d;
  }
}
Here the this is used to initialize member of current object.

NOTE : Call to this() must be the first statement in constructor.

this is used to call overloaded constructor (or constructor chaining) in java.

class Car
{
private String name;
public Car()
{
  this("BMW");    //oveloaded constructor is called.
}
public Car(Stting n)
{
  this.name=n;   //member is initialized using this.
}
}
this is also used to call Method of that class.

public void getName()
{
System.out.println("BeTheDeveloper");
}

public void display()
{
this.getName();
System.out.println();
}
this is used to return current Object.

public Car getCar()
{
return this;
}

2.static keyword

The static keyword in java is used for memory management mainly. We can apply java static keyword with variables, methods, blocks and nested class. The static keyword belongs to the class than instance of the class.

The static can be:

variable (also known as class variable).

method (also known as class method).

block.

nested class.

static variable

If you declare any variable as static, it is known static variable.

1. The static variable can be used to refer the common property of all objects (that is not unique for each object) e.g. company name of employees,college name of students etc.

2. The static variable gets memory only once in class area at the time of class loading.

Advantage of static variable

It makes your program memory efficient (i.e it saves memory).

NOTE : Java static property is shared to all objects.

Example: Program of counter by static variable.

class Counter{ 
static int count=0;//will get memory only once and retain its value 
 
Counter(){ 
count++; 
System.out.println(count); 
} 
 
public static void main(String args[]){ 
 
Counter c1=new Counter(); 
Counter c2=new Counter(); 
Counter c3=new Counter(); 
 
} 
} 
Output:
1     
2      
3
Static variable vs Instance Variable

Static variable Instance Variable
Represent common property. Instance Variable.
Accessed using class name. Accessed using object.
get memory only once. get new memory each time a new object is created.

Static Method

If you apply static keyword with any method, it is known as static method.

1. A static method belongs to the class rather than object of a class.

2. A static method can be invoked without the need for creating an instance of a class (or objects).

3. static method can access static data member and can change the value of it.

Example: Program to get cube of a given number by static method.

class Calculate{ 
  static int cube(int x){ 
  return x*x*x; 
  } 
 
  public static void main(String args[]){ 
  int result=Calculate.cube(5); 
  System.out.println(result); 
  } 
} 
Output: 125

Restrictions for static method

1. The static method can not use non static data member or call non-static method directly.

2. this and super cannot be used in static context.

class A{ 
int a=40;  //non static 
  
public static void main(String args[]){ 
  System.out.println(a); 
} 
}       
Output: Compile Time Error
Q. Why java main method is static??

Ans) because object is not required to call static method if it were non-static method, jvm create object first then call main() method that will lead the problem of extra memory allocation.

static Block

Is used to initialize the static data member.
It is executed before main method at the time of classloading.

Example of static block

class S2{ 
  static{System.out.println("static block is invoked");} 
  public static void main(String args[]){ 
   System.out.println("main method is invoked"); 
  } 
} 
Output:
static block is invoked
main method is invoked