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Chapter 7 - Constructors.pdf
1. Blue Ridge Public School
Standard X
Computer Applications – Chapter – 7 - Constructors
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2. Topics
• What are Constructors?
• Need for Constructors
• Types of Constructors
• Parameterized Constructors
• Constructor Overloading
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3. What are Constructors?
• A constructor in Java is a block of code similar to a method
that is called when an instance of an object is created
• It is a special method that is used to initialize a newly created
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
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4. What are Constructors – cont.
• Key differences between a constructor and a method
• A constructor doesn’t have a return type (not even void) since
the basic aim is to place the value in the object. If we write the
return type for the constructor then that constructor will be
treated as ordinary method
• The name of the constructor must be the same as the name of
the class
• Constructor definitions should not be static as constructors will
be called each and every time, whenever an object is created
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5. What are Constructors – cont.
• Unlike methods, constructors are not considered members of a
class
• A constructor is called automatically when a new instance of an
object is created
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Method Constructor
Method can be any user
defined name
Constructor must be class
name
Method should have return
type
It should not have any return
type (even void)
Method should be called
explicitly either with object
reference or class reference
It will be called automatically
whenever object is created
Method is not provided by
compiler in any case
The java compiler provides a
default constructor if we do
not have any constructor
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6. Need for Constructors
• It can be cumbersome to initialize all variables in a call each
time an instance of a class in created
• Constructors are mainly created for initializing the object
• Initialization is a process of assigning user defined values at
the time of allocation of memory space
• An advantage of constructors in Java is it eliminates placing
the default values
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7. Need for Constructors – cont.
• Whenever we create an object of any class, memory is
allocated memory for all the data members and their
values are initialized to their default values
• To eliminate these default values by user defined values
we use constructor
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8. Constructor Example
class Programming
{ //constructor method
Programming()
{
System.out.println("Constructor method called.");
}
public static void main(String[] args)
{
Programming object = new Programming(); //creating object
}
}
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9. Types of Constructors
• Based on creating objects in Java, constructor are classified in
two types. They are:
• Default or no argument Constructor
• Parameterized Constructor
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10. Types of Constructors – cont.
Default or no argument Constructor
• A constructor that has no parameter is known as default
constructor
• If we do not define a constructor in a class, then compiler
creates default constructor(with no arguments) for the class
• 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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11. Types of Constructors – cont.
• Example of default constructor
• In this example, we are creating the no-argument constructor in
the Bike class
• It will be invoked at the time of object creation
class Bike
{
Bike()
{
System.out.println("Bike is created");
}
public static void main(String args[])
{
Bike b=new Bike ();
}
}
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12. Types of Constructors – cont.
• If there is no constructor in a class, compiler automatically
creates a default constructor
• The default constructor is a constructor that the Java
compiler adds to your code if no explicit constructor is
available
• If you have added your own constructor (no matter whether
it's without parameters or with parameters) the compiler will
not add the default constructor in this case
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13. Types of Constructors – cont.
• 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
• This type of constructor accepts parameters and initializes the
data members based on the arguments passed to it
• A parameter is a variable in a method definition. When a method is
called, the arguments are the data you pass into the method's
parameters
public void MyMethod(String myParam) { }
...
String myArg1 = "this is my argument"; myClass.MyMethod(myArg1);
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14. Types of Constructors – cont.
• Example of parameterized constructor
class Student4{
int id;
String name;
Student4(int i,String n){
id = i;
name = n;
}
void display()
{
System.out.println(id+" "+name);
}
public static void main(String args[]){
Student4 s1 = new Student4(111,"Karan");
Student4 s2 = new Student4(222,"Aryan");
s1.display();
s2.display();
}
}
• In this example, we have created the constructor of Student class that has two parameters
• We can have any number of parameters in the constructor
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15. Constructor Overloading
• Constructor overloading is a technique in Java in which a class
can have any number of constructors that differ in parameter
lists
• 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
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16. Constructor Overloading – cont.
• Example of Constructor Overloading
class Student5{
int id;
String name;
int age;
Student5(int i,String n){
id = i;
name = n;
}
Student5(int i,String n,int a){
id = i;
name = n;
age=a;
}
void display(){System.out.println(id+" "+name+" "+age);}
public static void main(String args[]){
Student5 s1 = new Student5(111,"Karan");
Student5 s2 = new Student5(222,"Aryan",25);
s1.display();
s2.display();
}
}
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17. Constructor Overloading – cont.
class Demo{
int value1;
int value2;
Demo(){
value1 = 10;
value2 = 20;
System.out.println("Inside 1st Constructor");
}
Demo(int a){
value1 = a;
System.out.println("Inside 2nd Constructor");
}
Demo(int a,int b){
value1 = a;
value2 = b;
System.out.println("Inside 3rd Constructor");
}
public void display(){
System.out.println("Value1 === "+value1);
System.out.println("Value2 === "+value2);
}
public static void main(String args[]){
Demo d1 = new Demo();
Demo d2 = new Demo(30);
Demo d3 = new Demo(30,40);
d1.display();
d2.display();
d3.display();
}
}
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Output
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