The document discusses the concept of constructors in Java. It defines a constructor as a special method that initializes an object immediately upon creation and has the same name as the class. The document explains different types of constructors - default, explicit, parameterized, and overloaded constructors. It provides examples to illustrate these constructor types and how they are used to initialize object properties. The key differences between methods and constructors are also outlined.

1. Concept of Constructors and Types of Constructors
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2. Objectives
On completion of this period, you would be able to know
• What is constructor
• How to implement constructors
• Different types of constructors
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3. Recap
• We have discussed in the previous class
• ‘new’ operator
• Usage of ‘new’ operator
• Methods in a class
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4. Concept of Constructors
• Constructor is special member of the class
• It initializes an object immediately upon creation
• It has the same name of the class
• It is automatically called after the object is create,
before the ‘new’ operator completes
• It has no return type, not even void
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5. Concept of Constructors
• Example program
class Point{
int x,y;
Point(int x, int y){
this.x = x;
this.y = y;
}
• Class name is Point
• It has 2 instance variables : x and y
• A constructor is Point defined : Point(int x, int y)
• The constructor initializes the instance variables x and y
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6. Concept of Constructors
class PointCreate{
public static void main(String args[]){
Point p = new Point(10,20);
System.out.println(“x=“+p.x + “y=“+p.y);
}
}
• The object name is ‘p’
• It is created using the constructor
• The constructor sets the values of x and y to 10 and 20
respectively
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7. Use of Constructors
• Constructors are used
• if we know the specific value to be assigned to a data member
when an object is created
• to initialize the data member of the class, when an object is
created
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8. Difference Between Classes
WITHOUT CONSTRCTOR WITH CONSTRUCTOR
class A { class A {
int x; // data member int x; // data member
void set_x() { public A() { // constructor
x = 10; x = 10;
} }
void display_x() { void display_x(){// method display
System.out.println(“x value is:”+x) ; System.out.println(“x value is:”+x) ;
} }
} // end of class A } // end of class A
public class B{
public static void main (String args[ ]) public class B {
{ public static void main (String args[
A ref_a = new A(); ]) {
ref_a.set_x(); // calling set _x A ref_a = new A();
ref_a.display_x();// calling disp ref_a.display_x();
} }
} // end of class B } // end of class B

9. Comparison
• Without Constructor
• Output of the above program is : x value is 10
• In the above program constructor is NOT used to initialize the
data member x
• But an explicit method (set_x) is used to initialize the data
member x , and also the set_x method invoked explicitly
(ref_a.set_x).

10. Comparison contd..
• With Constructor
• Output of the above program is : x value is 10
• In the above program constructor is used to initialize the
data member x
• No explicit invoking statement is used to call the constructor
• Except ( A ref_a = new A())
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11. Types of Constructors
• Default constructors
• Explicit constructors
• Parameterized constructors
• Overloaded constructors
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12. Default constructor
What is default constructor ?
• Compiler creates the default constructor when user
has not defined a constructor in a class
• It is also called as “do-nothing” or no-operation
constructor
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13. Example On Default Constructor
class A { • no explicit constructor provided so the
compiler will provide for you
// public • it will be like
A() { } public A() { }
• is called do –nothing constructor or no
} // end class A operation constructor or default constructor
class B {
public static void main(Strins[] args) {
A ref_a = new A();
} // end of main
} // end of class B
• Output of this program is : No output, because default
constructor will do nothing
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14. Example on Explicit Constructor
• When a user defines a constructor in a class,
• the compiler will not provide any default constructor
class A {
public A() {
System.out.println(“ constructor provided by programmer”);
}
}
class B {
public static void main(Strins[] args) {
A ref_a = new A();
}
}
Output of this program is http://improvejava.blogspot.in/ by programmer
: constructor provided
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15. Parameterized Constructor
• The constructors can have parameters, those constructors are
called “parameterized constructors”
class A {
int i; // data member of class A
public A(int fp) { // constructor
i = fp;
} // end of constructor A
void display_x() { // to display me
System.out.println(“i value is: ”+i);
} // end of display method
} // end of class A
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16. Parameterized Constructor contd..
class B {
public static void main(Strins[] args) {
A ref_a = new A(10); // 10 is a.p
ref_a.display_x();
} // end of main
} // end of class B
• Output of the above program is : i value is: 10
• In the above example the compiler will not provide any
constructor
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17. Overloaded Constructor
• If class contain one or more constructors, this is called
as ‘constructer overloading’
• If you have more than one constructor in a class, the
constructor must have different argument lists
• The argument list includes the order and type of the
arguments
• In the following example class A consist of three
constructors
• These constructors are called or invoked automatically
when the objects are created with the respective
arguments
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18. Example For Overloaded Constructor
class A {
int i; boolean b;
A() {
System. out. println (“ No argument constructor ”);
}
A( int fp ) {
i=fp;
S.o.p(“ one argument constr. of int”);
}
A( boolean fp1, int fp2 ) {
b=fp1;
i = fp2;
S.o.p(“two argument constr. of
boolean and int”);
}
} // end of class 18

19. Example For Overloaded Constructor
contd..
public class B {
public static void main (String args[]) {
A a1 = new A();
// calls no-argument constructor
A a2 = new A(10);
// calls one argument constructor
A a3 = new A(true,10);
// call two argument constructor
}
} // end of class B
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20. Another Example For Overloaded Constructor
class A {
int i; boolean b;
A(boolean fp1, int fp2) {
b=fp1;
i = fp2;
System. Out. println (“two argument constr. of
boolean and int”);
}
A( int fp1, boolean fp2) {
i = fp1;
b=fp2;
System. out. println(“two argument constr. of
int and boolean”);
}
} // end of class A
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21. Another Example For Overloaded Constructor
contd..
public class B {
public static void main .(String args[]) {
A a1 = new A(10, true);
A a2 = new A( false, 20);
}
} // end of class B
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22. Discussion
• What are the differences between method and
constructor
1. Name
• Class and constructor have the same name
• Different names
1. Return type
• Constructor have no return type
• Methods have return type
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23. Summary
• In this class, we have discussed
• Concepts of constructor
• How constructors are created ?
• Types of constructors
• Default
• Explicit
• Parameterized
• Overloaded
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24. Quiz
1. Default constructors provided by?
a. Developer
b. User
c. JVM
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25. Quiz Contd..
2. Overloaded constructor has?
a. same name
b. different names
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26. Frequently Asked Questions
• Define Constructor?
• What is default constructor ?
• When constructor method is executed?
• What is overloaded constructor ?
• Why do you need to write a constructor if the
compiler writes one for you?
• How can you differentiate a constructor from a
method?
• Can you also have a method that’s the same name as
the class?

27. Assignment
1. Write a class called Student consisting of
pin_no, sname, fname, city as instance variables,
have a constructor in your class that by default
assigns some values to the instance variables and
disp_stu_details as methods
Create an object for the above class, and access
display method to display the values of instance
variables
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28. Assignment Contd..
2. Perform the above operation using parameterized
constructor
3. Perform the above operation using overloaded
constructors for the two student objects
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