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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