1. Sem 4 Paper 3 SYIT
C++/Java
CONSTURCTOR OVERLOADING
Program 1: Write a program to demonstrate constructor overloading, with the help of a class called
Box, which could either be a rectangle parallelepiped taking in 3 dimensions or a cuboid taking in
single dimension, or simply an empty constructor
class Box
{
double length, width, height=0;
Box(double x)
{
length = x;
width = x;
height = x;
}
//Similar to copy constructor in C++
Box(Box ob)
{
width = ob.width;
height = ob.height;
length = ob.length;
}
Box()
{
width = length = height = -1;
}
Box(double len, double ht, double wd)
{
length = len;
height = ht;
width = wd;
}
double volume()
{
return(length*width*height);
}
//class BoxMass inherits all the properties of class
// Box
class BoxMass extends Box
{
double mass;
// weight of the box
BoxMass(double w, double h, double l,
double m)
{
width = w;
height = h;
length = l;
mass = m;
}
}
----------------------------------------------------------class BoxDemo
{
public static void main(String args[])
{
BoxMass bm1 = new BoxMass(10,20,30,12);
BoxMass bm2= new BoxMass(1,2,3,4.4);
System.out.println("Volume is:"+bm1.volume());
System.out.println("Mass is:"+bm1.mass);
System.out.println("Volume is:"+bm2.volume());
System.out.println("Mass is:"+bm2.mass);
}
}
}
/*D:SYITjava_demosinheritance>javac BoxDemo.java
D:SYITjava_demosinheritance>java BoxDemo
Volume is:6000.0
Mass is:12.0
Volume is:6.0
Mass is:4.4 */
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lydia_fdes@yahoo.co.in
1

2. Sem 4 Paper 3 SYIT
C++/Java
class ColorBox extends Box
{
String color;
//Inside main function
ColorBox(double w, double h,
double l, String c)
{
width = w;
height = h;
length = l;
color = c;
}
}
System.out.println("Volume is:"+cb1.volume());
System.out.println("Color is:"+cb1.color);
-----------------------------------------------O/p???????
ColorBox cb1 = new ColorBox(11,11,11,"red");
Box
BoxMass
ColorBox
----------------------------------------Exercise----------------------------------------Java Language Keywords
You cannot use any of the following as identifiers in your programs. The keywords const and goto
are reserved, even though they are not currently used.
true, false, and null might seem like keywords, but they are actually literals; you cannot use
them as identifiers in your programs.
abstract
***
assert
boolean
break
byte
case
catch
char
class
const
*
continue
default
do
double
else
For
*
enum
extends
final
finally
goto
If
implements
Import
instanceof
Int
interface
Long
float
Native
****
new
package
private
protected
public
return
short
static
strictfp
super
**
switch
synchronized
this
throw
throws
transient
try
void
volatile
while
*
not used
added in 1.2
***
added in 1.4
****
added in 5.0
[circle out the keywords that you find common in C++]
**
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2

3. Sem 4 Paper 3 SYIT
C++/Java
Referencing the Subclass Object via the Superclass variable
So far you have been studying of how the sub-class object initializes member variables and
member methods of Super class through inheritance. In the following program we see how a superclass object can be used to reference a sub-class object.
class RefDemo
{
public static void main(String[] args)
{
BoxMass massbox = new BoxMass(1,2,3,4.5);
Box plainbox = new Box();
double vol;
vol = massbox.volume();
System.out.println("Volume is:"+vol);
System.out.println("Mass of box:"+massbox.mass);
plainbox = massbox;
//Assigning the BoxMass reference to Box reference
vol = plainbox.volume();
System.out.println("Volume of plainbox is:"+vol);
System.out.println("Mass of plainbox is:"+plainbox.mass); //throws error
}
}
Compile time error:
D:SYITjava_demosinheritance>javac RefDemo.java
RefDemo.java:23: cannot find symbol
symbol : variable mass
location: class Box
System.out.println("Mass of plainbox is:"+plainbox.mass);
^
1 error
- - - -- --- -- ------------------------------------------------- ------------------------------------1] It is the type of reference variable (here: plainbox) and not the type of object that it refers to-that
determines, what members are accessed.
2] Hence when a reference to a sub-class object is assigned to a super-class reference variable, we
will have access to only those parts of the object that are defined by the superclass (here length,
width, height, volume())
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3

4. Sem 4 Paper 3 SYIT
C++/Java
3] Hence plainbox is unable to access member variable: mass of massbox object even though it
refers to BoxMass class.
Learning-Super class has no knowledge as to what new specialization a sub class adds to its class,
hence mass is not visible and not accessible to Super class reference variable.
Practical Application:
---------------------------------------------------------------------------------------------Q] How do we access the Parent class constructor from the Child class?
SUPER
Notice the constructor in class BoxMass, here even though the first 3 variables are already initialized
by parent class Box, yet they are done again.
This leads to duplicate code.
BoxMass(double w, double h, double l, double m)
{
It suggests that a subclass needs to be
width = w;
granted access to superclass members.
height = h;
length = l;
mass = m;
}
}
What if the superclass members are declared as private???????
class Box
{
private double length, width, height=0;
- ---- ---}
If this was the case, subclass objects would never be able to access the objects of the parent class.
Solution to this problem is keyword super.
-
Q] What are the two uses of super?
-
Super has 2 forms=
I Calls the superclass’ constructor
II Used to access a member of the superclass that has been hidden by a member of a subclass.
I] Using super to call Superclass Constructors
Super is used to call the constructor of the parent class.
Syntax:
Super(param-list);
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4

5. Sem 4 Paper 3 SYIT
C++/Java
1] The parameter list specifies any parameters that would be required by the constructor in the super
class.
2] The Java compiler automatically inserts the necessary constructor calls in the process of
constructor chaining, or you can do it explicitly.
3] When a subclass issues a call to super(), it is actually calling the constructor of the immediate
superclass above it.
4] super() must always be the first statement executed inside a subclass constructor.
Explanation with the help of code:
class BoxMass extends Box //class BoxMass inherits all the properties of class Box
{
double mass;
// weight of the box
BoxMass(double w, double h, double l, double m)
{
super(w,h,l);
/*width = w;
height = h;
length = l;*/
mass = m;
}
}
Learning:
1] We managed to achieve data hiding here by declaring length, width and height private.
2] A Sub class need not perform initialization of SuperClass member data, it simply needs to call
super to perform this task. Hence, the Subclass can simply be concerned with initialization of its own
member variables declared within in its own class.
Exercise: Complete program 1 by implementing all constructor calls within class BoxMass with the
help of super.
Code snippet:
BoxMass()
class BoxMass extends Box
{
{
super(); // super here calls the
double mass;
// weight of the box
empty constructor of class Box
mass = -1;
BoxMass(double l, double w, double h, double m)
}
{
BoxMass(double x, double m)
super(l,w,h);
{
mass = m;
super(x);
}
mass = m;
BoxMass(BoxMass obj)
}
{
}
super(obj);
mass = obj.mass; }
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5

6. Sem 4 Paper 3 SYIT
C++/Java
II] Used to access a member of the SuperClass that has been hidden by a
member of a subclass.
[Before delving into the topic a quick introduction to the “this” keyword]
Q] Explain the keyword: this
this:
Within an instance method or a constructor, this is a reference to the current object — the object
whose method or constructor is being called.
It is possible to refer to any member of the current object from within an instance method or a
constructor by using the keyword this.
> Using this with a member field:
The following is an example of how “this” can be used with a field
Q] What is shadowing (Instance variable hiding)?
class Triangle
{
double base;
double height;
Triangle(double b, double h) //constructor
{
base = b;
height = h;
}
double area()
{
double f=1/2d;
return(base*height*f);
}
}
class TriangleDemo
{
public static void main(String[] args)
{
Triangle t1 = new Triangle(20,30);
System.out.println("Area is:"+t1.area());
}
}
D:SYITjava_demosinheritance>java
TriangleDemo
Area is:300.0
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Triangle(double base,double height) //constructor
{
base = base;
height = height;
}
• Disadvantage:The above code would not
take in the value 20 and 30 but it would
print the area as 0.0, as 0 is the default value
of base and height.
• Advantage: here is that we are not
declaring new variables b and h but reusing
the same variable base and height
Triangle(double base,double height)//constructor
{
this.base = base;
this.height = height;
}
Learning:
Each argument to the constructor shadows the
object's fields — inside the constructor base is a
local copy of the constructor's first argument. In
order to refer to the Triangle field base, the
constructor must use this.base.
Output: Area is:300.0
6

7. Sem 4 Paper 3 SYIT
C++/Java
Using this with a Constructor:
The “this” keyword could be used from within a constructor to call another constructor.
The following is an example of how “this” can be used to call another constructor
class Triangle
{
double base;
double height;
Triangle() {
//no- argument constructor calls 2-argument constructor
this(0,0);
}
} //end of class
Triangle(double base,double height)
{
this.base = base;
this.height = height;
}
Triangle(double base)
{
//Explicit constructor invocation
this(base,0);
}
double area()
{
double f=1/2d;
return(base*height*f);
}
class TriangleDemo
{
public static void main(String[] args)
{
Triangle t1 = new Triangle(20,30);
Triangle t2 = new Triangle(20);
System.out.println("Area of t1 is:"+t1.area());
System.out.println("Area of t2 is:"+t2.area());
}
}//end of class
Output:
1. A class may contain several constructors, each initializing all or just a few member fields.
2. The compiler determines which constructor to call, based on the number and the type of
arguments.
3. If a value is not provided, then the constructor must initialize it with a default value, which is
not provided by the argument.
4. The invocation of another constructor must be the first line in the constructor, i.e. “this”
should be the first line of code within the constructor.
Thinking Cap:
What will happen if we type :
Triangle(double base)
{
this(base);
}
Will there be an error?
What would the error be?
What concept does this remind you of?
Compiled by L.Fernandes
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instead of:
Triangle(double base)
{
this(base,0);
}
7

8. Sem 4 Paper 3 SYIT
Coming back to the second use of Super
C++/Java
The second form of super acts similar to “this”.
1] It always refers to the SuperClass of the SubClass in which it is used.
Syntax:
super.member
Where, member can be either an instance variable or a method.
2] super can also be used for hiding members just the way “this” is used.
Super is most applicable to situations in which member names of a subclass hide members by the
same name in the SuperClass.
3] Super can also be used to call methods of the SuperClass that are hidden by the SubClass.
Explanation of the concept with the help of an example:
class SuperDemo
class ASuperClass
{
{
public static void main(String[] args)
int i;
{
}
ASubClass obj = new ASubClass(100,11);
class ASubClass extends ASuperClass
obj.show_fields();
}
{
int i; // this is hiding the i of the super class
}
---------------------------------------------/*D:SYITjava_demosinheritance>java
ASubClass(int a, int b)
{
SuperDemo
i in ASuperClass : 100
super.i=a; // i in ASuperClass
i in ASubClass : 11*/
i=b;
// i in ASubClass
}
void show_fields()
{
System.out.println("i in ASuperClass : "+super.i);
System.out.println("i in ASubClass : "+i);
}
}
In the above program the field “ i ” is common in both the Super class as well as the Sub class,
the sub class hides the field i of the super class, this value can be obtained with the help of
super.
Q] Is super a keyword?
-------------------------**************************************----------------------------------
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8