Syllabus:: Unit-3: Classes, Inheritance, Exceptions, Packages and Interfaces

Object Oriented Concepts 15CS/IS45
UNIT-3: CLASSES, INHERITANCE, EXCEPTIONS,

PACKAGES AND INTERFACES
Syllabus:
Classes, Inheritance, Exceptions, Packages and Interfaces:
Classes: Classes fundamentals; Declaring objects; Constructors, this

keyword, garbage collection.
Inheritance: inheritance basics, using super, creating multi level

hierarchy, method overriding.
Exception handling: Exception handling in Java. Packages, Access

Protection, Importing Packages, Interfaces.
.
Beautiful thought: “You have to grow from the inside out. None can teach you, none
can make you spiritual. There is no other teacher but your own soul.” ― Swami
Vivekananda
Prepared by Nagamahesh BS, Asst.Professor, CSE, Sai Vidya Institute of Technology 1

1. CLASSES:
Definition
A class is a template for an object, and defines the data fields and methods
of the object. The class methods provide access to manipulate the data fields. The
“data fields” of an object are often called “instance variables.”
Example Program:
Program to calculate Area of Rectangle

class Rectangle
{
int length; //Data Member or instance Variables
int width;
void getdata(int x,int y) //Method
{
length=x;
width=y;
}
int rectArea() //Method
{
return(length*width);
}
}
class RectangleArea
{
public static void main(String args[])
{
Rectangle rect1=new Rectangle(); //object creation
rect1.getdata(10,20); //calling methods using object with dot(.)
int area1=rect1.rectArea();
System.out.println("Area1="+area1);
}
}

 After defining a class, it can be used to create objects by instantiating the
class. Each object occupies some memory to hold its instance variables (i.e.
its state).
 After an object is created, it can be used to get the desired functionality
together with its class.
Creating instance of a class/Declaring objects:
Rectangle rect1=new Rectangle()
Rectangle rect2=new Rectangle()
 The above two statements declares an object rect1 and rect2 is of type
Rectangle class using new operator , this operator dynamically allocates
memory for an object and returns a refernce to it.in java all class objects
must be dynamically allocated.
We can also declare the object like this:
Rectangle rect1; // declare reference to object.
rect1=new Rectangle() // allocate memory in the Rectangle object.
The Constructors:
 A constructor initializes an object when it is created. It has the same
name as its class and is syntactically similar to a method. However,
constructors have no explicit return type.
 Typically, you will use a constructor to give initial values to the
instance variables defined by the class, or to perform any other
startup procedures required to create a fully formed object.

 All classes have constructors, whether you define one or not, because
Java automatically provides a default constructor that initializes all
member variables to zero. However, once you define your own
constructor, the default constructor is no longer used.
Example:
Here is a simple example that uses a constructor:
// A simple constructor.
class MyClass
{
int x;
// Following is the constructor

MyClass()
{
x = 10;
}
}
You would call constructor to initialize objects as follows:
class ConsDemo
{
{
MyClass t1 = new MyClass();
MyClass t2 = new MyClass();
System.out.println(t1.x + " " + t2.x);
}
}

Parameterized Constructor:
 Most often you will need a constructor that accepts one or
more parameters. Parameters are added to a constructor in the
same way that they are added to a method: just declare them
inside the parentheses after the constructor's name.
Example:
Here is a simple example that uses a constructor:
// A simple constructor.
class MyClass
{
int x;
// Following is the Parameterized constructor

MyClass(int i )
{
x = 10;
}
}
You would call constructor to initialize objects as follows:
class ConsDemo
{
{
MyClass t1 = new MyClass( 10 );
MyClass t2 = new MyClass( 20 );
System.out.println(t1.x + " " + t2.x);
}
}
This would produce following result:
10 20

static keyword
The static keyword is used in java mainly for memory management.

We may apply 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:
1. variable (also known as class variable)

2. method (also known as class method)
3. block
4. nested class
static variable
Example Program without static variable
In this example, we have created an instance variable named count

which is incremented in the constructor. Since instance variable gets the
memory at the time of object creation, each object will have the copy of the
instance variable, if it is incremented, it won't reflect to other objects. So
each objects will have the value 1 in the count variable.
class Counter
{
int count=0;//will get memory when instance is created
Counter()
{
count++;
System.out.println(count);
}
}
Class MyPgm
{
{
Counter c1=new Counter();


}
}
Output: 1
1
1
Example Program with static variable
As we have mentioned above, static variable will get the memory only
once, if any object changes the value of the static variable, it will retain its
value.
class Counter
{
static int count=0;//will get memory only once and retain its value
Counter()
{
count++;
System.out.println(count);
}
}
Class MyPgm
{
{
}
}
Output:1
2
3

static method
If you apply static keyword with any method, it is known as static method
 A static method belongs to the class rather than object of a class.

 A static method can be invoked without the need for creating an instance
of a class.
 static method can access static data member and can change the value of
it.
//Program to get cube of a given number by static method
class Calculate
{
static int cube(int x)
{
return x*x*x;
}
Class MyPgm
{
{
//calling a method directly with class (without creation of object)
int result=Calculate.cube(5);
System.out.println(result);
}
}
Output:125
this keyword
 this keyword can be used to refer current class instance variable.
 If there is ambiguity between the instance variable and parameter,
this keyword resolves the problem of ambiguity.

Understanding the problem without this keyword

Let's understand the problem if we don't use this keyword by the example
given below:
class student
{
int id;
String name;
student(int id,String name)

{
id = id;
name = name;
}
void display()
{
System.out.println(id+" "+name);
}
}
Class MyPgm
{
{
student s1 = new student(111,"Anoop");
student s2 = new student(321,"Arayan");
s1.display();
s2.display();
}
}
Output: 0 null
0 null
In the above example, parameter (formal arguments) and instance variables are
same that is why we are using this keyword to distinguish between local variable and
instance variable.

Solution of the above problem by this keyword

//example of this keyword
class Student
{
int id;
String name;
student(int id,String name)

{
this.id = id;
this.name = name;
}
void display()
{
System.out.println(id+" "+name);
}
}
Class MyPgm
{
{
Student s1 = new Student(111,"Anoop");
Student s2 = new Student(222,"Aryan");
s1.display();
s2.display();
}
}
Output111 Anoop
222 Aryan
Inner class
 It has access to all variables and methods of Outer class and may refer to
them directly. But the reverse is not true, that is, Outer class cannot
directly access members of Inner class.
 One more important thing to notice about an Inner class is that it can be
created only within the scope of Outer class. Java compiler generates an
error if any code outside Outer class attempts to instantiate Inner class.

Example of Inner class

class Outer
{
public void display()
{
Inner in=new Inner();
in.show();
}
class Inner
{
public void show()
{
System.out.println("Inside inner");
}
}
}
class Test
{
public static void main(String[] args)
{
Outer ot=new Outer();
ot.display();
}
}
Output:
Inside inner
Garbage Collection
In Java destruction of object from memory is done automatically by the JVM.
When there is no reference to an object, then that object is assumed to be no
longer needed and the memories occupied by the object are released. This
technique is called Garbage Collection. This is accomplished by the JVM.

Can the Garbage Collection be forced explicitly?
No, the Garbage Collection cannot be forced explicitly. We may request JVM for
garbage collection by calling System.gc() method. But this does not guarantee that
JVM will perform the garbage collection.
Advantages of Garbage Collection
1. Programmer doesn't need to worry about dereferencing an object.
2. It is done automatically by JVM.
3. Increases memory efficiency and decreases the chances for memory leak.
finalize() method
Sometime an object will need to perform some specific task before it is destroyed
such as closing an open connection or releasing any resources held. To handle such
situation finalize() method is used. finalize() method is called by garbage collection
thread before collecting object. It’s the last chance for any object to perform
cleanup utility.
Signature of finalize() method
protected void finalize()
//finalize-code

gc() Method
gc() method is used to call garbage collector explicitly. However gc() method does
not guarantee that JVM will perform the garbage collection. It only requests the
JVM for garbage collection. This method is present in System and Runtime class.
Example for gc() method

public class Test
{

{
Test t = new Test();
t=null;
System.gc();
}
public void finalize()
{
System.out.println("Garbage Collected");
}
}
Output :
Garbage Collected

Inheritance:
 As the name suggests, inheritance means to take something that is
already made. It is one of the most important features of Object
Oriented Programming. It is the concept that is used for reusability
purpose.
 Inheritance is the mechanism through which we can derive classes
from other classes.
 The derived class is called as child class or the subclass or we can say
the extended class and the class from which we are deriving the
subclass is called the base class or the parent class.
 To derive a class in java the keyword extends is used. The following
kinds of inheritance are there in java.
Types of Inheritance
1. Single level/Simple Inheritance
2. Multilevel Inheritance
3. Multiple Inheritance (Java doesn’t support Multiple inheritance
but we can achieve this through the concept of Interface.)
Pictorial Representation of Simple and Multilevel Inheritance
Simple Inheritance Multilevel Inheritance

Single level/Simple Inheritance
 When a subclass is derived simply from its parent class then this
mechanism is known as simple inheritance. In case of simple
inheritance there is only a sub class and its parent class. It is also
called single inheritance or one level inheritance.
Example
class A
{
int x;
int y;
int get(int p, int q)
{
x=p;
y=q;
return(0);
}
void Show()
{
System.out.println(x);
}
}
class B extends A
{
{
A a = new A();
a.get(5,6);
a.Show();
}
void display()
{
System.out.println("y"); //inherited “y” from class A
}
}

 The syntax for creating a subclass is simple. At the beginning of your class
declaration, use the extends keyword, followed by the name of the class to
inherit from:
class A
{
class B extends A //B is a subclass of super class A.

{
Multilevel Inheritance
 When a subclass is derived from a derived class then this mechanism
is known as the multilevel inheritance.
 The derived class is called the subclass or child class for it's parent
class and this parent class works as the child class for it's just above
( parent ) class.
 Multilevel inheritance can go up to any number of level.
class A
{
int x;
int y;
int get(int p, int q)
{
x=p;
y=q;
return(0);
}
void Show()
{
System.out.println(x);
}
}

class B extends A
{
void Showb()
{
System.out.println("B");
}
}
class C extends B
{
void display()
{
System.out.println("C");
}
{
A a = new A();
a.get(5,6);
a.Show();
}
}
OUTPUT
5
Multiple Inheritance
 The mechanism of inheriting the features of more than one base class into a
single class is known as multiple inheritance. Java does not support multiple
inheritance but the multiple inheritance can be achieved by using the
interface.
 Here you can derive a class from any number of base classes. Deriving a
class from more than one direct base class is called multiple inheritance.

Java does not support multiple Inheritance
In Java Multiple Inheritance can be achieved through use of Interfaces by
implementing more than one interfaces in a class.
super keyword
 The super is java keyword. As the name suggest super is used to access the
members of the super class. It is used for two purposes in java.
 The first use of keyword super is to access the hidden data variables of the
super class hidden by the sub class.
Example: Suppose class A is the super class that has two instance variables
as int a and float b. class B is the subclass that also contains its own data members
named a and b. then we can access the super class (class A) variables a and b inside
the subclass class B just by calling the following command.
super.member;
 Here member can either be an instance variable or a method. This form of
super most useful to handle situations where the local members of a
subclass hides the members of a super class having the same name. The
following example clarifies all the confusions.

Example:
class A
{
int a;
float b;
void Show()
{
System.out.println("b in super class: " + b);
}
}
class B extends A
{
int a;
float b;
B( int p, float q)
{
a = p;
super.b = q;
}
void Show()
{
super.Show();
System.out.println("b in super class: " + super.b);
System.out.println("a in sub class: " + a);
}
}
class Mypgm
{
{
B subobj = new B(1, 5);
subobj.Show();
}
}
OUTPUT
b in super class: 5.0
b in super class: 5.0
a in sub class: 1

Use of super to call super class constructor: The second use of the keyword
super in java is to call super class constructor in the subclass. This functionality can
be achieved just by using the following command.
super(param-list);
 Here parameter list is the list of the parameter requires by the constructor
in the super class. super must be the first statement executed inside a
super class constructor. If we want to call the default constructor then we
pass the empty parameter list. The following program illustrates the use of
the super keyword to call a super class constructor.
Example:
class A
{
int a;
int b;
int c;
A(int p, int q, int r)
{
a=p;
b=q;
c=r;
}
}
class B extends A
{
int d;
B(int l, int m, int n, int o)
{
super(l,m,n);
d=o;
}
void Show()

{
System.out.println("a = " + a);
System.out.println("b = " + b);
System.out.println("c = " + c);
System.out.println("d = " + d);
}
}
class Mypgm
{
{
B b = new B(4,3,8,7);
b.Show();
}
}
OUTPUT
a=4
b=3
c=8
d=7
Method Overriding
 Method overriding in java means a subclass method overriding a super class
method.
 Superclass method should be non-static. Subclass uses extends keyword to
extend the super class. In the example class B is the sub class and class A
is the super class. In overriding methods of both subclass and superclass
possess same signatures. Overriding is used in modifying the methods of
the super class. In overriding return types and constructor parameters of
methods should match.
Below example illustrates method overriding in java.
Example:

class A
{
int i;
A(int a, int b)
{
i = a+b;
}
void add()
{
System.out.println("Sum of a and b is: " + i);
}
}
class B extends A
{
int j;
B(int a, int b, int c)
{
super(a, b);
j = a+b+c;
}
void add()
{
super.add();
System.out.println("Sum of a, b and c is: " + j);
}
}
class MethodOverriding
{
{
B b = new B(10, 20, 30);
b.add();
}
}
OUTPUT
Sum of a and b is: 30
Sum of a, b and c is: 60

Method Overloading
 Two or more methods have the same names but different argument lists.
The arguments may differ in type or number, or both. However, the return
types of overloaded methods can be the same or different is called method
overloading. An example of the method overloading is given below:
Example:
class MethodOverloading
{
int add( int a,int b)
{
return(a+b);
}
float add(float a,float b)
{
return(a+b);
}
double add( int a, double b,double c)
{
return(a+b+c);
}
}
class MainClass
{
public static void main( String arr[] )
{
MethodOverloading mobj = new MethodOverloading ();
System.out.println(mobj.add(50,60));
System.out.println(mobj.add(3.5f,2.5f));
System.out.println(mobj.add(10,30.5,10.5));
}
}
OUTPUT
110
6.0
51.0

Abstract Class
 abstract keyword is used to make a class abstract.

 Abstract class can’t be instantiated with new operator.
 We can use abstract keyword to create an abstract method; an abstract
method doesn’t have body.
 If classes have abstract methods, then the class also needs to be made
abstract using abstract keyword, else it will not compile.
 Abstract classes are used to provide common method implementation to all
the subclasses or to provide default implementation.
Example Program:
abstract Class AreaPgm

{
double dim1,dim2;
AreaPgm(double x,double y)
{
dim1=x;
dim2=y;
}
abstract double area();
}
class rectangle extends AreaPgm
{
rectangle(double a,double b)
{
super(a,b);
}
double area()
{
System.out.println("Rectangle Area");
return dim1*dim2;
}
}
class triangle extends figure
{
triangle(double x,double y)
{
super(x,y);

}
double area()
{
System.out.println("Traingle Area");
return dim1*dim2/2;
}
}
class MyPgm
{
{
AreaPgm a=new AreaPgm(10,10); // error, AreaPgm is a abstract class.
rectangle r=new rectangle(10,5);

System.out.println("Area="+r.area());
triangle t=new triangle(10,8);

AreaPgm ar;
ar=obj;
System.out.println("Area="+ar.area());
}
}
final Keyword In Java
The final keyword in java is used to restrict the user. The final keyword can be
used in many context. Final can be:
1. variable
2. method
3. class
1) final variable: If you make any variable as final, you cannot change the value of
final variable(It will be constant).
Example:There is a final variable speedlimit, we are going to change the value of

this variable, but It can't be changed because final variable once assigned a value
can never be changed.

class Bike
{
final int speedlimit=90;//final variable
void run()
{
speedlimit=400;
}
}
Class MyPgm
{
{
Bike obj=new Bike();
obj.run();
}
}
Output:Compile Time Error
2) final method: If you make any method as final, you cannot override it.
Example:
class Bike
{
final void run()
{
System.out.println("running");
}
}
class Honda extends Bike
{
void run()
{
System.out.println("running safely with 100kmph");
}
}

Class MyPgm
{
{
Honda honda= new Honda();
honda.run();
}
}
3) final class:If you make any class as final, you cannot extend it.
Example:
final class Bike

{
class Honda extends Bike

{
void run()
{
System.out.println("running safely with 50kmph");
}
}
Class MyPgm
{

{
Honda honda= new Honda();
honda.run();
}
}

Exception handling:
Introduction
An Exception, It can be defined as an abnormal event that occurs during program
execution and disrupts the normal flow of instructions. The abnormal event can be
an error in the program.
Errors in a java program are categorized into two groups:
1. Compile-time errors occur when you do not follow the syntax of a
programming language.
2. Run-time errors occur during the execution of a program.
Concepts of Exceptions
An exception is a run-time error that occurs during the exception of a java
program.
Example: If you divide a number by zero or open a file that does not exist, an
exception is raised.
In java, exceptions can be handled either by the java run-time system or by a user-
defined code. When a run-time error occurs, an exception is thrown.
The unexpected situations that may occur during program execution are:
 Running out of memory
 Resource allocation errors
 Inability to find files
 Problems in network connectivity

Exception handling techniques:
Java exception handling is managed via five keywords they are:
1. try:
2. catch.
3. throw.
4. throws.
5. finally.
Exception handling Statement Syntax
Exceptions are handled using a try-catch-finally construct, which has the Syntax.
try
{
<code>
}
catch (<exception type1> <parameter1>)
{
// 0 or more<statements>
}
finally
{
// finally block<statements>
}
1. try Block: The java code that you think may produce an exception is placed
within a try block for a suitable catch block to handle the error.
If no exception occurs the execution proceeds with the finally block else it
will look for the matching catch block to handle the error.

Again if the matching catch handler is not found execution proceeds with
the finally block and the default exception handler throws an exception.
2. catch Block: Exceptions thrown during execution of the try block can be caught
and handled in a catch block. On exit from a catch block, normal execution
continues and the finally block is executed (Though the catch block throws an
exception).
3. finally Block: A finally block is always executed, regardless of the cause of exit
from the try block, or whether any catch block was executed. Generally finally
block is used for freeing resources, cleaning up, closing connections etc.
Example:
The following is an array is declared with 2 elements. Then the code tries to access
the 3rd element of the array which throws an exception.
// File Name : ExcepTest.java

import java.io.*;
public class ExcepTest
{
{
try
{
int a[] = new int[2];
System.out.println("Access element three :" + a[3]);
}
catch(ArrayIndexOutOfBoundsException e)
{
System.out.println("Exception thrown :" + e);
}
System.out.println("Out of the block");
}
}

This would produce following result:
Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3

Out of the block
Multiple catch Blocks:
A try block can be followed by multiple catch blocks. The syntax for multiple catch
blocks looks like the following:
try
{
// code
}
catch(ExceptionType1 e1)
{
//Catch block
}
{
//Catch block
}
{
//Catch block
}
The previous statements demonstrate three catch blocks, but you can have any
number of them after a single try.

Example: Here is code segment showing how to use multiple try/catch
statements.
class Multi_Catch
{
public static void main (String args [])
{
try
{
int a=args.length;
System.out.println(“a=”+a);
int b=50/a;
int c[]={1}
}
catch (ArithmeticException e)
{
System.out.println ("Division by zero");
}
catch (ArrayIndexOutOfBoundsException e)
{
System.out.println (" array index out of bound");
}
}
}
OUTPUT
Division by zero
array index out of bound
Nested try Statements
 Just like the multiple catch blocks, we can also have multiple try blocks.
These try blocks may be written independently or we can nest the try blocks
within each other, i.e., keep one try-catch block within another try-block.
The program structure for nested try statement is:

Syntax
try
{
// statements
// statements
try
{
// statements
// statements
}
catch (<exception_two> obj)
{
// statements
}
// statements
// statements
}
catch (<exception_two> obj)
{
// statements
}
 Consider the following example in which you are accepting two numbers from
the command line. After that, the command line arguments, which are in the
string format, are converted to integers.
 If the numbers were not received properly in a number format, then during
the conversion a NumberFormatException is raised otherwise the control
goes to the next try block. Inside this second try-catch block the first
number is divided by the second number, and during the calculation if there
is any arithmetic error, it is caught by the inner catch block.

Example
class Nested_Try
{
public static void main (String args [ ] )
{
try
{
int a = Integer.parseInt (args [0]);
int b = Integer.parseInt (args [1]);
int quot = 0;
try
{
quot = a / b;
System.out.println(quot);
}
catch (ArithmeticException e)
{
System.out.println("divide by zero");
}
}
catch (NumberFormatException e)
{
System.out.println ("Incorrect argument type");
}
}
}
The output of the program is: If the arguments are entered properly in the
command prompt like:
OUTPUT
java Nested_Try 2 4 6
4
If the argument contains a string than the number:

OUTPUT
java Nested_Try 2 4 aa
Incorrect argument type
If the second argument is entered zero:
OUTPUT
java Nested_Try 2 4 0
divide by zero
throw Keyword
 throw keyword is used to throw an exception explicitly. Only object of

Throwable class or its sub classes can be thrown.
 Program execution stops on encountering throw statement, and the closest
catch statement is checked for matching type of exception.
Syntax : throw ThrowableInstance
Creating Instance of Throwable class
There are two possible ways to get an instance of class Throwable,
1. Using a parameter in catch block.

2. Creating instance with new operator.
new NullPointerException("test");
This constructs an instance of NullPointerException with name test.
Example demonstrating throw Keyword

class Test
{
static void avg()
{
try
{
throw new ArithmeticException("demo");
}
catch(ArithmeticException e)
{
System.out.println("Exception caught");
}
}
{
avg();
}
}
In the above example the avg() method throw an instance of ArithmeticException,
which is successfully handled using the catch statement.
throws Keyword
 Any method capable of causing exceptions must list all the exceptions
possible during its execution, so that anyone calling that method gets a prior
knowledge about which exceptions to handle. A method can do so by using
the throws keyword.
Syntax :
type method_name(parameter_list) throws exception_list

{
//definition of method
}

NOTE : It is necessary for all exceptions, except the exceptions of type Error and
RuntimeException, or any of their subclass.
Example demonstrating throws Keyword
class Test
{
static void check() throws ArithmeticException
{
System.out.println("Inside check function");
throw new ArithmeticException("demo");
}

{
try
{
check();
}
{
System.out.println("caught" + e);
}
}
}
finally
 The finally clause is written with the try-catch statement. It is guaranteed
to be executed after a catch block or before the method quits.
Syntax
try
{
// statements
}

catch (<exception> obj)

{
// statements
}
finally
{
//statements
}
 Take a look at the following example which has a catch and a finally block.
The catch block catches the ArithmeticException which occurs for
arithmetic error like divide-by-zero. After executing the catch block the
finally is also executed and you get the output for both the blocks.
Example:
class Finally_Block
{
static void division ( )
{
try
{
int num = 34, den = 0;
int quot = num / den;
}
{
System.out.println ("Divide by zero");
}
finally
{
System.out.println ("In the finally block");
}
}
class Mypgm
{
{

Finally_Block f=new Finally_Block();

f.division ( );
}
}
OUTPUT
Divide by zero
In the finally block
Java’s Built in Exceptions
Java defines several exception classes inside the standard package java.lang.
 The most general of these exceptions are subclasses of the standard type
RuntimeException. Since java.lang is implicitly imported into all Java
programs, most exceptions derived from RuntimeException are
automatically available.
Java defines several other types of exceptions that relate to its various class
libraries. Following is the list of Java Unchecked RuntimeException.
Exception Description
ArithmeticException Arithmetic error, such as divide-by-zero.
ArrayIndexOutOfBoundsException Array index is out-of-bounds.
Assignment to an array element of an

ArrayStoreException
incompatible type.
ClassCastException Invalid cast.
IllegalArgumentException Illegal argument used to invoke a method.
Illegal monitor operation, such as waiting on an

IllegalMonitorStateException
unlocked thread.

Environment or application is in incorrect

IllegalStateException
state.
Requested operation not compatible with

IllegalThreadStateException
current thread state.
IndexOutOfBoundsException Some type of index is out-of-bounds.
NegativeArraySizeException Array created with a negative size.
NullPointerException Invalid use of a null reference.
Invalid conversion of a string to a numeric

NumberFormatException
format.
SecurityException Attempt to violate security.
Attempt to index outside the bounds of a

StringIndexOutOfBounds
string.
UnsupportedOperationException An unsupported operation was encountered.
Following is the list of Java Checked Exceptions Defined in java.lang.
Exception Description
ClassNotFoundException Class not found.
Attempt to clone an object that does not implement

CloneNotSupportedException
the Cloneable interface.
IllegalAccessException Access to a class is denied.
Attempt to create an object of an abstract class or

InstantiationException
interface.
InterruptedException One thread has been interrupted by another thread.
NoSuchFieldException A requested field does not exist.
NoSuchMethodException A requested method does not exist.

Creating your own Exception Subclasses
 Here you can also define your own exception classes by extending Exception.
These exception can represents specific runtime condition of course you will
have to throw them yourself, but once thrown they will behave just like
ordinary exceptions.
 When you define your own exception classes, choose the ancestor carefully.
Most custom exception will be part of the official design and thus checked,
meaning that they extend Exception but not RuntimeException.
Example: Throwing User defined Exception
public class MyException extends Exception

{
String msg = "";
int marks=50;
public MyException()
{
}
public MyException(String str)
{
super(str);
}
public String toString()
{
if(marks <= 40)
msg = "You have failed";
if(marks > 40)
msg = "You have Passed";
return msg;
}
}

class test
{
{
test t = new test();
t.dd();
}
public void add()
{
try
{
int i=0;
if( i<40)
throw new MyException();
}
catch(MyException ee1)
{
System.out.println("Result:"+ee1);
}
}
}
OUTPUT
Result: You have Passed
Chained Exception
 Chained exceptions are the exceptions which occur one after another i.e.
most of the time to response to an exception are given by an application by
throwing another exception.
 Whenever in a program the first exception causes an another exception,
that is termed as Chained Exception. Java provides new functionality for
chaining exceptions.
 Exception chaining (also known as "nesting exception") is a technique for
handling the exception, which occur one after another i.e. most of the time

is given by an application to response to an exception by throwing another
exception.
 Typically the second exception is caused by the first exception. Therefore
chained exceptions help the programmer to know when one exception causes
another.
The constructors that support chained exceptions in Throwable class are:
Throwable initCause(Throwable)
Throwable(Throwable)
Throwable(String, Throwable)
Throwable getCause()

Packages in JAVA
 A java package is a group of similar types of classes, interfaces and sub-

packages.
 Package in java can be categorized in two form,
o built-in package and
o user-defined package.
There are many built-in packages such as java, lang, awt, javax, swing, net, io, util,
sql etc.
Advantage of Java Package
1) Java package is used to categorize the classes and interfaces so that they can be
easily maintained.
2) Java package provides access protection.
3) Java package removes naming collision.
The package keyword is used to create a package in java.
//save as Simple.java
package mypack;
public class Simple
{
{
System.out.println("Welcome to package");
}
}
How to access package from another package?
There are three ways to access the package from outside the package.
1. import package.*;
2. import package.classname;
3. fully qualified name.

1) Using packagename.*
If you use package.* then all the classes and interfaces of this package will be
accessible but not subpackages.
The import keyword is used to make the classes and interface of another package
accessible to the current package.
Example of package that import the packagename.*
//save by A.java
package pack;
public class A
{
public void msg(){System.out.println("Hello");}
}
//save by B.java
package mypack;
import pack.*;
class B
{
{
A obj = new A();
obj.msg();
}
}
Output:Hello
2) Using packagename.classname
If you import package.classname then only declared class of this package will be
accessible.
Example of package by import package.classname
//save by A.java
package pack;
public class A
{
public void msg(){System.out.println("Hello");

}
}
//save by B.java
package mypack;
import pack.A;
class B
{
{
A obj = new A();
obj.msg();
}
}
Output:Hello
3) Using fully qualified name
If you use fully qualified name then only declared class of this package will be
accessible. Now there is no need to import. But you need to use fully qualified name
every time when you are accessing the class or interface.
It is generally used when two packages have same class name e.g. java.util and
java.sql packages contain Date class.
Example of package by import fully qualified name
//save by A.java
package pack;
public class A
{
public void msg()
{
System.out.println("Hello");
}
}
//save by B.java
package mypack;
class B
{

{
pack.A obj = new pack.A();//using fully qualified name
obj.msg();
}
}
Output:Hello
Access Modifiers/Specifiers
The access modifiers in java specify accessibility (scope) of a data member,

method, constructor or class.
There are 4 types of java access modifiers:
1. private
2. default
3. protected
4. public
1) private access modifier

The private access modifier is accessible only within class.
2) default access modifier

If you don't use any modifier, it is treated as default by default. The default
modifier is accessible only within package.
3) protected access modifier
The protected access modifier is accessible within package and outside the
package but through inheritance only.
The protected access modifier can be applied on the data member, method and
constructor. It can't be applied on the class.
4) public access modifier

The public access modifier is accessible everywhere. It has the widest scope
among all other modifiers.

Understanding all java access modifiers by a simple table.

Access within within outside package by outside
Modifier class package subclass only package
Private Y N N N
Default Y Y N N
Protected Y Y Y N
Public Y Y Y Y
Interface in java
 An interface in java is a blueprint of a class. It has static final variables
and abstract methods.
 The interface in java is a mechanism to achieve abstraction. There can be
only abstract methods in the java interface does not contain method body.
It is used to achieve abstraction and multiple inheritance in Java.
 It cannot be instantiated just like abstract class.
 Interface fields are public, static and final by default, and methods are
public and abstract.
There are mainly three reasons to use interface. They are given below.
 It is used to achieve abstraction.
 By interface, we can support the functionality of multiple inheritance.
Understanding relationship between classes and interfaces
As shown in the figure given below, a class extends another class, an interface
extends another interface but a class implements an interface.

Example 1
In this example, Printable interface has only one method, its implementation is
provided in the Pgm1 class.
interface printable
{
void print();
}
class Pgm1 implements printable

{
public void print()
{
}
}
class IntefacePgm1
{
{
Pgm1 obj = new Pgm1 ();
obj.print();
}
}

Output:
Hello
Example 2
In this example, Drawable interface has only one method. Its implementation is
provided by Rectangle and Circle classes. In real scenario, interface is defined by
someone but implementation is provided by different implementation providers.
And, it is used by someone else. The implementation part is hidden by the user
which uses the interface.
//Interface declaration: by first user
interface Drawable
{
void draw();
}
//Implementation: by second user
class Rectangle implements Drawable

{
public void draw()
{
System.out.println("drawing rectangle");
}
}
class Circle implements Drawable

{
public void draw()
{
System.out.println("drawing circle");
}
}

//Using interface: by third user

class TestInterface1
{
{
//In real scenario, object is provided by method e.g. getDrawable()
Drawable d=new Circle();
d.draw();
}
}
Output:
drawing circle
Multiple inheritance in Java by interface
 If a class implements multiple interfaces, or an interface extends multiple

interfaces i.e. known as multiple inheritance.

Example
interface Printable
{
void print();
}
interface Showable
{
void show();
}
class Pgm2 implements Printable,Showable

{
public void print()
{
}
public void show()

{
System.out.println("Welcome");
}
}
Class InterfaceDemo
{
{
Pgm2 obj = new Pgm2 ();
obj.print();
obj.show();
}
}
Output:
Hello
Welcome

Multiple inheritance is not supported through class in java but it is
possible by interface, why?
 As we have explained in the inheritance chapter, multiple inheritance is not
supported in case of class because of ambiguity.
 But it is supported in case of interface because there is no ambiguity as
implementation is provided by the implementation class. For example:
Example
interface Printable
{
void print();
}
interface Showable
{
void print();
}
class InterfacePgm1 implements Printable, Showable

{
public void print()
{
}
}
class InterfaceDemo
{
{
InterfacePgm1 obj = new InterfacePgm1 ();
obj.print();
}
}
Output:
Hello

 As you can see in the above example, Printable and Showable interface have
same methods but its implementation is provided by class TestTnterface1,
so there is no ambiguity.
Interface inheritance
 A class implements interface but one interface extends another interface .
interface Printable
{
void print();
}
interface Showable extends Printable

{
void show();
}
class InterfacePgm2 implements Showable

{
public void print()
{
}
public void show()
{
System.out.println("Welcome");
}
Class InterfaceDemo2
{
{
InterfacePgm2 obj = new InterfacePgm2 ();
obj.print();
obj.show();
}
}
Output:
Hello
Welcome

Program to implement Stack

public class StackDemo
{
private static final int capacity = 3;
int arr[] = new int[capacity];
int top = -1;
public void push(int pushedElement)

{
if (top < capacity - 1)
{
top++;
arr[top] = pushedElement;
System.out.println("Element " + pushedElement + " is pushed to Stack !")
;
printElements();
}
else
{
System.out.println("Stack Overflow !");
}
}
public void pop()

{
if (top >= 0)
{
top--;
System.out.println("Pop operation done !");
}
else
{
System.out.println("Stack Underflow !");
}
}
public void printElements()

{
if (top >= 0)
{
System.out.println("Elements in stack :");
for (int i = 0; i <= top; i++)
{
System.out.println(arr[i]);
}

}
}
class MyPgm
{

{
StackDemo stackDemo = new StackDemo();
stackDemo.pop();
stackDemo.push(23);
stackDemo.push(2);
stackDemo.push(73);
stackDemo.push(21);
stackDemo.pop();
stackDemo.pop();
stackDemo.pop();
stackDemo.pop();
}
Output

Questions
1. Distinguish between Method overloading and Method

overriding in JAVA, with suitable examples.(Jan 2014) 6marks
2. What is super? Explain the use of super with suitable example
(Jan 2014) 6marks
3. Write a JAVA program to implement stack operations.
(Jan 2014) 6marks
4. What is an Exception? Give an example for nested try
statements? (Jan 2013) 6 Marks
5. WAP in java to implement a stack that can hold 10 integers
values (Jan 2013) 6 Marks
6. What is mean by instance variable hiding ?how to overcome it?
(Jan 2013) 04 Marks
7. Define exception .demonstrate the working of nested try blocks
with suitable example? (Dec 2011)08Marks
8. Write short notes on (Dec 2011)04Marks

i) Final class ii) abstract class
9. Write a java program to find the area and volume of a room.

Use a base class rectangle with a constructor and a method for
finding the area. Use its subclass room with a constructor that
gets the value of length and breadth from the base class and
has a method to find the volume. Create an object of the class
room and obtain the area and volume. (Jan-2006) 8Marks
10. Explain i) Instance variables ii) Class Variables iii) Local

variables (Jan-2009) 06 marks

11. Distinguish between method overloading and method

overriding? How does java decide the method to call?
(Jan-2008-8Marks) 6Marks
12. Explain the following with example.
i) Method overloading ii) Method overriding (jun-2006) 8Marks
13. Write a java program to find the distance between two points
whose coordinates are given. The coordinates can be 2-
dimensional or 3-dimensional (for comparing the distance
between 2D and a 3D point, the 3D point, the 3D x and y
components must be divided by z). Demonstrate method
overriding in this program. (May-2007)10 marks
14. What is an interface? Write a program to illustrate multiple
inheritance using interfaces. (Jan-2010) 8Marks
15. Explain packages in java.
16. What are access specifiers? Explain with an example.
17. With an example explain static keyword in java.
18. Why java is not support concept of multiple inheritance?
Justify with an example program.
19. Write a short note on:
1. this keyword
2. super keyword
3. final keyword
4. abstract
20. Illustrate constructors with an example program

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Object Oriented Concepts 15CS/IS45

UNIT-3: CLASSES, INHERITANCE, EXCEPTIONS,

Classes, Inheritance, Exceptions, Packages and Interfaces:

Classes: Classes fundamentals; Declaring objects; Constructors, this

Inheritance: inheritance basics, using super, creating multi level

Exception handling: Exception handling in Java. Packages, Access

Prepared by Nagamahesh BS, Asst.Professor, CSE, Sai Vidya Institute of Technology 1

“data fields” of an object are often called “instance variables.”

Program to calculate Area of Rectangle

Prepared by Nagamahesh BS, Asst.Professor, CSE, Sai Vidya Institute of Technology 2

 After defining a class, it can be used to create objects by instantiating the

 After an object is created, it can be used to get the desired functionality

together with its class.

Creating instance of a class/Declaring objects:

Rectangle rect1=new Rectangle()

Rectangle rect2=new Rectangle()

Rectangle class using new operator , this operator dynamically allocates

must be dynamically allocated.

We can also declare the object like this:

Rectangle rect1; // declare reference to object.

rect1=new Rectangle() // allocate memory in the Rectangle object.

 A constructor initializes an object when it is created. It has the same

name as its class and is syntactically similar to a method. However,

constructors have no explicit return type.

 Typically, you will use a constructor to give initial values to the

instance variables defined by the class, or to perform any other

startup procedures required to create a fully formed object.

Prepared by Nagamahesh BS, Asst.Professor, CSE, Sai Vidya Institute of Technology 3

Java automatically provides a default constructor that initializes all

member variables to zero. However, once you define your own

constructor, the default constructor is no longer used.

Here is a simple example that uses a constructor:

// Following is the constructor

You would call constructor to initialize objects as follows:

Prepared by Nagamahesh BS, Asst.Professor, CSE, Sai Vidya Institute of Technology 4

 Most often you will need a constructor that accepts one or

more parameters. Parameters are added to a constructor in the

inside the parentheses after the constructor's name.

Here is a simple example that uses a constructor:

// Following is the Parameterized constructor

You would call constructor to initialize objects as follows:

This would produce following result:

Prepared by Nagamahesh BS, Asst.Professor, CSE, Sai Vidya Institute of Technology 5

The static keyword is used in java mainly for memory management.

The static can be:

1. variable (also known as class variable)

Example Program without static variable

In this example, we have created an instance variable named count

Prepared by Nagamahesh BS, Asst.Professor, CSE, Sai Vidya Institute of Technology 6

Counter c2=new Counter();

Example Program with static variable

Prepared by Nagamahesh BS, Asst.Professor, CSE, Sai Vidya Institute of Technology 7

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

//Program to get cube of a given number by static method

 this keyword can be used to refer current class instance variable.

 If there is ambiguity between the instance variable and parameter,

this keyword resolves the problem of ambiguity.

Prepared by Nagamahesh BS, Asst.Professor, CSE, Sai Vidya Institute of Technology 8