2. 2
Exception Handling in Java
throw
It is possible to throw an exception explicitly.
Syntax:
throw ThrowableInstance
throwableInstance must be an object of type Throwable or a
subclass of Throwable.
By 2 ways v can obtain a Throwable object
1.Using parameter into a catch clause
2.Creating one with new operator
3. 3
class throwDemo
{
public static void main(String s[])
{
int size;
int arry[]=new int[3];
size=Integer.parseInt(s[0]);
try
{
if(size<=0)
throw new NegativeArraySizeException("Illegal Array size");
for(int i=0;i<3;i++)
arry[i]+=i+1;
}
catch(NegativeArraySizeException e)
{
System.out.println(e);
throw e; //rethrow the exception
}
}
}
new operator used
parameter used into catch clause
4. 4
throws
If a method causing an exception that it doesn't
handle, it must specify this behavior that callers of the
method can protect themselves against the exception.
This can be done by using throws clause.
throws clause lists the types of exception that a
method might throw.
Form
type methodname(parameter list) throws Exception list
{//body of method}
6. 6
finally
It creates a block of code that will b executed after try/catch block has
completed and before the code following try/catch block.
It will execute whether or not an exception is thrown
finally is useful for:
•Closing a file
•Closing a result set
•Closing the connection established with db
This block is optional but when included is placed after the last catch
block of a try
15. 15
Checked Exceptions
Class not found.
Attempt to clone an object that does not implement the
Cloneable interface.
Access to a class is denied.
Attempt to create an object of an abstract class or interface.
One thread has been interrupted by another thread.
A requested field does not exist.
A requested method does not exist.
ClassNotFoundException
CloneNotSupportedException
IllegalAccessException
InstantiationException
InterruptedException
NoSuchFieldException
NoSuchMethodException
Exception Meaning
16. • A checked exception is any subclass of Exception (or
Exception itself), excluding class RuntimeException
and its subclasses.
• Making an exception checked forces client
programmers to deal with the possibility that the
exception will be thrown.
• eg, IOException thrown by java.io.FileInputStream's
read() method .
• Unchecked exceptions are RuntimeException and any
of its subclasses. Class Error and its subclasses also
are unchecked.
16
Java’s Built-in Exceptions: inside java.lang package
17. • With an unchecked exception, however, the compiler
doesn't force client programmers either to catch the
exception or declare it in a throws clause.
• In fact, client programmers may not even know that
the exception could be thrown.
• eg, StringIndexOutOfBoundsException thrown by
String's charAt() method.
• Checked exceptions must be caught at compile time.
Runtime exceptions do not need to be.
17
18. 18
Creating our own Exception class
For creating an exception class our own simply make
our class as subclass of the super class Exception.
Eg:
class MyException extends Exception
{
MyException(String msg)
{
super(msg);
}
}
19. 19
class TestMyException
{
public static void main(String d[])
{
int x=5,y=1000;
try
{
float z=(float)x/(float)y;
if(z<0.01)
{
throw new MyException("too small number");
}
}
20. 20
catch(MyException me)
{
System.out.println("caught my exception");
System.out.println(me.getMessage());
}
finally
{
System.out.println("from finally");
}
}
}
• E:JAVAPGMS>java TestMyException
• caught my exception
• too small number
• from finally
Output
21. 21
Making safer program by
providing special mechanism
Objective of Exception Handling
Summary
