Exception
Exception
handling
Exception can be
generated by Java-runtime system or they can be manually generated by code.
Error-Handling becomes a necessary while developing an application to account
for exceptional situations that may occur during the program execution, such as
§ Run out of memory
§ Resource allocation Error
§ Inability to find a file
§ Problems in Network connectivity.
If the Resource file is not present in the disk,
you can use the Exception handling mechanisim to handle such abrupt termination
of program.
§ Exception class: is used for the exceptional conditions that are trapped by the
program.
An exception is an abnormal condition or error that occur during the execution
of
the program.
§ Error: the
error class defines the conditions that do not occur under normal conditions.
Eg: Run out of memory,
Stack overflow error.
Java.lang.Object
+….Java.Lang.Throwable Throwable
+…. Java.lang.Error
| +…. A whole
bunch of errors
| Exception Error
+….Java.Lang.Exception (Unchecked, Checked)
+….Java.Lang.RuntimeException
| +…. Various Unchecked Exception
|
+…. Various checked Exceptions.
Two
types of exceptions:
(1).Checked
Exceptions: must be declare in
the method declaration or caught in a catch block.
Checked exception
must be handled at Compile Time. Environmental error that cannot necessarly be
detected by Testing, Eg: disk full, brocken Socket, Database unavailable etc.
(2).Un-checked
Exceptions: Run-time Exceptions
and Error, does’t have to be declare.(but can be caught).
Run-time
Exceptions : programming errors
that should be detectd in Testing ,
Arithmetic,
Null pointer, ArrayIndexOutofBounds, ArrayStore, FilenotFound, NumberFormate,
IO, OutofMemory.
Errors: Virtual mechine error – class not found , out of memory, no such method
, illegal access to private field , etc.
Java Exception handling can
be managed by five keywords:
§ Try :
The try block governs the statements that are enclosed within it and defines
the scope of exception handler associated with it. Try block follows
catch or finally or both.
§ Catch:
This is a default exception handler. since the exception class is the base
class for all the exception class, this handler id capable of catching any type
of exception.
The
catch statement takes an Object of exception class as a parameter,
if an exception is thrown the statement in the catch block is
executed. The catch block is restricted to the statements in
the proceeding try block only.
Try {
// statements that may cause exception
}
catch(Exception obj)
{
}
§ Finally : when an exception is raised, the statement in the try block
is ignored, some times it is necessary to process certain
statements irrespective of wheather an exception is raised or not,
the finally block is used for this purpose.
§ Throw :
The throw class is used to call exception explicitly. You may want
to throw an exception when the user enters a wrong login ID and pass word, you
can use throw statement to do so.
The throw statement takes
an single argument, which is an Object of exception class.
Throw<throwable
Instance>
If the Object does not
belong to a valid exception class the compiler gives error.
§ Throws: The throws statement species the list of exception that has thrown
by a method.
If a method is capable of raising an exception that is does not handle, it must
specify the exception has to be handle by the calling method,
this is done by using the throw statement.
[<access specifier>] [<access modifier>] <return type>
<method name> <arg-list> [<exception-list>]
Eg: public void accept
password( ) throws illegalException
{
System.out.println(“Intruder”);
Throw new illegalAccesException;
}
Multi
Programming
A
multithreaded program contains two or more parts that can run concurrently,
Each part a program is called thread and each part that defines a separate path
of excution.
Thus multithreading
is a specified from of multitasking .
There are two distinct
types of multitasking .
Process: A Process is , in essence , a program that is
executing.
v Process-based :is heavy weight- allows you run two or more programs
concurrently.
Eg: you can use JAVA compiler at the same time you
are using text editor.
Here a program
is a small unit of code that can be dispatched by scheduler .
Thread-based: is Light weight- A Program can perform two or
more tasks simultaneously.
Creating a thread:
Eg: A text editor can
formate at the same time you can print, as long as these two tasks are being
perform separate treads.
Thread: can be defined
as single sequential flow of control with in a program.
Single Thread :
Application can perform only one task at a time.
Multithreaded : A
process having more than one thread is said to be multithreaded.
The multiple threads
in the process run at the same time, perform different task and interact with
each other.
v Daemon Thread: Is a low priority thread which runs immedeatly on the back ground
doing the Garbage Collection operation for the Java Run time System.
SetDaemon( ) – is used to
create DaemonThread.
Creating
a Thread:
(1). By implementing the
Runnable Interface.
(2). By extending the
thread Class.
v Thread Class: Java.lang.Threadclass is used to construct and access the
individual threads in a multithreaded application.
Syntax: Public Class <class name> extends
Thread { }
The Thread class define
several methods .
o Getname() – obtain a thread name.
o Getname() – obtain thread priority.
o Start( ) - start a thread by calling a Run( ).
o Run( ) - Entry point for the thread.
o Sleep( ) - suspend a thread for a period of
time.
o IsAlive( ) - Determine if a thread is still running.
o Join( ) - wait for a thread to
terminate.
v Runable Interface : The Runnable interface consist of a Single
method Run( ), which is executed when the thread is activated.
When
a program need ti inherit from another class besides the thread Class,
you need to implement the Runnable interface.
Syntax: public
void <Class-name> extends <SuperClass-name> implements Runnable
Eg: public Class myapplet
extends Japplet implements Runnable
{
// Implement the Class
}
* Runnable interface is the
most advantageous method to create threads because we need not extend thread
Class here.
v Life Cycle of Thread :
|
Runnable
|
New Thread --à
----à
Not Runnable
ß----
Dead The Run( ) terminates .
New
Thread: When an instance of a
thread class is created, a thread enters the new thread
state. Thread newThread = new
Thread(this);
You have to invoke the Start( ) to start the thread. ie,
newThread.Start( );
Runnable: when the Start( ) of the thread is invoked
the thread enters into the Runnable State.
Not
Runnable: A thread is said to
be not runnable state if it
à Is Slleping
à Is Waiting
à Is being blocked by
another thread.
sleep(long t); where t= no: of milliseconds for which the
thread is inactive.
The sleep( ) is a static
method because it operates on the current thread.
Dead: A thread can either die natuarally or be
killed.
- A thread dies a natural
death when the loop in the Run( ) is complete.
- Assigning null to the
thread Object kills the thread.
- If th loop in the Run( )
has a hundread iterations , the life of the thread is a hundread iterators of
the loop.
IsAlive(
): of the thread class
is used to determine wheather a thread has been started or stopped. If isAlive(
) returns true the thread is still running otherwise running completed.
Thread
Priorities: are used by the
thread scheduler to decide when each thread should ne allowed to run.To set a
thread priority, use te setpriority( ), which is a member of a
thread.
final void setpriority(int level) - here level
specifies the new priority seting for the calling thread.
The value
level must be with in the range :-
MIN_PRIORITY
= 1
NORM_PRIORITY = 5
MAX_PRIORITY
= 10
You can obtain the current
priority setting by calling getpriority( ) of thread.
final
int getpriority( )
No comments:
Post a Comment