Basic concept of java (1)

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BASIC CONCEPT OF JAVA
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Q Can we override static method?  A No, static
method cannot be overridden. It can be proved by
runtime polymorphism, so we will learn it
later.  Q Why we cannot override static
method?  A because static method is bound with
class whereas instance method is bound with
object. Static belongs to class area and instance
belongs to heap area.  Q Can we override java
main method?  A No, because main is a static
method.        
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• Difference between method overloading and method
  overriding in java
• Method Overloading
• Method overloading is used to increase the
  readability of the program.
• Method overloading is performed within class.
• In case of method overloading, parameter must be
  different
• Method overloading is the example of compile time
  polymorphism.
• 5)In java, method overloading can't be performed
  by changing return type of the
  
  
  method only.
  Return type can be same or different in method
  overloading. But you must have to change the
  parameter.

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• Method Overriding
• Method overriding is used to provide the specific
  implementation of the method that is already
  provided by its super class.
• 2) Method overriding occurs in two classes
  (inheritance).
• 3) In case of method overriding, parameter must
  be same.
• 4) Method overriding is the example of run time
  polymorphism.
• 5) Return type must be same or covariant in
  method overriding.

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/this keyword in java/ To refer the current
class instance variable To invoke the current
class constructor To invoke the current class
method To pass as an argument in the method
call To pass as an argument in the constructor
call To return the current class
instance Proving this keyword There can be a lot
of usage of java this keyword. In java, this is a
reference variable that refers to the current
object.
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• /Usage of java this keyword/
• Here is given the 6 usage of java this keyword.
• This() keyword can be used to refer current class
  instance variable.
• 2. This() can be used to invoke current class
  constructor.
• 3 .This() keyword can be used to invoke current
  class method (implicitly)
• 4. This() can be passed as an argument in the
  method call.
• 5. This() can be passed as argument in the
  constructor call.
• 6 .This() keyword can also be used to return the
  current class instance.

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Multithreading in java with examples A thread is
a light-weight smallest part of a process that
can run concurrently With the other parts(other
threads) of the same process. Threads are
independent because they all have separate path
of execution thats the reason if an exception
occurs in one thread, it doesnt affect the
execution of other threads. All threads of a
process share the common memory. The process of
executing multiple threads simultaneously is
known as multithreading. The main purpose of
multithreading is to provide simultaneous
execution of two or more parts of a program to
maximum utilize the CPU time. A multithreaded
program contains two or more parts that can run
concurrently. Each such part of a program called
thread.
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2. Threads are lightweight sub-processes, they
share the common memory space. In Multithreaded
environment, programs that are benefited from
multithreading, utilize the maximum CPU time so
that the idle time can be kept to minimum. 3. A
thread can be in one of the following states A
thread can be in only one state at a given point
in time. Multitasking vs Multithreading vs
Multiprocessing vs parallel processing
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• If you are new to java you may get confused among
  these terms as they are used quite frequently
  when we discuss multithreading. Lets talk about
  them in brief.
• Multitasking Ability to execute more than one
  task at the same time is known as multitasking.
• Multithreading We already discussed about it. It
  is a process of executing multiple threads
  simultaneously. Multithreading is also known as
  Thread-based Multitasking.
• Multiprocessing It is same as multitasking,
  however in multiprocessing more than one CPUs are
  involved. On the other hand one CPU is involved
  in multitasking.
• Parallel Processing It refers to the utilization
  of multiple CPUs in a single computer system.
• Creating a thread in Java
• There are two ways to create a thread in Java
• By extending Thread class.
• 2) By implementing Runnable interface.

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Before we begin with the programs (code) of
creating threads, lets have a look at these
methods of Thread class. We have used few of
these methods in the example below. getName()
It is used for Obtaining a threads
name getPriority() Obtain a threads
priority isAlive() Determine if a thread is
still running join() Wait for a thread to
terminate run() Entry point for the
thread sleep() suspend a thread for a period
of time start() start a thread by calling its
run() method
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• Methods isAlive() and join()
• In all the practical situations main thread
  should finish last else other threads which have
  spawned from the main thread will also finish.
• To know whether the thread has finished we can
  call is Alive() on the thread which returns true
  if the thread is not finished.
• Another way to achieve this by using join()
  method, this method when called from the parent
  thread makes parent thread wait till child thread
  terminates.
• These methods are defined in the Thread class.
• We have used is Alive() method in the above
  examples too.

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• Synchronization
• Multithreading introduces asynchronous behavior
  to the programs. If a thread is writing some data
  another thread may be reading the same data at
  that time. This may bring inconsistency.
• When two or more threads need access to a shared
  resource there should be some way that the
  resource will be used only by one resource at a
  time. The process to achieve this is called
  synchronization.
• To implement the synchronous behavior java has
  synchronous method. Once a thread is inside a
  synchronized method, no other thread can call any
  other synchronized method on the same object. All
  the other threads then wait until the first
  thread come out of the synchronized block.

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When we want to synchronize access to objects of
a class which was not designed for the
multithreaded access and the code of the method
which needs to be accessed synchronously is not
available with us, in this case we cannot add the
synchronized to the appropriate methods. In java
we have the solution for this, put the calls to
the methods (which needs to be synchronized)
defined by this class inside a synchronized block
in following manner. Synchronized (object)
// statement to be synchronized
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To make a method synchronized, simply add the
synchronized keyword to its declaration. Then no
two invocations of synchronized methods on the
same object can interleave with each
other. 1.Synchronized statements must specify
the object that provides the intrinsic
synchronized keyword is used for exclusive
accessing. 2.Lock. When synchronized (this) is
used, you have to avoid to synchronizing
invocations of other objects' methods. 3.Wait()
tells the calling thread to give up the monitor
and go to sleep until some other thread enters
the same monitor and calls notify( ). 4.Notify()
wakes up the first thread that called wait() on
the same object.
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Synchronized ,Wait, notify and notifyall When a
thread running in a synchronized method of an
object is calling the wait() method of the same
object, that thread releases the lock of the
object and is added to that object's waiting
queue. As long as it's there, it sits idle. Note
also that wait() forces the thread to release its
lock. This means that it must own the lock of an
object before calling the wait() method of that
(same) object. Hence the thread must be in one of
the object's synchronized methods or synchronized
block before calling wait().
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When a thread invokes an object's notify() or
notifyAll() method, one (an arbitrary thread) or
all of the threads in its waiting queue are
removed from the waiting queue to the entry
queue. They then actively contend for the
object's lock, and the one that gets the lock
goes on to execute. If no threads are waiting in
the waiting queue, then notify() and notifyAll()
have no effect. Before calling the notify() or
notifyAll() method of an object, a thread must
own the lock of the object. Hence it must be in
one of the object's synchronized methods or
synchronized block.
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