Thread in Java

Multitasking:

Executing several tasks simultaneously is the concept of multitasking.

There are 2 types of multitasking

Process based
Thread based

1. Process Based:-

Executing several task simultaneously where each task is a separate independent program ( process ).

Ex-

Make a program
Listen music
Download a file

(Same time in system, each task independent to each other in different process)

2. Thread Based:-

Executing several task simultaneously where each task is a separate independent part of the same program, is called thread based multitasking and each independent is called thread.

Thread based multitasking is best at programming level.

When compared with old language developing multi-threaded in java is very easy because java provide inbuilt support for multi-threading with reach API [ Thread, Runnable, ThreadGroup... ]

Thread:

Thread is a separate flow of execution. For every thread there is a separate job.

We can define a thread in the following 2 ways.

By extending Thread class.
By implementing Runnable interface.

1. By extending Thread class:-

Case 1:-

Thread Scheduler

It is the part of JVM.
It is responsible to schedule threads. If multiple threads are waiting to get chance of execution, then in which order thread will be executed is decided by thread scheduler.
We cannot expect exact algorithm followed by thread scheduler. It is varied from JVM to JVM. Hence we cannot accept threads execution order and output.
Hence whenever situations comes to multi-threading there is no guarantee for exact output but we can provide several possible output.

Case 2:-

Difference between t.start() and t.run() method

In the case of t.start(), a new thread will be created which is responsible for the execution of run() method.
But in the case of t.run() method, a new thread would not be created run method will be executed just like a normal method call by main thread.
In the above program if we replace t.start() with t.run() then the output is [ child thread 10 times and main thread 10 times ]
Total output produce by only main thread.

Case 3:-

Importance of Thread class start() method

Thread class start() method is responsible to register the thread with Thread scheduler and all other mandatory activities.
Hence without executing Thread class start(), there is no chance of starting a new thread in java.
Due to this Thread class start() is considered of heart multi-threading.

start ()
{
Register this thread with Thread Scheduler.
Perform all other mandatory activities.
Invoke run().
}

Case 4:-

Overloading of run() method

Overloading of run() method is always possible but thread class start() method can invoke no-argument run() method.
The other overloaded method we have to call explicitly like a normal method call.

Ex:

Case 5:-

If we are not overriding run() method

If we are not overriding run() method then thread class run method will be executed which has empty implementation. Hence we will not get any output.

Ex:

Note:

It is highly recommended to override run method otherwise do not go for multi-threaded concept.

Case 6:-

Overriding of start() method

If we override start() method, then our start() will be executed just like a normal method call and new thread would not be created

Case 7:-

Life cycle of thread

Case 8:-

After starting a thread if we are trying to restart the same thread then, we will get runtime exception saying - IllegalThreadException

Ex:

Thread t = new Thread();

t.start();

t.start(); --> RE: IllegalThreadException

2.Defining a thread by implementing Runnable ( Interface ):-

We can define a thread by implementing Runnable interface.

Runnable interface present in java.lang package and its contains only one method

public void run()

Ex: