A thread is light weight process that can execute simultaneously. Multithreading in java allows multiple threads to run simultaneously.

• A program can be divided into two or more parts that can run concurrently and each part of such program is known as thread.
• In a simple word, thread is a lightweight sub-process.
•  It is a smallest unit of processing.
• When these threads execute concurrently for accomplishing a task is known multithreading.
• In multithreading, each thread defines a separate path of execution. 
• In multithreading, every thread has a priority between 1 to 10. Thread with highest priority get preference over lower priority thread for execution.

 In a java programming, Thread is a predefined class within a java.lang package.

Operating system (OS) can contain multiple process, in the above figure, there are three process  P1,P2 & P3, within OS and within P3 process there are three threads T1, T2& T3. There is context-switching between the threads.  

Difference between Process and Thread

Process	Thread
A process has their own virtual address space.	Threads are entities within a process it uses its virtual address space of process.
Two processes running on the same system at the same time do not uses address space.	Two threads running within a process uses the same address space.
Every process has its own data segment	All threads within the process uses the same data segment of process.
In multiprocessing, processes use inter process communication techniques to interact with other processes.	In multithreading, threads do not uses inter process communication techniques because they not uses separate address spaces. They share the same address space therefore, they can communicate directly with other threads within process.
Processes have considerable overhead.	Threads have almost no overhead;
Process is heavy weight.	Threads are light weight process (LWP).
Process contains much information about system, resource.	Threads carry less state information than process
When we create a child process from a parent process then it requires duplication of the resources of parent process.	No duplication of resources is required when we create a child thread from main thread.

Thread Creation in Java

We can create a thread by two ways:

1. By extending Thread
2. By implementing Runnable interface

Lets see multithreading in java with example

Creating Thread By Implementing Runnable Interface

1. We can create a thread by creating a class that implements the Runnable interface.
2. To implement Runnable, a class only need to implement a single method called run().
3. Inside run () we will define the code that constitute the new thread.    
4. When a new thread creates, it do not executes until we call its start () method. When start () method executes it call to run() method.  

Result

Description:

Passing “this” as a argument, “DemoThread” is the name of the thread.  

In this program, main thread fineshes last because the main thread sleeps for 1000 milliseconds (1 seconds) between iteration, but the child thread sleeps for only 500 milliseconds.

This cause the child thread terminates earlier than main thread.

Creating Thread By Extending Thread Class

1. To create a thread we will create a thread a class that extends Thread class.
2. To extend Thread class, a class only needs to implement a single method called run (). This extending class must override the run() method
3. Inside run () we will define the code that constitute the new thread.                                                                                 
4. When a new thread creates, it do not executes until we call its start () method. When start () method executes it call to run() method.

Result

Thread Methods in Java

Commonly used methods of Thread class are as follows

Sr No	Method and Description
1	void run() Work execute as a thread is defined in this method .
2	void start() Used to start a thred which Calls the run() method of the thread or child class.
3	static void sleep(long miliseconds) Suspend the thread for specified amount of time.
4	static void sleep(long miliseconds,int nanos) Suspend the thread for specified amount of time.
5	void join() This method is used to pause the ececution of current thread until it die. void join().
6	void join(long miliseconds) Pause the execution of current thread for the pecified miliseconds.
7	void join(long miliseconds,int nanos) Pause the execution of current thread for the pecified miliseconds and nano seconds.
8	boolean isAlive() To checks a thread is alive or not.
9	String getName() Returns the name of the thread.
10	setName(String name) Used to set the name of thread.
11	int getPriority() Get the priority of the thread.
12	void setPriority(int priority) Set the priority of thread.
13	static Thread currentThread() Returns the reference of currently executing thread
14	long getId() Returns the id of the thread.
15	static void yield() Temporarily pause the currently executing thread and allow other threads to execute.
16	void suspend() Used to suspend the thread.
17	void resume() Used to resume the suspended thread.
18	void stop() Used to stop the thread.
19	void interrupt() Interrupts the thread.
20	boolead isInterrupted() Check whether the thead is interrupted or not