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first you learn what is draw back of thread....why we we use thread... in previous we use sequential flow of execution...but problem is...if task 1 start second task2 is stop..but this stop time..we not. use another task... that is we use thread... multiple task run at a time parallel way ..is called as thread. run method is reserve method.. if we call start all run method run parallel..way. I hope you understand.
To understand thread Let me give an example.. In earlier days and in some OS now also when we open a web page data gets loaded and it comes in a proper sequence... like first text..then gif..then video... and thus it is the work of thread to present everything in a sequential maner and one task precedes the other.
Threads are useful for multitasking purpose.. Here is an example: Assume that you are playing a game at the same time you want to listen music.. that means multitasking at a same period of time.. Threads are useful to run multiple tasks concurrently like as to play music and video game and alot more.