Why we call start() method which in turns calls run method, why not we directly call run method ?

In both the above examples, the run() method is the most important method in the thread classes, it is also the only method that we need to implement in both cases. Why it's only proper to call the start() method to start the thread instead of calling the run() method directly? Because the run() method is not a regular class method. It should only be called by the JVM. Writing thread classes is not about a single sequential thread, it's about the use of multiple threads running at the same time and performing different tasks in a single program. The JVM needs to work closely with the underneath operating system for the actual implementation of concurrent operations. This is how the performance can be improved, and all other benefits mentioned above can be achieved.

You should not invoke the run() method directly. If you call the run() method directly, it will simply execute in the caller's thread instead of as its own thread. Instead, you need to call the start() method, which schedules the thread with the JVM. The JVM will call the corresponding run() method when the resources and CPU is ready. The JVM is not guaranteed to call the run() method right way when the start() method is called, or in the order that the start() methods are called. Especially for the computers have a single processor, it is impossible to run all running threads at the same time. The JVM must implement a scheduling scheme that shares the processor among all running threads. This is why when you call the start() methods from more than one thread, the sequence of execution of the corresponding run() methods is random, controlled only by the JVM.