creating-threads.md

Creating Threads

We know that the execution of a Java application starts from the main() method.

Similarly, the execution of a thread starts from the run() method.

There are two ways to create a thread in Java :

1. By extending the Thread class
2. By implementing the Runnable interface --> recommended approach

To perform any task in a separate thread, you need to override the run() method and invoke the start() method on the thread.

If we need to add any feature to the Thread class implementation then only use approach #1.

For regular thread creation scenarios, always use #2.

Using #1 we create a thread directly.

But using #2 we create a Task which implements the Runnable interface.

So for #2 once we create a task and override the run() method, we need to create a Thread class object and submit this task to run inside that thread.

And then submit the thread for execution by doing thread.start()

thread.start() does not necessarily start the thread execution immediately. It just submits the thread for execution and whenever the thread gets the CPU cycle, it starts getting executed.

Runnable or Thread?

Simply put, it is encouraged to use Runnable instead of Thread class. Reasons:

• When extending the Thread class, we're not overriding any of its methods. Instead, we override the method of Runnable (which Thread happens to implement). This is a clear violation of is-a Thread principle.

• Creating an implementation of Runnable and passing it to the Thread class utilizes composition and not inheritance which is more flexible.

• After extending the Thread class, we can't extend any other class.

• From Java 8 onwards, Runnables can be represented as lambda expressions.