There are basically three methods of Observable conceptually as below:
1. next(): this method define that how to process data which is sent by observable
2. error(): this method define that how to manage error handling activities.
3. complete(): this method define course of action need to perform after the observable has completed producing and emitting data.
4. next() method cannot be executed after the observable has errored or completed.
5. next(), error() or complete() method cannot be called after unsubscribe.
unsubscribe is called on error or complete to free the resources used by the subscription and the observable.