What does it mean that an operator by default does not operate on any particular Scheduler

Question

What does it mean that an operator by default does not operate on any particular Scheduler?

Example: CombineLatest.

RxJava implements this operator as combineLatest. It may take between two and nine Observables (as well as the combining function) as parameters, or a single List of Observables (as well as the combining function). It does not by default operate on any particular Scheduler.

Answer 1

Operators do not require a particular Scheduler when they do not perform thread management operations.

That doesn't mean the code is not thread-safe. Thread safety is achieved by using scope containment, stack variables, volatile values and Atomic variables, with minimal use of synchronized clauses.

Thread management is more sophisticated, and means moving operations or data between threads. In the code for such operators, you will see them creating Runnables, or TimerTasks, which require knowing the scheduler.

Answer 2

It means that the operator follows this guideline stated in the Scheduler docs --

By default, an Observable and the chain of operators that you apply to it will do its work, and will notify its observers, on the same thread on which its Subscribe method is called.

Everything it does will happen on the thread that subscribe was called on. Most operators work this way. Some cannot perform their work on same thread (like sample), so need a second thread to perform the work. These operators generally have a default Scheduler that they'll use for work, as well as a version that accepts a Scheduler as a parameter to let you choose where they get the second thread to work on.