What is the difference between using subscribeOn and observeOn in RxJava, and how do they affect the execution thread?

In RxJava, the methods subscribeOn and observeOn control the threads on which different stages of the reactive chain are executed.

• subscribeOn sets the thread where the subscription and the execution of the source data (Observable) will occur. It affects where data generation begins.

• observeOn switches the thread on which subsequent operators and result processing will be performed after this method is called.

For example, if you need to perform heavy work in a background thread and process results in the UI thread, you can write:

observable
    .subscribeOn(Schedulers.io())       // Source runs in a background thread
    .observeOn(AndroidSchedulers.mainThread()) // Result processing in UI thread
    .subscribe(...);

Thus, subscribeOn influences the source thread, and observeOn influences the downstream processing thread.