Explain the working principle and main features of the RxJava library for reactive programming.

RxJava is a library for reactive programming in Java, based on the concept of data streams and asynchronous events.

Main principles and features:

• Observable — a data source that can emit a sequence of events (data).
• Observer (Subscriber) — a subscriber that receives these events and processes them.
• Operators — functions for transforming, filtering, and combining data streams.
• Asynchrony and thread management — RxJava allows easy switching of execution threads (Schedulers), simplifying multithreading.

Example:

Observable.just("Hello", "RxJava")
    .map(String::toUpperCase)
    .subscribe(System.out::println);

RxJava helps write clean, declarative, and easily scalable code for handling events and asynchronous operations.