What is the main idea of reactive programming and what problems does it solve?

The main idea of reactive programming is the processing of data streams and events asynchronously with the ability to reactively respond to changes. Instead of writing code that executes sequentially and blocks the thread, reactive programming allows describing chains of transformations and reactions to data that arrive over time.

Tasks solved by reactive programming:

• Efficient processing of asynchronous events and data streams.
• Managing complex event streams using transformation, filtering, and merging operators.
• Improving responsiveness and scalability of applications through non-blocking I/O.
• Simplifying working with data streams that can arrive at different speeds (backpressure).

In Java, reactive programming is often implemented through libraries supporting the Reactive Streams specification (e.g., Reactor, RxJava).

Example on Reactor:

Flux.just(1, 2, 3, 4)
    .filter(i -> i % 2 == 0)
    .map(i -> i * 10)
    .subscribe(System.out::println); // Will output 20 and 40

Here, a data stream is created that filters even numbers, multiplies them by 10, and outputs the result — all happening asynchronously and reactively.