Java中多线程编程是什么，提供一个使用多线程编程的实际案例

import java.io.*;
import java.net.URL;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class MultiThreadDownloader {
    private static final String fileUrl = "http://example.com/largefile.zip";
    private static final int NUM_THREADS = 4;

    public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(NUM_THREADS);

        try {
            URL url = new URL(fileUrl);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            int fileSize = connection.getContentLength();

            int chunkSize = fileSize / NUM_THREADS;
            int remainingBytes = fileSize % NUM_THREADS;

            for (int i = 0; i < NUM_THREADS; i++) {
                int startByte = i * chunkSize;
                int endByte = (i + 1) * chunkSize - 1;
                if (i == NUM_THREADS - 1) {
                    endByte += remainingBytes;
                }
                executor.execute(new DownloadTask(fileUrl, startByte, endByte, "part" + i + ".tmp"));
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

        executor.shutdown();
    }

    static class DownloadTask implements Runnable {
        private String fileUrl;
        private int startByte;
        private int endByte;
        private String fileName;

        public DownloadTask(String fileUrl, int startByte, int endByte, String fileName) {
            this.fileUrl = fileUrl;
            this.startByte = startByte;
            this.endByte = endByte;
            this.fileName = fileName;
        }

        @Override
        public void run() {
            try {
                URL url = new URL(fileUrl);
                HttpURLConnection connection = (HttpURLConnection) url.openConnection();
                connection.setRequestProperty("Range", "bytes=" + startByte + "-" + endByte);
                InputStream inputStream = connection.getInputStream();

                FileOutputStream outputStream = new FileOutputStream(fileName);
                byte[] buffer = new byte[1024];
                int bytesRead;
                while ((bytesRead = inputStream.read(buffer)) != -1) {
                    outputStream.write(buffer, 0, bytesRead);
                }

                inputStream.close();
                outputStream.close();
                System.out.println("Downloaded " + fileName + " from " + startByte + " to " + endByte);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}