在Java中實現多線程下載文件可以通過使用多線程來同時下載不同部分的文件，提高下載速度。以下是一個簡單的示例代碼：

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

public class MultiThreadFileDownloader {
    private static final String FILE_URL = "http://example.com/file.zip";
    private static final int NUM_THREADS = 4;

    public static void main(String[] args) {
        try {
            URL url = new URL(FILE_URL);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            int fileSize = connection.getContentLength();

            ExecutorService executor = Executors.newFixedThreadPool(NUM_THREADS);
            int chunkSize = fileSize / NUM_THREADS;

            for (int i = 0; i < NUM_THREADS; i++) {
                int startByte = i * chunkSize;
                int endByte = (i == NUM_THREADS - 1) ? fileSize - 1 : (i + 1) * chunkSize - 1;
                executor.execute(new Downloader(FILE_URL, startByte, endByte, i));
            }

            executor.shutdown();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static class Downloader implements Runnable {
        private String fileUrl;
        private int startByte;
        private int endByte;
        private int threadId;

        public Downloader(String fileUrl, int startByte, int endByte, int threadId) {
            this.fileUrl = fileUrl;
            this.startByte = startByte;
            this.endByte = endByte;
            this.threadId = threadId;
        }

        @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();
                BufferedInputStream bufferedInputStream = new BufferedInputStream(inputStream);

                FileOutputStream fileOutputStream = new FileOutputStream("part_" + threadId + ".tmp");

                byte[] buffer = new byte[1024];
                int bytesRead;

                while ((bytesRead = bufferedInputStream.read(buffer)) != -1) {
                    fileOutputStream.write(buffer, 0, bytesRead);
                }

                fileOutputStream.close();
                bufferedInputStream.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

在上面的代碼中，我們首先創建一個Downloader類來實現下載文件的邏輯。然后在主函數中創建一個固定大小的線程池，根據線程數量將文件分為不同的部分，并為每個部分創建一個下載線程。每個下載線程通過HTTP請求下載文件的指定部分，并將其保存到一個臨時文件中。

請注意，這只是一個簡單的示例代碼，實際中可能需要做更多的異常處理和錯誤處理。