public class Main {

    private static final String TEXT = "abcdefghijklmnopqrstuvwxyz";
    
    public static void main(String[] args) throws Exception {
        byte[] bytes = TEXT.getBytes();
        File[] files = { new File("output1.txt"), new File("output2.txt") };
        CountDownLatch countDownLatch = new CountDownLatch(files.length);
        ExecutorService executorService = Executors.newCachedThreadPool();
        for (File file : files) {
            executorService.submit(new ICallable(countDownLatch, file, bytes));
        }
        executorService.shutdown();
        countDownLatch.await();
        System.out.println("END");
    }
}

class ICallable implements Callable<Object> {

    private CountDownLatch countDownLatch;
    private File file;
    private byte[] bytes;

    public ICallable(CountDownLatch countDownLatch, File file, byte[] bytes) {
        this.countDownLatch = countDownLatch;
        this.file = file;
        this.bytes = bytes;
    }

    public Object call() throws Exception {
        BufferedOutputStream bufferedOutputStream = null;
        try {
            bufferedOutputStream = new BufferedOutputStream(new FileOutputStream(file));
            bufferedOutputStream.write(bytes);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (bufferedOutputStream != null) {
                bufferedOutputStream.close();
            }
            countDownLatch.countDown();
        }
        return null;
    }

}