CountDownLatch

The main thread initializes a CountDownLatch, e.g.:

    CountDownLatch countDownLatch = new CountDownLatch(3);

It passes countDownLatch to n (in this example 3) sub-threads, for instance via constructor.
Finally, the main thread waits for the execution of the sub threads:

    countDownLatch.await();

Once ready, every thread finally calls

    countDownLatch.countDown();

Every countDown() call decrements the initial number, and the main thread finally will resume with the operations defined after the wait() statement.

See more here: https://howtodoinjava.com/java/multi-threading/when-to-use-countdownlatch-java-concurrency-example-tutorial/