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There are several ways to ensure that all threads have completed their tasks before proceeding in Java:

1. Join() method: You can call the join() method on all the threads you want to wait for. This method blocks the executing thread until the join() on each thread has completed.

2. CountDownLatch: A CountDownLatch is a synchronization aid that allows a thread to wait until a set of operations being performed on other threads completes. You can create a CountDownLatch with a count of the number of threads you want to wait for. Each thread should call countDown() on the latch when it completes its task. The main thread can then call await() on the latch to block until all tasks are complete.

3. ExecutorService: You can use an ExecutorService to manage a collection of threads. The ExecutorService provides a shutdown() method that blocks until all tasks have completed.

4. CompletableFuture: A CompletableFuture is a class introduced in Java 8 that represents a future result of an asynchronous computation. You can create a CompletableFuture for each task and then use the allOf() method to wait for all CompletableFuture objects to complete.

Overall, the approach you choose depends on the specific requirements of your application.