The Java parent of or the main thread waits for an instance where all the child threads exit

Examples are as follows:

static void testLock1(){
   
   final AtomicInteger waitCount = new AtomicInteger(30000);
   final Object waitObj = new Object();
   System.out.println("start"+System.currentTimeMillis());
   for (int i=0;i<30000;i++) {
    new Thread(new Runnable() {
     @Override
     public void run() {
      try {
       Thread.sleep(10);
      } catch (InterruptedException e) {
       e.printStackTrace();
      }
      waitCount.decrementAndGet();
      synchronized(waitObj){
       waitObj.notifyAll();
      }
     }
    }).start();
   }
   while( waitCount.intValue()>0) {
    synchronized (waitObj) {
     if(waitCount.intValue()>0){
      try {
       waitObj.wait();
      } catch (InterruptedException e) {
       e.printStackTrace();
      }
     }
    }
   }
   System.out.println("ok"+System.currentTimeMillis());
  }
  static void testLock2(){
   
   final CountDownLatch workLauch = new CountDownLatch(30000);// counter 
   System.out.println("start2"+System.currentTimeMillis());
   for (int i=0;i<30000;i++) {
    new Thread(new Runnable() {
     @Override
     public void run() {
      try {
       Thread.sleep(10);
      } catch (InterruptedException e) {
       e.printStackTrace();
      }
      workLauch.countDown();
     }
    }).start();
   }
   try {
    workLauch.await();
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
   System.out.println("ok2"+System.currentTimeMillis());
  }
  public static void main(String[] args) {
   testLock1();
   testLock2();
  }

The first one is the implementation that I just wrote here, which is kind of rough. The second is a friend notifies you of a class, java's concurrent, which is said to have several similar functional class implementations. The 30,000 threads time difference is probably less than 200ms