Java Thread How to - Start one thread, then start another one and first thread keeps running when thread 2 running

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Question

We would like to know how to start one thread, then start another one and first thread keeps running when thread 2 running.

Answer

import java.util.concurrent.CountDownLatch;
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public class Main {
  static CountDownLatch cdl;

  public static void main(String... s) {
    cdl = new CountDownLatch(1);
    Thread a = new Thread(() -> {
      System.out.println("started a");
      try {
        Thread.sleep(4000);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      cdl.countDown();
      System.out.println("stoped a");
    });
    Thread b = new Thread(() -> {
      System.out.println("started b");
      System.out.println("wait a");
      try {
        cdl.await();
      } catch (Exception e) {
        e.printStackTrace();
      }
      System.out.println("stoped b");
    });
    b.start();
    a.start();
  }
}

The code above generates the following result.

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