title: Android Thread complete notify
body: <p>I create 4 threads to do 4 operations.
  When all threads complete execution,I want to notify main thread.
  How to know if all thread complete?</p>
  
  <p>Here is my code:</p>
  
  <pre><code>public class TestActivity extends AppCompatActivity {
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_test);
  
          /*
          * create 4 threads to do 4 operations
          * */
          Thread thread1 = new Thread(new Runnable() {
              @Override
              public void run() {} // do operation 1
          });
          Thread thread2 = new Thread(new Runnable() {
              @Override
              public void run() {} // do operation 2
          });
          Thread thread3 = new Thread(new Runnable() {
              @Override
              public void run() {} // do operation 3
          });
          Thread thread4 = new Thread(new Runnable() {
              @Override
              public void run() {} // do operation 4
          });
  
          thread1.start();
          thread2.start();
          thread3.start();
          thread4.start();
  
      }
  }
  </code></pre>