MainObj.java

package com.inferdata.examples.simplethread;

public class MainObj implements Runnable {
  
  private int finished_actions;

  public void run() {
    System.out.println("starting secondary threads...");
    new Thread(new SecondaryThread(this, "action 1")).start();
    new Thread(new SecondaryThread(this, "action 2")).start();
    new Thread(new SecondaryThread(this, "action 3")).start();
    waiting();
    System.out.println("finishing MainObj run method");
  }
  
  synchronized void waiting() {
    while (finished_actions < 3) {
      try {
        System.out.println("in MainObj 'waiting' method, waiting...");
        wait();
        System.out.println("in MainObj 'waiting' method, got a notification- " + finished_actions + " finished actions.");
      }
      catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
  }

  synchronized public void processResponse(String action, String resp) {
    System.out.println("in MainObj.processResponse: got response " + resp + " for action " + action);
    finished_actions++;
    notifyAll();
  }
  
	public static void main(String[] args) {
		MainObj mt = new MainObj();
		new Thread(mt).start();
	}
  
}

SecondaryThread.java

package com.inferdata.examples.simplethread;

public class SecondaryThread implements Runnable {
  
  MainObj callback;
  String action;

  SecondaryThread(MainObj cb, String act) {
    callback = cb;
    action = act;
  }

  public void run() {
    // generate random sleep time in ms between 0 and 15 seconds
    int sleeptime = ((int)(Math.random() * 15)) * 1000;
    System.out.println("pretending to work on " + action + " (" + sleeptime + ")");
    try {
      Thread.sleep(sleeptime);
    }
    catch (java.lang.InterruptedException e) {
      e.printStackTrace();
    }
    System.out.println("finished " + action);
    callback.processResponse(action, ""+sleeptime);
  }
}