Observables must issue notifications to observers serially (not in parallel). They may issue these notifications from different threads, but there must be a formal happens-before relationship between the notifications.

final PublishSubject<Integer> subject = PublishSubject.create();

subject.subscribe(new Subscriber<Integer>() {
 @Override
 public void onCompleted() {

 }

 @Override
 public void onError(Throwable e) {

 }

 @Override
 public void onNext(Integer integer) {
  unSafeCount = unSafeCount + integer;
  Log.d("TAG", "onNext: " + unSafeCount);
 }
});

findViewById(R.id.send).setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View v) {
  final int unit = 1;
  for(int i = 0;i < 10;i++) {
   new Thread(new Runnable() {
    @Override
    public void run() {
     for (int j = 0; j < 1000; j++) {
      subject.onNext(unit);
     }
    }
   }).start();
  }
 }
});

final PublishSubject<Integer> subject = PublishSubject.create();

subject.subscribe(new Subscriber<Integer>() {
 @Override
 public void onCompleted() {

 }

 @Override
 public void onError(Throwable e) {

 }

 @Override
 public void onNext(Integer integer) {
  unSafeCount = unSafeCount + integer;
  count.addAndGet(integer);

  Log.d("TAG", "onNext: " + count);
 }
});

final SerializedSubject<Integer, Integer> ser = new SerializedSubject<Integer, Integer>(subject);

findViewById(R.id.send).setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View v) {
  final int unit = 1;

  for(int i = 0;i < 10;i++){
   new Thread(new Runnable() {
    @Override
    public void run() {
     for(int j = 0;j < 1000;j++){
      ser.onNext(unit);
     }
    }
   }).start();
  }
 }
});

@Override
public void onNext(T t) {
 if (terminated) {
  return;
 }
 synchronized (this) {
  if (terminated) {
   return;
  }
  if (emitting) {
   FastList list = queue;
   if (list == null) {
    list = new FastList();
    queue = list;
   }
   list.add(nl.next(t));
   return;
  }
  emitting = true;
 }
 try {
  actual.onNext(t);
 } catch (Throwable e) {
  terminated = true;
  Exceptions.throwOrReport(e, actual, t);
  return;
 }
 for (;;) {
  for (int i = 0; i < MAX_DRAIN_ITERATION; i++) {
   FastList list;
   synchronized (this) {
    list = queue;
    if (list == null) {
     emitting = false;
     return;
    }
    queue = null;
   }
   for (Object o : list.array) {
    if (o == null) {
     break;
    }
    try {
     if (nl.accept(actual, o)) {
      terminated = true;
      return;
     }
    } catch (Throwable e) {
     terminated = true;
     Exceptions.throwIfFatal(e);
     actual.onError(OnErrorThrowable.addValueAsLastCause(e, t));
     return;
    }
   }
  }
 }
}

Observable o1 = Observable.create(new Observable.OnSubscribe<Integer>() {
 @Override
 public void call(final Subscriber<? super Integer> subscriber) {
  new Thread(new Runnable() {
   @Override
   public void run() {
    try {
     Thread.sleep(1000);
     subscriber.onNext(1);
     subscriber.onCompleted();
    } catch (InterruptedException e) {
     e.printStackTrace();
    }
   }
  }).start();
 }
});
Observable o2 = Observable.create(new Observable.OnSubscribe<Integer>() {
 @Override
 public void call(Subscriber<? super Integer> subscriber) {
  subscriber.onNext(2);
  subscriber.onCompleted();
 }
});

Observable.merge(o1,o2)
  .subscribe(new Subscriber<Integer>() {
   @Override
   public void onCompleted() {

   }

   @Override
   public void onError(Throwable e) {

   }

   @Override
   public void onNext(Integer i) {
    Log.d("TAG", "onNext: " + i);
   }
  });

@Override
public void onNext(Observable<? extends T> t) {
  if (t == null) {
    return;
  }
  if (t == Observable.empty()) {
    emitEmpty();
  } else
  if (t instanceof ScalarSynchronousObservable) {
    tryEmit(((ScalarSynchronousObservable<? extends T>)t).get());
  } else {
    InnerSubscriber<T> inner = new InnerSubscriber<T>(this, uniqueId++);
    addInner(inner);
    t.unsafeSubscribe(inner);
    emit();
  }
}

Concat waits to subscribe to each additional Observable that you pass to it until the previous Observable completes.

if (wip.getAndIncrement() != 0) {
  return;
}

final int delayErrorMode = this.delayErrorMode;

for (;;) {
  if (actual.isUnsubscribed()) {
    return;
  }

  if (!active) {
    if (delayErrorMode == BOUNDARY) {
      if (error.get() != null) {
        Throwable ex = ExceptionsUtils.terminate(error);
        if (!ExceptionsUtils.isTerminated(ex)) {
          actual.onError(ex);
        }
        return;
      }
    }

    boolean mainDone = done;
    Object v = queue.poll();
    boolean empty = v == null;

    if (mainDone && empty) {
      Throwable ex = ExceptionsUtils.terminate(error);
      if (ex == null) {
        actual.onCompleted();
      } else
      if (!ExceptionsUtils.isTerminated(ex)) {
        actual.onError(ex);
      }
      return;
    }

    if (!empty) {

      Observable<? extends R> source;

      try {
        source = mapper.call(NotificationLite.<T>instance().getValue(v));
      } catch (Throwable mapperError) {
        Exceptions.throwIfFatal(mapperError);
        drainError(mapperError);
        return;
      }

      if (source == null) {
        drainError(new NullPointerException("The source returned by the mapper was null"));
        return;
      }

      if (source != Observable.empty()) {

        if (source instanceof ScalarSynchronousObservable) {
          ScalarSynchronousObservable<? extends R> scalarSource = (ScalarSynchronousObservable<? extends R>) source;

          active = true;

          arbiter.setProducer(new ConcatMapInnerScalarProducer<T, R>(scalarSource.get(), this));
        } else {
          ConcatMapInnerSubscriber<T, R> innerSubscriber = new ConcatMapInnerSubscriber<T, R>(this);
          inner.set(innerSubscriber);

          if (!innerSubscriber.isUnsubscribed()) {
            active = true;

            source.unsafeSubscribe(innerSubscriber);
          } else {
            return;
          }
        }
        request(1);
      } else {
        request(1);
        continue;
      }
    }
  }
  if (wip.decrementAndGet() == 0) {
    break;
  }
}