Subject：它既是Observable，又是observer。也就是既可以傳送事件，也可以接收事件。

下面是四個子類PublishSubject、ReplaySubject、BehaviorSubject、AsyncSubject的區別：

PublishSubject<Integer> source = PublishSubject.create();　　//PublicSubject：接收到訂閱之後的所有資料。
// It will get 1, 2, 3, 4 and onComplete
source.subscribe(getFirstObserver()); 
source.onNext(
 
1);
source.onNext(2);
source.onNext(3);
// It will get 4 and onComplete for second observer also.
source.subscribe(getSecondObserver());
source.onNext(4);
source.onComplete();

ReplaySubject<Integer> source = ReplaySubject.create();　　//ReplaySubject：接收到所有的資料，包括訂閱之前的所有資料和訂閱之後的所有資料。
// It will get 1, 2, 3, 4
 

source.subscribe(getFirstObserver());
source.onNext(1);
source.onNext(2);
source.onNext(3);
source.onNext(4);
source.onComplete();
// It will also get 1, 2, 3, 4 as we have used replay Subject
source.subscribe(getSecondObserver());

BehaviorSubject<Integer> source = BehaviorSubject.create();　　//BehaviorSubject：接收到訂閱前的最後一條資料和訂閱後的所有資料。
 
// It will get 1, 2, 3, 4 and onComplete
source.subscribe(getFirstObserver());
source.onNext(1);
source.onNext(2);
source.onNext(3);
// It will get 3(last emitted)and 4(subsequent item) and onComplete
source.subscribe(getSecondObserver());
source.onNext(4);
source.onComplete();

AsyncSubject<Integer> source = AsyncSubject.create();　　//AsyncSubject：不管在什麼位置訂閱，都只接接收到最後一條資料
// It will get only 4 and onComplete
source.subscribe(getFirstObserver());
source.onNext(1);
source.onNext(2);
source.onNext(3);
// It will also get only get 4 and onComplete
source.subscribe(getSecondObserver());
source.onNext(4);
source.onComplete();