rxjava，如何检查单个

Observable.just(1, 2, 3)
    .isEmpty()
    .subscribe(new Consumer<Boolean>() {
        @Override
        public void accept(Boolean isEmpty) throws Exception {
            if (isEmpty) {
                System.out.println("Observable没有发射任何数据");
            } else {
                System.out.println("Observable发射了数据");
            }
        }
    });

Observable.just(1, 2, 3)
    .contains(2)
    .subscribe(new Consumer<Boolean>() {
        @Override
        public void accept(Boolean contains) throws Exception {
            if (contains) {
                System.out.println("Observable发射了指定的数据项");
            } else {
                System.out.println("Observable没有发射指定的数据项");
            }
        }
    });

Observable.just(1, 2, 3)
    .elementAt(1)
    .subscribe(new Consumer<Integer>() {
        @Override
        public void accept(Integer value) throws Exception {
            System.out.println("Observable发射的指定位置的数据项为：" + value);
        }
    }, new Consumer<Throwable>() {
        @Override
        public void accept(Throwable throwable) throws Exception {
            System.out.println("Observable没有发射指定位置的数据项");
        }
    });