RxJavaOperatorExample.java

/* Here first of all, we get the list of users from server.
* Then for each userId from user, it makes the network call to get the detail 
* of that user. 
* Finally, we get the userDetail for the corresponding user one by one
*/

/*
* This observable return the list of users.
*/
private Observable<List<User>> getUserListObservable() {
return RxAndroidNetworking.get("https://fierce-cove-29863.herokuapp.com/getAllUsers/{pageNumber}")
        .addPathParameter("pageNumber", "0")
        .addQueryParameter("limit", "10")
        .build()
        .getObjectListObservable(User.class);
}

/*
* This observable return the userDetail corresponding to the user.
*/
private Observable<UserDetail> getUserDetailObservable(long userId) {
return RxAndroidNetworking.get("https://fierce-cove-29863.herokuapp.com/getAnUserDetail/{userId}")
        .addPathParameter("userId", String.valueOf(userId))
        .build()
        .getObjectObservable(UserDetail.class);
}

/*
* This method do the magic - first gets the list of users
* from server.Then, for each user, it makes the network call to get the detail 
* of that user.
* Finally, we get the UserDetail for the corresponding user one by one
*/
public void flatMap() {
    getUserListObservable()
            .flatMap(new Func1<List<User>, Observable<User>>() { // flatMap - to return users one by one
                @Override
                public Observable<User> call(List<User> usersList) {
                    return Observable.from(usersList); // returning user one by one from usersList.
                }
            })
            .flatMap(new Func1<User, Observable<UserDetail>>() {
                @Override
                public Observable<UserDetail> call(User user) {
                    // here we get the user one by one
                    // and returns corresponding getUserDetailObservable
                    // for that userId
                    return getUserDetailObservable(user.id);
                }
            })
            .subscribeOn(Schedulers.newThread())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(new Observer<UserDetail>() {
                @Override
                public void onCompleted() {
                    // do something onCompleted
                }

                @Override
                public void onError(Throwable e) {
                    // handle error
                }

                @Override
                public void onNext(UserDetail userDetail) {
                    // here we get userDetail one by one for all users
                    Log.d(TAG, "userDetail id : " + userDetail.id);
                    Log.d(TAG, "userDetail firstname : " + userDetail.firstname);
                    Log.d(TAG, "userDetail lastname : " + userDetail.lastname);
                }
            });
}