为什么最终变量没有内部类就返回？

Question

I want to return final ArrayList in Android, but the ArrayList always return size 0. I do add() in inner class, but the return log faster then inner class log.

My code :

public static ArrayList<SportsData> getSportsFavorite(){
    final ArrayList<SportsData> sportsDatas = new ArrayList<>();

    DBRequestData reqData = new DBRequestData();
    reqData.cmdId = CmdConst.DISCIPLINE_DATA.SELECT_ALL.ordinal();
    DisciplineCmd cmd = new DisciplineCmd();
    cmd.requestCmd(reqData, new OnDatabaseListener() {
        @Override
        public void onDBResponse(BaseCmd command) {
            if (command == null || command.getResponseData() == null) {
                return;
            }

            ArrayList<DisciplineTable> dbDisciplineList = command.getResponseData().disciplineTableList;
            if (dbDisciplineList == null) {
                return;
            }

            if(sportsFavoriteList.size() == 0) {
                init();
            }

            for (DisciplineTable disciplineData : dbDisciplineList){
                for (String favoriteCode : sportsFavoriteList){
                    String[] codeArr = favoriteCode.split("[|]");
                    String discCode = codeArr[0];
                    String compeCode = codeArr[1];

                    if(disciplineData.disciplineCode.equals(discCode) && disciplineData.competitionCode.equals(compeCode)){
                        int imgRes = SportsUtil.getSportsImgResource(disciplineData.disciplineCode, disciplineData.competitionCode);
                        sportsDatas.add(new SportsData(imgRes, disciplineData));
                        Log.e("SportsUtil", "inner Size : "+sportsDatas.size()); // RETURN SECOND -
                    }
                }
            }
        }
    });

    Log.e("SportsUtil", "Return Size : "+sportsDatas.size()); //RETURN FIRST
    return sportsDatas;
}

Logcat :

07-22 14:33:32.337      651-651/com.samsung.riowow.result E/SportsUtil﹕ Return Size : 0
07-22 14:33:32.347      651-651/com.samsung.riowow.result E/SportsUtil﹕ inner Size : 1
07-22 14:33:32.347      651-651/com.samsung.riowow.result E/SportsUtil﹕ inner Size : 2
07-22 14:33:32.347      651-651/com.samsung.riowow.result E/SportsUtil﹕ inner Size : 3
07-22 14:33:32.347      651-651/com.samsung.riowow.result E/SportsUtil﹕ inner Size : 4
07-22 14:33:32.347      651-651/com.samsung.riowow.result E/SportsUtil﹕ inner Size : 5

The result that want to is ArrayList what size is 5. But always return ArrayList 's size is 0. Please help me :) Thanks.

SOLVE

If you want to know reason of this situation, please check the Florian Schaetz's answer.

My solution for this, I change the return type of method and add interface listener parameter. Please check my solution.

First, Create interface and method has parameter you want to return.

public static interface SportsDataListener{
    public void onSportsDataResponse(ArrayList<SportsData> sportsDataList);
}

Second, Change method's return type to void and add interface parameter. And call interface's method instead return

public static void getSportsFavorite(final SportsDataListener listener){
    final ArrayList<SportsData> sportsDatas = new ArrayList<>();
    DBRequestData reqData = new DBRequestData();
    reqData.cmdId = CmdConst.DISCIPLINE_DATA.SELECT_ALL.ordinal();
    DisciplineCmd cmd = new DisciplineCmd();
    cmd.requestCmd(reqData, new OnDatabaseListener() {
        @Override
        public void onDBResponse(BaseCmd command) {
            if (command == null || command.getResponseData() == null) {
                return;
            }

            ArrayList<DisciplineTable> dbDisciplineList = command.getResponseData().disciplineTableList;
            if (dbDisciplineList == null) {
                return;
            }

            if(sportsFavoriteList.size() == 0) {
                init();
            }

            for (DisciplineTable disciplineData : dbDisciplineList){
                for (String favoriteCode : sportsFavoriteList){
                    String[] codeArr = favoriteCode.split("[|]");
                    String discCode = codeArr[0];
                    String compeCode = codeArr[1];

                    if(disciplineData.disciplineCode.equals(discCode) && disciplineData.competitionCode.equals(compeCode)){
                        int imgRes = SportsUtil.getSportsImgResource(disciplineData.disciplineCode, disciplineData.competitionCode);
                        sportsDatas.add(new SportsData(imgRes, disciplineData));
                    }
                }
            }
            listener.onSportsDataResponse(sportsDatas);
        }
    });
}

Last, You can use

SportsUtil.getSportsFavorite(new SportsUtil.SportsDataListener() {
    @Override
    public void onSportsDataResponse(ArrayList<SportsData> sportsDataList) {
        //Some action you want to do. You can use parameter
    }
}

Answer 1

Seems pretty asynchronous to me. You are calling a method and give it a callback that will be called AFTER the action is done, here probably after the database returns. The main code isn't blocked, but goes one, while the other OnDatabaseListener waits in the background for an answer, thus your method returns the original list which then suddenly grows as the database answer returns.

Answer 2

I guess your onDBResponse method is running in different thread. You are modifying the arraylist inside that method. So before modifying the arraylist is getting returned in this case. Always it will give you arraylist of size 0.

Answer 3

您应该为getSportsFavorite()提供一个回调接口，该接口将数组列表作为参数，并在OnDatabaseListener方法完成时调用它。

Answer 4

Yes the problem is because your onDBResponse() is called after the method has returned the array list. You should suspect this from the order of your logs.

SOLUTION

Assuming that it is your requirement that you get the array list synchronously. Just before your cmd.requestCmd do:

final Object lock = new Object();

then add this before your return sportsDatas; :

synchronized(lock) {
    lock.wait();
}

and in the end of onDBResponse() add:

synchronized(lock) {
    lock.notify();
}

Essentially, now your method will wait until the response from DB has come before returning the array list.

You will need to catch the InterruptedException and that's it! Hope it helps! :)

Answer 5

you must run onDBResponse once at least.

public static void main(String[] args) {
    final List<String> sportsDatas = new ArrayList<>();
    Commond commond = new Commond() {
        @Override
        public void onDBResponse() {
            sportsDatas.add("1");
            sportsDatas.add("2");
            sportsDatas.add("3");
        }
    };
    System.out.println(sportsDatas.size());

    commond.onDBResponse();

    System.out.println(sportsDatas.size());
    }
}

output:

0
3