如何在方法外部访问String

Question

I'm trying to access a string outside the parse query, it's not working outside query but it works inside the query. I declared the global variable and and tried to access it but it's still not working, instead of string it returns null.

I'm new to Java and Android so please forgive me if it's stupid question.

Any help would be appreciated.

public class ActivityQuiz extends AppCompatActivity {

ArrayList<String> answerList = new ArrayList<>();

int c=0;

String Answer = "correct";
String selectedOption;
public static String  checkAns;


@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_quiz);


    ParseQuery<ParseObject> query = ParseQuery.getQuery("Question");
    query.findInBackground(new FindCallback<ParseObject>() {
        @Override
        public void done(List<ParseObject> questions, ParseException e) {
            if (e == null) {
                for (ParseObject question : questions) {                        
                    answerList.add(question.getString(Answer));
                     checkAns = getanswerList.get(c);
                     Log.d("String check",""+checkAns); //here it's working
                }

            } else {
                e.printStackTrace();
            }
        }

    });

    try{

    String checkanswer = checkAns ; // here returns null instead of string
    Log.d("String checkans",""+checkanswer);}
    catch (Exception e)
    {
        e.printStackTrace();
        Log.d("Error", "Error, Not Working");
    }
    }

Answer 1

There is very simple problem. In first instance you are getting the value of checkAns from the data after hitting the API (in background). So if data come from API it will be non-null.

In later part, since the API response takes time, try-catch block will be executed before it sets value in checkAns So checkAns is equal to initialised value which is null .

Ideally, you should use the value of checkAns only inside done() method.

Answer 2

add your try-catch code inside ParseQuery like this:

ParseQuery<ParseObject> query = ParseQuery.getQuery("Question");
        query.findInBackground(new FindCallback<ParseObject>() {
            @Override
            public void done(List<ParseObject> questions, ParseException e) {
                if (e == null) {
                    for (ParseObject question : questions) {                        
                        answerList.add(question.getString(Answer));
                         checkAns = getanswerList.get(c);
                         Log.d("String check",""+checkAns); //here it's working

                         // ....Add your try catch code.....
                         try{
                              String checkanswer = checkAns ; // also working well
                              Log.d("String checkans",""+checkanswer);}
                              catch (Exception e)
                              {
                                  e.printStackTrace();
                                  Log.d("Error", "Error, Not Working");
                              }
                           }

                } else {
                    e.printStackTrace();
                }
            }

        });

Answer 3

That is normal behavior for asynchronous functions.

By the time your try/catch block is executed checkanswer is still null because your query takes time to execute. Once done() is executed you'll have your string.

If you must have your value in that block, you can sleep a second or two and check if checkanswer is no longer null. Be careful with sleeps as they will hang your program if you do use them in your main thread

Answer 4

The Problem is that your try catch block will execute befor your query is done means the overriden done method will call and set the checkAns after you execute the try catch block.
If you will use the findInBackground method than make a method in your done method in which you will do all your stuff like:

query.findInBackground(new FindCallback<ParseObject>() {
    @Override
    public void done(List<ParseObject> questions, ParseException e) {
      if (e == null) {
        successful(questions);

      } else {
        e.printStackTrace();
      }
    }

  });

  private void successful(List<ParseObject> questions){
    for (ParseObject question : questions) {
      answerList.add(question.getString(Answer));
      Log.d("String checkans",""+checkanswer);
    }
    String checkanswer = getanswerList.get(c);
  }

else you can use the find() method:

List<ParseObject> questions = query.find();

    for (ParseObject question : questions) {
      answerList.add(question.getString(Answer));
      Log.d("String checkans",""+checkanswer);
    }
    String checkanswer = getanswerList.get(c);