HashMap: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.util.HashMap.put'

Question

I've initialized HashMap#onCreate method and i want to use HashMap in addLocation method. Everything looks okay but I don't understand why I get this error:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.util.HashMap.put(java.lang.Object, java.lang.Object)' on a null object reference

when I call locationList#put method.

Here's my code:

public class InitLocationsOnStartup extends Application implements LocationListener {
    HashMap < String, Double[] > locationList;
    List < StationViewModel > stations;

    @Override
    public void onCreate() {
        super.onCreate();
        stations = new ArrayList < StationViewModel > ();
        locationList = new HashMap < > (); // 
    }

    public void getStations(List < StationViewModel > list) {
        this.stations = list;
        addLocation(locationList);
    }

    public HashMap < String, Double[] > addLocation(HashMap < String, Double[] > locationList) {
        for (int i = 0; i < stations.size(); i++) {
            latLong = new Double[] {
                Double.parseDouble(stations.get(i).getLatitude()),
                    Double.parseDouble(stations.get(i).getLongitude())
            };

            if (stations.get(i).getStationName() != null && latLong != null) {
                locationList.put(stations.get(i).getStationName(), latLong);

            }
        }
        return locationList;;
    }

have you tried debugging? It seems like your locationList is null for some reason (even if you did initialize it in onCreate()) — Cargeh, Apr 28 '18 at 10:47
The whole mistake is about the top.But i add more code to addLocation() @Pavneet_Singh — more, Apr 28 '18 at 10:49
Does that make sense? `if(locationList==nul){locationList = new HashMap<>();}` — more, Apr 28 '18 at 10:59
@more why are you not doing this `HashMap locationList = new HashMap<>()` after the class declaration ? — Lokesh Pandey, Apr 28 '18 at 11:01
I did and problem solved.But i got **same error** at another method in this class.I think i have different problem. @Lokesh — more, Apr 28 '18 at 11:04
@more In that case it's being used some where else. Can you include that function where it's being used — Lokesh Pandey, Apr 28 '18 at 11:07
@Lokesh he wants to do lazy initialization which is the correct way of initialization, IMHO. — Nishit, Apr 28 '18 at 12:16
Since the HashMap locationList is a field level variable, you don't need to explicitly pass it to the method addLocation. — Nishit, Apr 28 '18 at 12:18

gil.fernandes · Accepted Answer · 2018-04-28T11:51:21.740

1

In this case the addLocation is being called before the onCreate() method is executed.

I guess you are accessing an instance of InitLocationsOnStartup which is not managed by the Android framework.

If you have code like:

InitLocationsOnStartup startup = new InitLocationsOnStartup();
startup.addLocation(..., ...);

You are going to get this NullPointerException.

The best way to access this class should be in Android activities using the

getApplication()

method. You could create a static factory method to access this object in a safe way, like e.g:

public static InitLocationsOnStartup getInitLocationsOnStartup(Activity activity) {
    return (InitLocationsOnStartup) activity.getApplication();
}

This should get you the right instance of the InitLocationsOnStartup class which has been correctly initialized by Android.

This method could then be used from Android activities like this:

InitLocationsOnStartup startup = getInitLocationsOnStartup(this);

edited Apr 28 '18 at 11:51

answered Apr 28 '18 at 11:04

gil.fernandes

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Thanks for your answer.I want use in Runnable class.So what i need change this code? Maybe like getInitLocationsOnStartup(Runnable runnable) – more Apr 28 '18 at 11:11
@more You will always need to have a reference to an `android.app.Activity` or `android.app.Service` to be able to call the `getApplication()` method. `Runnable` will not help you, because it is neither an Activity or Service via which you can get the right instance of `InitLocationsOnStartup`. Yet, if the Runnable you implement has a reference to an `android.app.Activity` or `android.app.Service` then you can use that reference to call `getApplication()`. – gil.fernandes Apr 28 '18 at 11:18
@more in other words: make sure that the `Runnable` object you create has a reference to an Android activity or Android service, so that you can call the `getApplication()` method from either of these. – gil.fernandes Apr 28 '18 at 11:19
Thanks for everything! – more Apr 28 '18 at 11:45
@more You are most welcome :) – gil.fernandes Apr 28 '18 at 11:50

HashMap: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.util.HashMap.put'

1 Answers1