Android Fragment Manager Null Object Reference

Issue

@JavascriptInterface
public void switchView() {
    //sync the BottomBar Icons since a different Thread is running
    Handler refresh = new Handler(Looper.getMainLooper());
    refresh.post(new Runnable() {
        public void run()
        {
            MapFragment mapFragment = new MapFragment();
            FragmentManager fragmentManager = ((MainActivity) mContext).getFragmentManager();
            FragmentTransaction transaction = fragmentManager.beginTransaction();
            transaction.replace(R.id.content, mapFragment);
            transaction.commit();
        }
    });
}

When i run this code everything is fine, but when i add the line

mapFragment.setUrl("www.examplestuff.com");

the app crashes with Attempt to invoke virtual method 'void android.webkit.WebView.loadUrl(java.lang.String)' on a null object reference

My Fragment class looks like this

public WebView mapView;
private String thisURL;

public void setUrl(String url) {
    thisURL = url;
    mapView.loadUrl(thisURL);
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    View view = inflater.inflate(R.layout.fragment_map,container, false);

    mapView = (WebView)view.findViewById(R.id.mapView);
    this.setUrl("file:///android_asset/www/MapView.html");

    mapView.setWebViewClient(new WebViewClient());

    WebSettings webSettings = mapView.getSettings();
    webSettings.setJavaScriptEnabled(true);
    //allow cross origin - like jsonP
    webSettings.setAllowUniversalAccessFromFileURLs(true);

    return view;
}

Also call there the method this.setURL() and works fine.

What I am doing wrong? Has the FragmentManager no access of the instance WebView of the fragment???

Solution

This be because when you call setUrl it invokes this method:

public void setUrl(String url) {
    thisURL = url;
    mapView.loadUrl(thisURL);
}

the line mapView.loadUrl(thisURL); accesses the mapView. However you are likely calling setUrl before the Android system has called onCreateView, therefore mapView is null, causing said crash.

    public void setUrl(String url) {
        thisURL = url;
        if(mapView != null) {
            mapView.loadUrl(thisURL);
        } 
    }

and

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

     View view = inflater.inflate(R.layout.fragment_map,container, false);

     mapView = (WebView)view.findViewById(R.id.mapView);
     if(thisUrl != null) {
        mapView.loadUrl(thisURL);
     }
     ... other code

Then mapFragment.setUrl("www.examplestuff.com"); would work

---

A better solution would be to understand more the Activity & Fragment lifecycles and not call setUrl when the Fragment is in an invalid state :-) You are probably calling setUrl when really you should be passing the Url as an intent extra when the fragment is created. https://developer.android.com/training/basics/fragments/communicating.html

Answered By - Blundell