Context null Pointer

Question

I've been following this tutorial: http://developer.android.com/resources/tutorials/views/hello-mapview.html but in onTap mContext is throwing a NullPointerException.. anyone know why? Here's my code..

    public class Mapitems extends ItemizedOverlay{
Context mContext;

private ArrayList<OverlayItem> mOverlays = new ArrayList<OverlayItem>();

public Mapitems(Drawable defaultMarker) {
      super(boundCenterBottom(defaultMarker));
    }

public Mapitems(Drawable defaultMarker, Context context) {
      super(defaultMarker);
      mContext = context;
    }
@Override
protected OverlayItem createItem(int i) {
    return mOverlays.get(i);
}

public void addOverlay(OverlayItem overlay) {
    mOverlays.add(overlay);
    populate();
}

@Override
protected boolean onTap(int index) {
  OverlayItem item = mOverlays.get(index);
  AlertDialog.Builder dialog = new AlertDialog.Builder(mContext);
  dialog.setTitle(item.getTitle());
  dialog.setMessage(item.getSnippet());
  dialog.show();
  return true;
}

@Override
public int size() {
    return mOverlays.size(); 
}

  }

//edit: I'm still having problems with this. Bounty is for anyone who can give me an explanation as to why I'm getting this sort of error and how would I correct it?

//edit2: It seems past answer allows me to click the item but doesnt show its icon in the mapview.. anyone know why??

Answer 1

There is no reference set when you are using constructor with one parameter, and method AlertDialog.Buidler(/*param*/) need not null value.

Search in your code where You initialize the object of class Mapitems.

Please add link to "this" tutorial

Answer 2

To not get a NPE, your client code will need to:

• use the 2 argument constructor only, that is, the one taking in a Context
• call the constructor with a non-null Context. If passing this from an activity, make sure the constructor call occurs within the onCreate() method or later in the activity lifecycle. That means you can't directly initialize a Mapitems object as a field of an activity, for example.

I took a look at the tutorial you're referencing and indeed they forgot to tell you to call the constructor with the context. In HelloItemizedOverlay.java, this:

HelloItemizedOverlay itemizedoverlay = new HelloItemizedOverlay(drawable);

really should be:

HelloItemizedOverlay itemizedoverlay = new HelloItemizedOverlay(drawable, this);

Change the relevant reference in your code (where you are instantiating Mapitems) and it should work.

Answer 3

Looking at your code, you probably call the simple constructor

public Mapitems(Drawable defaultMarker)

This constructor does not set the mContext and that's why you get a NullPointerException.
Adding a line like mContext = new Context() or mContext = android.content.getApplicationContext() might solve the problem.

It is also possible that a null argument is supplied to the other constructor

public Mapitems(Drawable defaultMarker, Context context)

Inserting a null check when assigning mContext and if necessary providing a default context may then solve the problem.

The constructors would look like this:

public Mapitems(Drawable defaultMarker) {
    super(boundCenterBottom(defaultMarker));
    mContext = android.content.getApplicationContext();
    // or: mContext = new Context();
}

public Mapitems(Drawable defaultMarker, Context context) {
    super(defaultMarker);
    if(context==null)
        mContext = android.content.getApplicationContext();
        // or: mContext = new Context();
    mContext = context;
}

Hope this solves your problem.

Answer 4

as to the image not showing, after changing the call to set the context correctly

HelloItemizedOverlay itemizedoverlay = new HelloItemizedOverlay(drawable, this);

the called constructor does not treat the image in the same way as the single parameter constructor. Changing the 2 parameter constructor to wrap the drawable with boundCenterBottom now works for me

    public HelloItemizedOverlay(Drawable defaultMarker, Context context){
    super(boundCenterBottom(defaultMarker));
    mContext = context;
}