Getting null when trying to access variables from another class, in Java

Question

I have two classes and I want to pass a (geo-point) variable on touch from one to the other, so that I can calculate the distance between user current location and the geo-point.

Long story short:

My first class is named Session and the second Calculation - they are in the same package.

I declare the variables as public in the main class. On touch, I save the users current location into the (public) geo-point variable called userposition .

Then in the second class I use

Session pass = new Session();

to create an instance of the Session. After which I try to retrieve the userposition variable:

Geopoint position = pass.userposition;

But it keeps returning null and throws exception even if I tested the variable on the main class and works great ... What am I missing here??

The main code for Session is:

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.maps);
    // Blah blah blah

    // And here i declare the GestureDetector...I working with google maps.
    Gest = new GestureDetector(new SimpleOnGestureListener());

    mapView.getOverlays().add(new Overlay() {
        @Override
        public boolean onTouchEvent(MotionEvent e, MapView mapView) {
            Gest.onTouchEvent(e);
            return super.onTouchEvent(e, mapView);
        }
    });

    Gest.setOnDoubleTapListener(new OnDoubleTapListener() {

        @Override
        public boolean onDoubleTap(MotionEvent e) {
            // and in here i set the variable which works fine i tested it
            // in a textView
            userposition = myLocationOverlay.getMyLocation();
        }
    });
}

And for my Calculation class:

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

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

@Override
protected OverlayItem createItem(int i) {
    // TODO Auto-generated method stub
    return mOverlays.get(i);
}

@Override
public int size() {
    // TODO Auto-generated method stub
    return mOverlays.size();
}

@Override
public boolean onTap(int index) {
    // and here i try to get the variable by taping a marker on map (its a
    // lot of code no need to post works fine too)

    Session pass = new Session();
    Geopoint position = pass.userposition;
}

And then I get the null pointer exception.

What I noticed (I'm not so good at Java so I will try to explain) is that I can pass a variable like an integer (I tried it) from the main class Session. The thing is that to pass the geo-point, I need to pass the variable into the Gesture Detection onDoubleTap method inside the Session ... something like pass.onDoubleTap.userposition because that is the variable I need, and in that class the variable is not null. But how I go about doing that??

Answer 1

我通过声明变量static来解决它，以便它可以在包中的所有类之间共享。