java - Android调用数组列表导致程序崩溃

Question

我已经编写了一些代码，在一个类中我创建并填充了一个数组列表，我可以从另一个类调用并打印它，但是当我尝试从第三个类调用它时，会导致移动应用程序关闭。这是数组列表的代码:

public class MyPositionOverlay extends ItemizedOverlay<OverlayItem> {

    public ArrayList<OverlayItem> overlayItemList = new ArrayList<OverlayItem>();
    Context context;


    public MyPositionOverlay(Drawable marker, Context c) {
        super(boundCenterBottom(marker));
        // TODO Auto-generated constructor stub
        populate();
        context = c;


    }
    private final int mRadius = 5;

    Location location;

    public Location getLocation() {
        return location;
    }
    public void setLocation(Location location) {;
    this.location = location;
    }

    //Method for user intputs - fill in
    @Override
    public boolean onTap(GeoPoint point, MapView mapView) {

        if(super.onTap(point, mapView)){
            return true;
        }
        String title = "Point:" + String.valueOf(overlayItemList.size()+ 1) + "\n" +
                "Lat:" + "\n" + String.valueOf(point.getLatitudeE6() + 
                        "Long:" + "\n" + String.valueOf(point.getLongitudeE6()));
        String snippet = "geo:\n"
                + String.valueOf(point.getLatitudeE6()) + "\n"
                + String.valueOf(point.getLongitudeE6());

        addItem(point, title, snippet);



        //return true;
        //if(hit test is true)
        //  perform task
        // return true;
        //if not handled

        return false;
    }

    @Override
    protected boolean onTap(int index) {
        // TODO Auto-generated method stub
        //return super.onTap(index);

        Toast.makeText(context,
                "Touch on marker: \n" + overlayItemList.get(index).getTitle(),
                Toast.LENGTH_LONG).show();

        return true;
    }

    public void addItem(GeoPoint point, String title, String snippet){
        OverlayItem newItem = new OverlayItem(point, title, snippet);
        overlayItemList.add(newItem);
        populate();
    }



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

我只是尝试从下面的类中调用它:

public class GeneticAlgorithm2  {

    //public int populationSize;
    //private MyPositionOverlay popList;

    MyPositionOverlay popSize;
    MyPositionOverlay positionOverlay;
    HomeScreen homeScreenPop;
    //MyPositionOverlay overlaySize = new MyPositionOverlay(null, null);

    //int populationSize2 = popSize.size();

    int populationSize = 800;
    public int route[] = new int[populationSize +1];
    int pointLimit;
    int iterations;
    double mutationRate = 0.10;

    //Canvas C = null;
    //Dimension CD;
    //Label L;
    //Graphics GC;
    //int Initial_population = 800;


    int matingPopulation = populationSize/2;
    int favPopulation = matingPopulation/2;
    int numOfCities = 30;//populationSize2;
    //int numOfCities = popSize.overlayItemList.size();
    int cutLength = numOfCities/5;
    //double Mutation_probability = 0.10;

    double loops=1.5;
    int ePoch;
    double xLow = 0.0;
    double yLow = 0.0;
    double xHigh = 100.0;
    double yHigh = 100.0;
    double xRange, yRange;
    double minCost = 5.0;
    double timeStart,timeEnd;
    Thread T = null;
    double costPerfect = 0.;

    boolean started = false;

    City [] cities;
    Chromosome [] chromosomes;



/*
    public GeneticAlgorithm2() {

        // Calling My Position Overlay class
        //popList = new MyPositionOverlay(null, null);

        //Sets the population size to the number of markers
        //populationSize = popSize.overlayItemList.size();
        //start();
        //test();

        //int populationSize3 = popSize.size();

    }*/



    public void test(){
        System.out.println("test");
    }

    public void start() {

        // Calling My Position Overlay class
        //popSize = new MyPositionOverlay(null, null);

        //Sets the population size to the number of markers
        //populationSize = popSize.overlayItemList.size();

        //popSize.size();

        //int populationSize2 = popSize.size();
        //int populationSize3 = positionOverlay.size();



        System.out.println("Start ...");
        // generate a set of cities

        System.out.println(positionOverlay.size());
        //System.out.println( homeScreenPop.positionOverlay.size());

}
}

这是我收到的错误消息:

06-06 17:14:05.210: W/dalvikvm(1301): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
06-06 17:14:05.220: E/AndroidRuntime(1301): FATAL EXCEPTION: main
06-06 17:14:05.220: E/AndroidRuntime(1301): java.lang.NullPointerException
06-06 17:14:05.220: E/AndroidRuntime(1301):     at android.Maps.GeneticAlgorithm2.start(GeneticAlgorithm2.java:97)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at android.Maps.HomeScreen$3.onClick(HomeScreen.java:152)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at android.view.View.performClick(View.java:2408)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at android.view.View$PerformClick.run(View.java:8816)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at android.os.Handler.handleCallback(Handler.java:587)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at android.os.Handler.dispatchMessage(Handler.java:92)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at android.os.Looper.loop(Looper.java:123)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at android.app.ActivityThread.main(ActivityThread.java:4627)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at java.lang.reflect.Method.invokeNative(Native Method)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at java.lang.reflect.Method.invoke(Method.java:521)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
06-06 17:14:05.220: E/AndroidRuntime(1301):     at dalvik.system.NativeStart.main(Native Method)

据我所知，它说数组列表为空，但就像我说的，我可以愉快地从另一个类输出它。

请帮忙。

Answer 1

在您的 start 方法中，positionOverlay 为 null，您刚刚声明了它，但在访问此字段之前尚未初始化它，因此是 NPE。