Android調用數組列表會導致程序崩潰

Question

我編寫了一些代碼，並在一個類中創建並填充了一個數組列表，並且我可以從另一個類中調用並打印出來，但是當我嘗試調用它時從第三課開始就導致移動應用程序關閉。下面是數組列表代碼：

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(); 
    } 
} 

，我只是試圖從下面的類稱之爲：

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()); 

} 
} 

以下是錯誤消息我得到：

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) 

從我可以看到它是說數組列表爲空，但就像我說我可以愉快地從另一個類輸出它。

請幫忙。

Answer 1

在您的start方法中，positionOverlay爲空，您剛剛聲明瞭它，但在訪問此字段之前尚未初始化它，因此NPE。