Android、Google Maps API、JSON 问题。获取空指针异常

Question

我正在开发一个小型人员应用程序来帮助我学习 Android 开发和 Java。

首先让我让大家知道 Google 地图部分正在应用中运行。它在应用启动并获取手机的正确当前位置时加载。

另外，我的 JSON 文件也可以正常工作。数据是使用他们的 API 从 www.wunderground.com 提取的，我的代码抓取了我所要求的内容并创建了一个 Arraylist 并很好地填充它并在应用程序中按应有的方式显示它。

问题是，我正在尝试获取一些 JSON 数据，并创建一个谷歌地图标记对象并将其显示在地图上。

下面是我的代码：

这个方法是从它从 JsonData.java 获得的 JSON 数据创建一个 Cyclone 对象，我将在下面展示。

Cyclone.java

public class Cyclone {

// Category of the cyclone (e.g. -2, -1, 0, 1, 2, 3, 4, or 5)
private int category;

// Latitude & Longitude of Cyclone
private float latitude, longitude;

// Cyclone name
private String name;

// Cyclone heading (N, S, E, or W)
private String heading;

// Wind speed in Cyclone
private String windSpeedKnots;

// Url to website with Cyclone data
private String url;

/**
* CONSTRUCTOR
* Create a new Cyclone object.
*
* @param constCategory is the category of the cyclone (e.g. -2, -1, 0, 1, 2, 3, 4, or 5)
* @param constLatitude is the Latitude of the cyclone
* @param constLongitude is the Longitude of the cyclone
* @param constName is the name of the Cyclone
* @param constHeading is the direction the cyclone is moving
* @param constWindSpeedKnots is the speed of the wind the Cyclone is producing
* @param constUrl is the url of the website that is providing the data
*/
public Cyclone(int constCategory, float constLatitude, float constLongitude,String constName,
               String constHeading, String constWindSpeedKnots, String constUrl) {

    category = constCategory;
    latitude = constLatitude;
    longitude = constLongitude;
    name = constName;
    heading = constHeading;
    windSpeedKnots = constWindSpeedKnots;
    url = constUrl;
}

/**
 * Setting public getters for the private variables above for use of other classes
 */
// Get the storm category
public int getCategory() {
    return category;
}

// Get the cyclones Latitude
public float getLatitude() {
    return latitude;
}

//Get the cyclones Longitude
public float getLongitude() {
    return longitude;
}

// Get the cyclones name
public String getName() {
    return name;
}

// Get the cyclones heading
public String getHeading() {
    return heading;
}

// Get the cyclones wind speed
public String getWindSpeedKnots() {
    return windSpeedKnots;
}

// Get the Url
public String getUrl() {
    return url;
}

}

此类从 www.wunderground.com 提取 JSON 数据。我已经从底部切断了一堆代码。如果没有返回，它主要只是处理异常。 JsonData.java

/**
 * Return a list of {@link Cyclone} objects that has been built up from
 * parsing a JSON response.
 */
public static ArrayList<Cyclone> extractFeatureFromJson(String cycloneJSON) {

    // Create an empty ArrayList to start adding Cyclones to
    ArrayList<Cyclone> cyclones = new ArrayList<>();

    // Try to parse the cycloneJSON response string. If there's a problem with the way the JSON
    // is formatted, a JSONException exception object will be thrown.
    // Catch the exception , and print the error message to the logs.

    try {

        JSONObject rootJsonObject = new JSONObject(cycloneJSON);

        // Create JSONArray associated with the key called "currenthurricane", which represents
        // a list of cyclones from JSON response.
        JSONArray currentHurricaneArray = rootJsonObject.getJSONArray("currenthurricane");

        /**
         * Loop through each section in the currentHurricaneArray array & create an
         * {@link Cyclone} object for each one
         *
         */
        for (int i = 0; i < currentHurricaneArray.length(); i++) {
            //Get cyclone JSONObject at position i in the array
            JSONObject cycloneProperties = currentHurricaneArray.getJSONObject(i);

            // Extract "stormInfo" object
            JSONObject stormInfo = cycloneProperties.optJSONObject("stormInfo");
            //Extract “stormName_Nice” & "requesturl" for Cyclone's name and url
            String name = stormInfo.optString("stormName_Nice");
            String url = stormInfo.optString("wuiurl");

            // Extract "Current" object
            JSONObject current = cycloneProperties.optJSONObject("Current");
            // Extract "SaffirSimpsonCategory" key
            int category = current.optInt("SaffirSimpsonCategory");

            //Extract "Lat" & "Lon" keys to get cyclone position
            float latitude = (float) current.optDouble("lat");
            float longitude = (float) current.optDouble("lon");

            //Extract "WindSpeed" object
            JSONObject windSpeed = current.optJSONObject("WindSpeed");
            //Extract wind speed in "Knots" Key
            int Kts = windSpeed.optInt("Kts");
            String knots = Kts + " Knots";

            //Extract "Movement" object
            JSONObject movement = current.optJSONObject("Movement");
            //Extract movement direction in "Text" key
            String direction = movement.optString("Text");

            Cyclone cyclone = new Cyclone(category, latitude, longitude, name, direction, knots, url);
            //Add new cyclone to list
            cyclones.add(cyclone);
        }

我将在这里展示相关的部分。

MainActivity.java

public class MainActivity extends AppCompatActivity implements
    LoaderManager.LoaderCallbacks<List<Cyclone>>,
    OnMapReadyCallback,
    GoogleApiClient.ConnectionCallbacks,
    GoogleApiClient.OnConnectionFailedListener,
    LocationListener {

....

//Creating a Cyclone object, calling it from Cyclone.java
    Cyclone trackedCyclone = null;

    //getting the three bits of info I need/want (Name, lat, lon)
    String name = trackedCyclone.getName();
    float lat = trackedCyclone.getLatitude();
    float lon = trackedCyclone.getLongitude();

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.cyclone_list);
....

// Obtain the SupportMapFragment and get notified when the map is ready
    //Calling up the map fragment from cyclone_list.xml
    MapFragment mapFragment = (MapFragment) getFragmentManager().findFragmentById(R.id.google_map);
    mapFragment.getMapAsync(this);

....

@Override
public void onMapReady(GoogleMap mapLocalInstance) {
    //Setting mapReady to true
    mapReady = true;

    //displaying cyclone map marker on the map
    LatLng cycloneMarker = new LatLng(lat, lon);
    googleMap.addMarker(new MarkerOptions().position(cycloneMarker).title(name));

    //Loading local instance map from Callback
    googleMap = mapLocalInstance;
    googleMap.setMapType(GoogleMap.MAP_TYPE_SATELLITE);

    //checks for permission using the Support library before enabling the My Location layer
    //Initialize Google Play Services
    if (android.os.Build.VERSION.SDK_INT >= M) {
        if (ContextCompat.checkSelfPermission(this,
                Manifest.permission.ACCESS_FINE_LOCATION)
                == PackageManager.PERMISSION_GRANTED) {
            buildGoogleApiClient();
            googleMap.setMyLocationEnabled(true);
        }
    } else {
        buildGoogleApiClient();
        googleMap.setMyLocationEnabled(true);
    }
}

当应用程序启动时，它会崩溃并显示以下内容：

04-17 16:11:29.546 7028-7028/com.palarran.cycloops E/AndroidRuntime: 致命异常: main 进程：com.palarran.cycloops，PID：7028 java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“java.lang.String com.palarran.cycloops.Cyclone.getName()” 在 com.palarran.cycloops.MainActivity.onMapReady(MainActivity.java:266) 在 com.google.android.gms.maps.MapFragment$zza$1.zza（未知来源） 在 com.google.android.gms.maps.internal.zzt$zza.onTransact（未知来源） 在 android.os.Binder.transact(Binder.java:499) 在 com.google.android.gms.maps.internal.bw.a(:com.google.android.gms.DynamiteModulesB:82) 在 com.google.maps.api.android.lib6.impl.bf.run(:com.google.android.gms.DynamiteModulesB:1805) 在 android.os.Handler.handleCallback(Handler.java:751) 在 android.os.Handler.dispatchMessage(Handler.java:95) 在 android.os.Looper.loop(Looper.java:154) 在 android.app.ActivityThread.main(ActivityThread.java:6121) 在 java.lang.reflect.Method.invoke（本机方法） 在 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)

我只在 Android 和 Java 上玩了大约一年。所以OOP对我来说还是很新的。我不明白很多。我有一半认为问题在于，由于 JsonData.java 类正在创建 JsonArray，我可能无法使用 Cyclone.java 调用我需要的部分> 类。如果是这种情况，那么我要创建不同的 JsonArray 吗？这对我来说似乎很笨拙？但我真的不知道。

感谢您提供的任何帮助。

Answer 1

//Creating a Cyclone object, calling it from Cyclone.java
Cyclone trackedCyclone = null;

// Here is a problem. You need to init your trackedCyclone
trackedCyclone = new Cyclone(??????????);

//getting the three bits of info I need/want (Name, lat, lon)
String name = trackedCyclone.getName();
float lat = trackedCyclone.getLatitude();
float lon = trackedCyclone.getLongitude();