設定後の変数null（アンドロイド開発）

Question

次のコードに問題があります。私はかなり理解していないものがあるようです。私は、クラス変数requestArrayを初期化します。次に、getHelpRequests（）関数内にデータを追加します。しかし、私が他の関数（onMapReady）からデータにアクセスしようとすると、配列は空です。どうしてこれなの？

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback { 

private GoogleMap mMap; 
private List<HelpRequest> requestArray = new ArrayList<HelpRequest>(); 

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

    // Obtain the SupportMapFragment and get notified when the map is ready to be used. 
    SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager() 
      .findFragmentById(R.id.map); 
    mapFragment.getMapAsync(this); 
} 

private void getHelpRequests() { 
    // Instantiate the RequestQueue. 
    RequestQueue queue = Volley.newRequestQueue(this); 
    String url ="***/gethelprequests.php"; 

    // Request a string response from the provided URL. 
    StringRequest stringRequest = new StringRequest(Request.Method.GET, url, 
      new Response.Listener<String>() { 
       @Override 
       public void onResponse(String response) { 

        try { 
         JSONArray jsonArray = new JSONArray(response); 

         for(int i=0 ; i<jsonArray.length() ; i++) { 
          HelpRequest rq = new HelpRequest(); 
          rq.setLogo(Integer.parseInt(jsonArray.getJSONObject(i).get("logo").toString())); 
          rq.setUsername(jsonArray.getJSONObject(i).get("username").toString()); 
          rq.setAge(Integer.parseInt(jsonArray.getJSONObject(i).get("age").toString())); 
          rq.setReputation(Integer.parseInt(jsonArray.getJSONObject(i).get("age").toString())); 
          rq.setDescription(jsonArray.getJSONObject(i).get("description").toString()); 
          rq.setLat(Float.valueOf(jsonArray.getJSONObject(i).get("lat").toString())); 
          rq.setLng(Float.valueOf(jsonArray.getJSONObject(i).get("lng").toString())); 
          requestArray.add(rq); 

          /*Log.d("Person " + i, " logo: " + rq.getLogo()); 
          Log.d("Person " + i, " username: " + rq.getUsername()); 
          Log.d("Person " + i, " age: " + rq.getAge()); 
          Log.d("Person " + i, " reputation: " + rq.getReputation()); 
          Log.d("Person " + i, " description: " + rq.getDescription()); 
          Log.d("Person " + i, " latitude: " + rq.getLat()); 
          Log.d("Person " + i, " longitude: " + rq.getLng());*/ 

         } 

        } catch (final JSONException e) { 
         Log.e("ERROR: ", "Json parsing error: " + e.getMessage()); 

        } 



       } 
      }, new Response.ErrorListener() { 
     @Override 
     public void onErrorResponse(VolleyError error) { 
      Log.d("ERROR:", error.getMessage()); 
     } 
    }); 
    // Add the request to the RequestQueue. 
    queue.add(stringRequest); 
} 


/** 
* Manipulates the map once available. 
* This callback is triggered when the map is ready to be used. 
* This is where we can add markers or lines, add listeners or move the camera. In this case, 
* we just add a marker near Sydney, Australia. 
* If Google Play services is not installed on the device, the user will be prompted to install 
* it inside the SupportMapFragment. This method will only be triggered once the user has 
* installed Google Play services and returned to the app. 
*/ 
@Override 
public void onMapReady(GoogleMap googleMap) { 

    mMap = googleMap; 
    getHelpRequests(); 

    Log.d("EMPTY:", String.valueOf(requestArray.size())); 

    for(int i=0 ; i<requestArray.size() ; i++) { 
     Log.d("REQ:", requestArray.get(i).getUsername()); 
    } 
    LatLng stpetersburg = new LatLng(27.88, -82.83); 
    Marker burg = mMap.addMarker(new MarkerOptions().position(stpetersburg).title("Marker in St. Petersburg")); 
    burg.showInfoWindow(); 
    mMap.moveCamera(CameraUpdateFactory.newLatLng(stpetersburg)); 


} 

}

EDITは：問題は、誰もがこの問題を修正し、これについての知識を得るために正しい方向に私をヒントでした...など非同期とは何かを持っていると思われるので、私は理解して問題は次回私はこの種の問題にぶつかる？

Answer 1

まず、あなたのクラスでのインターフェイスでリスナーを作成します（または新しいインターフェイスファイルを作成する）：

public interface OnReceivedResponse{ 
      void onSuccessResponse(List<HelpRequest> myArray); 
     } 

がgetHelpRequestsにこのリスナーを追加します。

private void getHelpRequests(OnReceivedResponse mListener) 

あなたの配列の準備ができたらちょうど呼び出しますmListener.onSuccessResponse(requestArray)

getHelRequestsを呼び出すときに、新しいリスナーを作成します。

getHelpRequests(new OnReceivedResponse(){ 

       @Override 
       public void onSuccessResponse(List<HelpRequest> myArray){ 

       //Do whatever you want with myArray received 
       } 

     }); 

ここでは、完全なサンプルコード：

 public class MapsActivity extends FragmentActivity implements OnMapReadyCallback { 

     private GoogleMap mMap; 
     private List<HelpRequest> requestArray = new ArrayList<HelpRequest>(); 

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

      // Obtain the SupportMapFragment and get notified when the map is ready to be used. 
      SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager(OnReceivedResponse mListener) 
        .findFragmentById(R.id.map); 
      mapFragment.getMapAsync(this); 
     } 

     private void getHelpRequests(OnReceivedResponse mListener) { 
      // Instantiate the RequestQueue. 
      RequestQueue queue = Volley.newRequestQueue(this); 
      String url ="***/gethelprequests.php"; 

      // Request a string response from the provided URL. 
      StringRequest stringRequest = new StringRequest(Request.Method.GET, url, 
        new Response.Listener<String>() { 
         @Override 
         public void onResponse(String response) { 

          try { 
           JSONArray jsonArray = new JSONArray(response); 

           for(int i=0 ; i<jsonArray.length() ; i++) { 
            HelpRequest rq = new HelpRequest(); 
            rq.setLogo(Integer.parseInt(jsonArray.getJSONObject(i).get("logo").toString())); 
            rq.setUsername(jsonArray.getJSONObject(i).get("username").toString()); 
            rq.setAge(Integer.parseInt(jsonArray.getJSONObject(i).get("age").toString())); 
            rq.setReputation(Integer.parseInt(jsonArray.getJSONObject(i).get("age").toString())); 
            rq.setDescription(jsonArray.getJSONObject(i).get("description").toString()); 
            rq.setLat(Float.valueOf(jsonArray.getJSONObject(i).get("lat").toString())); 
            rq.setLng(Float.valueOf(jsonArray.getJSONObject(i).get("lng").toString())); 
            requestArray.add(rq); 

            /*Log.d("Person " + i, " logo: " + rq.getLogo()); 
            Log.d("Person " + i, " username: " + rq.getUsername()); 
            Log.d("Person " + i, " age: " + rq.getAge()); 
            Log.d("Person " + i, " reputation: " + rq.getReputation()); 
            Log.d("Person " + i, " description: " + rq.getDescription()); 
            Log.d("Person " + i, " latitude: " + rq.getLat()); 
            Log.d("Person " + i, " longitude: " + rq.getLng());*/ 

           } 
          mListener.onSuccessResponse(requestArray) 

          } catch (final JSONException e) { 
           Log.e("ERROR: ", "Json parsing error: " + e.getMessage()); 

          } 


         } 
        }, new Response.ErrorListener() { 
       @Override 
       public void onErrorResponse(VolleyError error) { 
        Log.d("ERROR:", error.getMessage()); 
       } 
      }); 
      // Add the request to the RequestQueue. 
      queue.add(stringRequest); 
     } 


     /** 
     * Manipulates the map once available. 
     * This callback is triggered when the map is ready to be used. 
     * This is where we can add markers or lines, add listeners or move the camera. In this case, 
     * we just add a marker near Sydney, Australia. 
     * If Google Play services is not installed on the device, the user will be prompted to install 
     * it inside the SupportMapFragment. This method will only be triggered once the user has 
     * installed Google Play services and returned to the app. 
     */ 
     @Override 
     public void onMapReady(GoogleMap googleMap) { 

      mMap = googleMap; 
      getHelpRequests(new OnReceivedResponse(){ 

       @Override 
       public void onSuccessResponse(List<HelpRequest> myArray){ 

       Log.d("EMPTY:", String.valueOf(myArray.size())); 

       for(int i=0 ; i<myArray.size() ; i++) { 
        Log.d("REQ:", myArray.get(i).getUsername()); 
       } 
       LatLng stpetersburg = new LatLng(27.88, -82.83); 
       Marker burg = mMap.addMarker(new MarkerOptions().position(stpetersburg).title("Marker in St. Petersburg")); 
       burg.showInfoWindow(); 
       mMap.moveCamera(CameraUpdateFactory.newLatLng(stpetersburg)); 
       } 
     }); 


     } 

     public interface OnReceivedResponse{ 
      void onSuccessResponse(List<HelpRequest> myArray); 
     }