[英]NullPointerException in adapter holding viewholders
ElementAdapter 類
package com.example.sierendeelementeninbreda;
import android.content.Context;
import android.view.ViewGroup;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.ImageView;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
import com.squareup.picasso.Picasso;
import java.util.ArrayList;
import java.util.List;
public class ElementAdapter extends RecyclerView.Adapter<ElementAdapter.ViewHolder> {
private static final String TAG = ElementAdapter.class.getSimpleName();
private List<Element> mElements = new ArrayList<>();
private final ElementOnClickHandler mElementClickHandler;
public ElementAdapter(List<Element> mElements, ElementOnClickHandler mElementClickHandler) {
this.mElements = mElements;
this.mElementClickHandler = mElementClickHandler;
}
@NonNull
@Override
public ElementAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int viewType) {
Log.v(TAG, "++++++++ onCreateViewHolder - viewGroup-class: " + viewGroup.getClass().getSimpleName());
Log.v(TAG, "++++++++ onCreateViewHolder - viewGroup-resourceName: " + viewGroup.getContext().getResources().getResourceName(viewGroup.getId()));
Log.v(TAG, "++++++++ onCreateViewHolder - viewGroup-resourceEntryName: " + viewGroup.getContext().getResources().getResourceEntryName(viewGroup.getId()));
Context context = viewGroup.getContext();
LayoutInflater inflator = LayoutInflater.from(context);
// create a new view
View elementListItem = inflator.inflate(R.layout.element_item, viewGroup, false);
ElementAdapter.ViewHolder viewHolder = new ViewHolder(elementListItem);
return viewHolder;
}
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
Log.v(TAG, "++++ onBindViewHolder-type: " + holder.getClass().getSimpleName());
holder.title.setText(mElements.get(position).getTitle());
holder.geographicalLocation.setText(mElements.get(position).getGeographicalLocation());
holder.identificationNumber.setText(mElements.get(position).getIdentificationNumber());
Picasso.get()
.load(mElements.get(position).getImage())
.into(holder.image);
}
@Override
public int getItemCount() {
return mElements.size();
}
public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
//one drinkItem has 3 views
// Provide a reference to each view in the drinkItem
private ImageView image;
private TextView title;
private TextView geographicalLocation;
private TextView identificationNumber;
public ViewHolder(View listItemView) {
super(listItemView);
title = (TextView) listItemView.findViewById(R.id.element_item_name);
geographicalLocation = (TextView) listItemView.findViewById(R.id.element_Location);
identificationNumber = (TextView) listItemView.findViewById(R.id.element_idNumber);
image = (ImageView) listItemView.findViewById(R.id.element_item_imageview);
// image.setOnClickListener(this);
// name.setOnClickListener(this);
// description.setOnClickListener(this);
// listItemView.setOnClickListener(this);
}
@Override
public void onClick(View view) {
int itemIndex = getAdapterPosition();
mElementClickHandler.onElementClick(view, itemIndex);
}
}
}
NetworkUtils 類
package com.example.sierendeelementeninbreda;
import android.os.AsyncTask;
import android.util.Log;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.Scanner;
public class NetworkUtils extends AsyncTask<String, Void, String> {
private static final String TAG = NetworkUtils.class.getSimpleName();
private OnElementApiListener listener;
private final static String url = "https://services7.arcgis.com/21GdwfcLrnTpiju8/arcgis/rest/services/Sierende_elementen/FeatureServer/0/query?where=1%3D1&outFields=*&outSR=4326&f=json";
public NetworkUtils(OnElementApiListener listener) {
this.listener = listener;
}
@Override
protected String doInBackground(String... input) {
//De methode begint met een log zodat je kunt zien wat er gebeurt in de run.
Log.d(TAG, "doInBackground was called");
String response = null;
HttpURLConnection httpURLConnection = null;
// ToDo: Url maken op basis van internet adres
try {
URL mUrl = new URL(url);
URLConnection mConnection = mUrl.openConnection();
httpURLConnection = (HttpURLConnection) mConnection;
httpURLConnection.setRequestMethod("GET");
httpURLConnection.setRequestProperty("Content-Type", "application/json");
httpURLConnection.connect();
int responseCode = httpURLConnection.getResponseCode();
if(responseCode != HttpURLConnection.HTTP_OK){
Log.e(TAG, "Aanroep naar de server is mislukt!");
} else {
InputStream in = httpURLConnection.getInputStream();
Scanner scanner = new Scanner(in);
scanner.useDelimiter("\\A");
boolean hasInput = scanner.hasNext();
if (hasInput) {
response = scanner.next();
}
}
Log.d(TAG, response);
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if(httpURLConnection != null)
httpURLConnection.disconnect();
}
return response;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
Log.d(TAG, "onPreExecute: ");
}
@Override
protected void onPostExecute(String response) {
super.onPostExecute(response);
Log.i(TAG, "createElementFromJson called");
ArrayList<Element> elements = new ArrayList<>();
if (response == null || response.equals("")) {
Log.e(TAG, "onPostExecute: empty response!");
return;
}
try {
JSONObject jsonResults = new JSONObject(response);
JSONArray elementList = jsonResults.getJSONArray("features");
Log.d(TAG, "onPostExecute: " + jsonResults);
Log.i(TAG, "elementArray length = " + elementList.length());
for(int i = 0; i < elementList.length(); i++) {
JSONObject element = elementList.getJSONObject(i);
//all info
JSONObject elementAttributes = element.getJSONObject("attributes");
String image = elementAttributes.getString("URL");
String title = elementAttributes.getString("AANDUIDINGOBJECT");
String geographicalLocation = elementAttributes.getString("GEOGRAFISCHELIGGING");
String identificationNumber = elementAttributes.getString("IDENTIFICATIE");
Element element_item = new Element(title, geographicalLocation, identificationNumber);
element_item.setImage(image);
elements.add(element_item);
Log.i(TAG, String.valueOf(elements.size()));
}
listener.onElementAvailable(elements);
} catch (JSONException e) {
e.printStackTrace();
}
}
public interface OnElementApiListener {
void onElementAvailable(ArrayList<Element> elements);
}
主活動類
package com.example.sierendeelementeninbreda;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.content.Context;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Toast;
import java.util.ArrayList;
public class MainActivity extends AppCompatActivity
implements View.OnClickListener,NetworkUtils.OnElementApiListener,
ElementOnClickHandler{
private static String TAG = MainActivity.class.getName();
private final static String url = "https://services7.arcgis.com/21GdwfcLrnTpiju8/arcgis/rest/services/Sierende_elementen/FeatureServer/0/query?where=1=1&outFields=&outSR=4326&f=json";
private ArrayList<Element> mElementList;
private RecyclerView elementRecyclerView;
private ElementAdapter mElementAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
elementRecyclerView= findViewById(R.id.recyclerView_element_list);
elementRecyclerView.setLayoutManager(new LinearLayoutManager(this));
mElementAdapter = new ElementAdapter(mElementList,this);
elementRecyclerView.setAdapter(mElementAdapter);
// Start background task
NetworkUtils networkingTask = new NetworkUtils(this);
networkingTask.execute(url);
}
@Override
public void onElementAvailable(ArrayList<Element> elements) {
Log.i(TAG, "elements = " + elements.size());
this.mElementList = new ArrayList<>();
mElementList.clear();
mElementList.addAll(elements);
mElementAdapter.notifyDataSetChanged();
Log.i(TAG, "new elements = " + mElementList.size());
}
@Override
public void onElementClick(View view, int itemIndex) {
Log.v(TAG, "clicked on " + view.getClass().getSimpleName());
int viewId = view.getId();
Context context = this;
String toastMessage = "";
// if (viewId == R.id.product_imageview) {
// toastMessage = "clicked on image of drink item";
// } else if (viewId == R.id.product_item_name || viewId == R.id.product_item_description) {
// toastMessage = "clicked on name or description of drink item";
// } else if (viewId == R.id.product_list_item) {
// toastMessage = "clicked on drink list-item nr: " + itemIndex;
// }
Toast.makeText(context, toastMessage, Toast.LENGTH_LONG)
.show();
}
@Override
public void onClick(View v) {
}
}
錯誤
D/NetworkUtils: onPreExecute: D/NetworkUtils: doInBackground was called D/HostConnection: HostConnection::get() New Host Connection established 0xd4a13c30, tid 27630 D/HostConnection: HostComposition ext ANDROID_EMU_CHECKSUM_HELPER_v1 ANDROID_EMU_native_sync_v2 ANDROID_EMU_native_sync_v3 ANDROID_EMU_native_sync_v4 ANDROID_EMU_dma_v1 ANDROID_EMU_direct_mem ANDROID_EMU_host_composition_v1 ANDROID_EMU_host_composition_v2 ANDROID_EMU_YUV420_888_to_NV21 ANDROID_EMU_YUV_Cache ANDROID_EMU_async_unmap_buffer GL_OES_EGL_image_external_essl3 GL_OES_vertex_array_object GL_KHR_texture_compression_astc_ldr ANDROID_EMU_gles_max_version_3_0 D/AndroidRuntime: Shutting down VM E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.sierendeelementeninbreda, PID: 27595 java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference at com.example.sierendeelementeninbreda.ElementAdapter.getItemCount(ElementAdapter.java:64) at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep1(RecyclerView.java:4044) at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3849) at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4404) at android.view.View.layout(View.java:21912) at android.view.ViewGroup.layout(ViewGroup.java:6260)
這是我運行應用程序時遇到的錯誤,我嘗試通過查找來解決此問題,但它不起作用。 NullPointerException 發生在我的適配器類的第 64 行。 該應用程序基本上使用 API 從站點獲取數據,然后在 recylerView 中展示它。
你在傳遞給適配器之前沒有初始化mElementList所以當適配器調用 size 方法時會拋出空指針
在此處編輯您的代碼
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
elementRecyclerView= findViewById(R.id.recyclerView_element_list);
elementRecyclerView.setLayoutManager(new LinearLayoutManager(this));
// init mElementList here
mElementList = new ArrayList<>();
mElementAdapter = new ElementAdapter(mElementList,this);
elementRecyclerView.setAdapter(mElementAdapter);
// Start background task
NetworkUtils networkingTask = new NetworkUtils(this);
networkingTask.execute(url);
}
做空檢查總是更好
@Override public int getItemCount() { if(mElements!=null) return mElements.size(); 否則返回 0 }
並在構造函數中初始化 mElements
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