我的应用尝试加载列表后立即崩溃，我进行了搜索，发现有些错误是因为他们没有，但是我知道：
implementation 'com.android.support:recyclerview-v7:28.0.0-alpha3'
implementation 'com.android.support:cardview-v7:28.0.0-alpha3'
这是我得到的错误代码：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.yemencar.yemencar, PID: 28558
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView$Adapter.notifyDataSetChanged()' on a null object reference
at com.yemencar.yemencar.homepageActivity.parseData(homepageActivity.java:144)
at com.yemencar.yemencar.homepageActivity.access$000(homepageActivity.java:26)
at com.yemencar.yemencar.homepageActivity$1.onResponse(homepageActivity.java:95)
at com.yemencar.yemencar.homepageActivity$1.onResponse(homepageActivity.java:91)
at com.android.volley.toolbox.JsonRequest.deliverResponse(JsonRequest.java:65)
at com.android.volley.ExecutorDelivery$ResponseDeliveryRunnable.run(ExecutorDelivery.java:99)
at android.os.Handler.handleCallback(Handler.java:789)
at android.os.Handler.dispatchMessage(Handler.java:98)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
E/Surface: queueBuffer: error queuing buffer to SurfaceTexture, -19
E/EGL_emulation: tid 28589: swapBuffers(550): error 0x300d (EGL_BAD_SURFACE)
我正在尝试使用标头制作RecyclerView，以便可以将sort选项和view作为Grid或list选项使用。这是我的主页活动代码：
package com.yemencar.yemencar;
import android.os.Bundle;
import android.support.v4.view.ViewCompat;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.View;
import android.widget.ProgressBar;
import android.widget.Toast;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.JsonArrayRequest;
import com.android.volley.toolbox.Volley;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.util.ArrayList;
import java.util.List;
public class homepageActivity extends AppCompatActivity implements RecyclerView.OnScrollChangeListener {
//Creating a List of superheroes
private List<SuperHero> listSuperHeroes;
//Creating Views
private RecyclerView recyclerView;
private RecyclerView.LayoutManager layoutManager;
private RecyclerView.Adapter adapter;
//Volley Request Queue
private RequestQueue requestQueue;
//The request counter to send ?page=1, ?page=2 requests
private int requestCount = 1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_homepage);
ViewCompat.setLayoutDirection(getWindow().getDecorView(),ViewCompat.LAYOUT_DIRECTION_RTL);
//Initializing Views
recyclerView = (RecyclerView) findViewById(R.id.recyclerView);
recyclerView.setHasFixedSize(true);
layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);
//Initializing our superheroes list
listSuperHeroes = new ArrayList<>();
requestQueue = Volley.newRequestQueue(this);
//Calling method to get data to fetch data
getData();
//Adding an scroll change listener to recyclerview
recyclerView.setOnScrollChangeListener(this);
//initializing our adapter
CardAdapter adapter = new CardAdapter(getHeader(),listSuperHeroes, this);
//Adding adapter to recyclerview
recyclerView.setAdapter(adapter);
}
public Header getHeader()
{
Header header = new Header();
header.setHeader("I'm header");
return header;
}
//Request to get json from server we are passing an integer here
//This integer will used to specify the page number for the request ?page = requestcount
//This method would return a JsonArrayRequest that will be added to the request queue
private JsonArrayRequest getDataFromServer(int requestCount) {
//Initializing ProgressBar
final ProgressBar progressBar = (ProgressBar) findViewById(R.id.progressBar1);
//Displaying Progressbar
progressBar.setVisibility(View.VISIBLE);
setProgressBarIndeterminateVisibility(true);
//JsonArrayRequest of volley
JsonArrayRequest jsonArrayRequest = new JsonArrayRequest(Config.DATA_URL + String.valueOf(requestCount),
new Response.Listener<JSONArray>() {
@Override
public void onResponse(JSONArray response) {
//Calling method parseData to parse the json response
parseData(response);
//Hiding the progressbar
progressBar.setVisibility(View.GONE);
}
},
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
progressBar.setVisibility(View.GONE);
//If an error occurs that means end of the list has reached
Toast.makeText(homepageActivity.this, "No More Items Available", Toast.LENGTH_SHORT).show();
}
});
//Returning the request
return jsonArrayRequest;
}
//This method will get data from the web api
private void getData() {
//Adding the method to the queue by calling the method getDataFromServer
requestQueue.add(getDataFromServer(requestCount));
//Incrementing the request counter
requestCount++;
}
//This method will parse json data
private void parseData(JSONArray array) throws RuntimeException {
for (int i = 0; i < array.length(); i++) {
//Creating the superhero object
SuperHero superHero = new SuperHero();
JSONObject json = null;
try {
//Getting json
json = array.getJSONObject(i);
//Adding data to the superhero object
superHero.setImageUrl(json.getString(Config.TAG_IMAGE_URL));
superHero.setName(json.getString(Config.TAG_NAME));
superHero.setPublisher(json.getString(Config.TAG_PUBLISHER));
} catch (JSONException e) {
e.printStackTrace();
}
//Adding the superhero object to the list
listSuperHeroes.add(superHero);
}
//Notifying the adapter that data has been added or changed
Log.d("ADebugTag", "Value: " + listSuperHeroes);
adapter.notifyDataSetChanged();
}
//This method would check that the recyclerview scroll has reached the bottom or not
private boolean isLastItemDisplaying(RecyclerView recyclerView) {
if (recyclerView.getAdapter().getItemCount() != 0) {
int lastVisibleItemPosition = ((LinearLayoutManager) recyclerView.getLayoutManager()).findLastCompletelyVisibleItemPosition();
if (lastVisibleItemPosition != RecyclerView.NO_POSITION && lastVisibleItemPosition == recyclerView.getAdapter().getItemCount() - 1)
return true;
}
return false;
}
//Overriden method to detect scrolling
@Override
public void onScrollChange(View v, int scrollX, int scrollY, int oldScrollX, int oldScrollY) {
//Ifscrolled at last then
if (isLastItemDisplaying(recyclerView)) {
//Calling the method getdata again
getData();
}
}
}
答案 0 :(得分：0)
进行这两个更改，您应该会很好。
public class homepageActivity ... {
// 1. Change this to a CardAdapter
private CardAdapter adapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
// ...
// 2. Change this to reference the field adapter.
adapter = new CardAdapter(getHeader(),listSuperHeroes, this);
}
}