java - 使用 RecyclerView 时我的应用程序崩溃

Question

我的应用程序在尝试加载列表后立即崩溃，我搜索并发现有些错误是因为他们没有，但我有：

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)

我正在尝试使用标题制作 RecyclerView，以便我可以放置排序选项并将视图作为网格或列表选项。所以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();
        }
    }
}

标签： javaandroidandroid-studioandroid-recyclerview

Answer 1

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);
    }
}