Question

实际上，在我找到它之前，从未在这里寻求帮助，但这是例外情况。我的应用程序应获取json文件（单个文件，基本上是静态信息），将其解析并将其放入不同片段的navigationDrawer中。 NavDrawer应该动态填充（json具有三个参数，其中之一是单个项的名称），内容应以不同的片段显示，并且所有这些都应同时适用于纵向和横向（应保存状态）。

所以我得到了NetworkOperations.java类，在其中我尝试使用enqueue（）类型的请求从放置json文件的url获得响应。由于这种类型，基本上，据我了解，我的所有逻辑都应使用onResponse方法，因为在其他情况下，由于此请求工作的异步类型，在大多数情况下，我将获得NPE。

要保存我的响应列表类型为List的数据，我试图通过意图进行传输。我的问题是，在getAplicationContext（）（下面的代码）期间出现了NullPointerException。有人可以提供帮助吗？ onCreate方法正在执行，就像声明了retrofitBuilder的App util方法一样。从理论上讲，这两种情况都应该出现，为什么我不能拥有它？

先谢谢您，抱歉我的英语不好。

编辑：这是提供NPE的代码

Intent intent = new Intent(mContext.getApplicationContext() , 
MainActivity.class);

MainActivity.java

public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener {

//Tags
private static final String TAG = "MainActivity";

//vars
private NetworkOperations operations = new NetworkOperations();
private List<com.example.coder.testapp.model.Menu> menuItemList = new ArrayList<>();
private ArrayList<com.example.coder.testapp.model.Menu> listForDrawer = new ArrayList<>();

//widgets


private NavigationView navigationView;
private ProgressBar mProgressBar;
private Menu menu;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    DrawerLayout drawer = findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.addDrawerListener(toggle);
    toggle.syncState();

    navigationView = findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);

    mProgressBar = findViewById(R.id.progress_bar);


    new DownloadTask().execute();

}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}


@Override
public boolean onNavigationItemSelected(MenuItem item) {
    int id = item.getItemId();

    if (id == FragmentKeys.FRAGMENT_ONE) {

    } else if (id == FragmentKeys.FRAGMENT_TWO) {

    } else if (id == FragmentKeys.FRAGMENT_THREE) {

    } else if (id == FragmentKeys.FRAGMENT_FOUR) {

    }

    DrawerLayout drawer = findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

public void addNewItem (String itemName){
    menu = navigationView.getMenu();
    menu.add(Menu.NONE,Menu.NONE,Menu.NONE,itemName);
    System.out.println("Item " + itemName + " added" );
}

public void initDrawer(List<com.example.coder.testapp.model.Menu> list){
    System.out.println(list);
    if (list != null) {
        System.out.println(list.size());
        addNewItem(list.get(FragmentKeys.FRAGMENT_ONE).getName());
        addNewItem(list.get(FragmentKeys.FRAGMENT_TWO).getName());
        addNewItem(list.get(FragmentKeys.FRAGMENT_THREE).getName());
        addNewItem(list.get(FragmentKeys.FRAGMENT_FOUR).getName());
    }
}

private void getMenuList (){
    menuItemList = new ArrayList<>();
    operations.getResponses(menuItemList);
    Intent intent = new Intent(this, MainActivity.class);
    listForDrawer = intent.getParcelableArrayListExtra("menus");

    System.out.println("get menu list response " + menuItemList);
}

class DownloadTask extends AsyncTask<Void, Void, Void> {

    protected void onPreExecute() {
        Log.d(TAG, "onPreExecute: ");
        publishProgress(true);
    }

    @Override
    protected Void doInBackground(Void... voids) {
        Log.d(TAG, "doInBackground: ...");
        getMenuList();
        return null;
    }

    @Override
    protected void onPostExecute(Void aVoid) {
        Log.d(TAG, "onPostExecute: completed...");
        initDrawer(listForDrawer);
        publishProgress(false);
    }

    public void publishProgress(boolean change){
        if (change) {
            mProgressBar.bringToFront();
            mProgressBar.setBackgroundColor(getResources().getColor(R.color.mainBackgroundblur));
            mProgressBar.setVisibility(View.VISIBLE);
            getWindow().setFlags(WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE,
                    WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE);
        } else {
            mProgressBar.setVisibility(View.GONE);

            getWindow().clearFlags(WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE);

        }
    }
}

NetworkOperations.java

public class NetworkOperations {

private static final String TAG = "Network operations";

private ArrayList<Menu> menuList;
private Example mRespons;
private Context mContext;

public void getResponses(final List<Menu> list) {

    App.getServerAPI().getExample().enqueue(new Callback<Example>() {
        @Override
        public void onResponse(@NonNull Call<Example> call,
                               @NonNull Response<Example> response) {
            if (response.body()!= null) {
                mRespons = response.body();
                System.out.println("Response is not empty " + mRespons);
                if (mRespons != null) {
                    System.out.println(mRespons.getMenu() + " shit");
                    list.addAll(mRespons.getMenu());
                    menuList= new ArrayList<>();
                    menuList.addAll(list);
                    System.out.println("This is global " + menuList);
                    System.out.println("This is inner " + list);

                   if (menuList.size()>0) {
                       System.out.println(mContext.getApplicationContext().toString());
                       Intent intent = new Intent(mContext.getApplicationContext() , MainActivity.class);
                       intent.putParcelableArrayListExtra("menus", menuList);
                       System.out.println("OLA " + menuList);
                   }
                }
            }
        }
        @Override
        public void onFailure(Call<Example> call, Throwable t) {
            Toast.makeText(mContext, "An error occurred during networking", Toast.LENGTH_SHORT).show();
        }
    });
}

}

编辑： 错误代码

07-30 20:50:38.715 4751-4751/com.example.coder.testapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.coder.testapp, PID: 4751
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
    at com.example.coder.testapp.operations.NetworkOperations$1.onResponse(NetworkOperations.java:51)
    at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run(ExecutorCallAdapterFactory.java:70)
    at android.os.Handler.handleCallback(Handler.java:815)
    at android.os.Handler.dispatchMessage(Handler.java:104)
    at android.os.Looper.loop(Looper.java:207)
    at android.app.ActivityThread.main(ActivityThread.java:5728)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)

改造onResponse方法中的getApplicationContext处出现NullPointerException

0 个答案: