僅當使用片段時，Tablet上才會出現NullPointerException

Question

我在平板電腦上使用我的應用程序時收到NullPointerException ，但在Emulater（平板電腦+手機）和手機上可以正常工作。

問題似乎出在與其他Fragment通信時。

我的平板電腦是1280x800，7英寸

我有五個XML文件：

activity_main.xml
activity_main.xml（土地）
activity_main.xml（sw600dp）

iteminfo_fragment.xml
listview_fragment.xml

Activity_Main.java

import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity implements ListViewFragment.OnitemClickedListener {

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

        if(findViewById(R.id.layout_container) != null){

            if (savedInstanceState != null){
                return;
            }

            ListViewFragment listViewFragment = new ListViewFragment();
            FragmentTransaction fragmentTransaction = getFragmentManager().beginTransaction();
            fragmentTransaction.add(R.id.layout_container,listViewFragment);
            fragmentTransaction.commit();
        }
    }

    @Override
    public void onItemClick(int position) {
            ItemInfo_Fragment itemInfo_fragment = (ItemInfo_Fragment) getFragmentManager().
                    findFragmentById(R.id.ListView_ItemFragmentID);

        if(itemInfo_fragment != null) {
            itemInfo_fragment.updateInfo(position);
        } else {
            ItemInfo_Fragment itemInfo_fragment1 = new ItemInfo_Fragment();
            Bundle args = new Bundle();
            args.putInt("position", position);
            itemInfo_fragment1.setArguments(args);

            FragmentTransaction fragmentTransaction = getFragmentManager()
                    .beginTransaction();
            fragmentTransaction.replace(R.id.layout_container, itemInfo_fragment1);
            fragmentTransaction.addToBackStack(null);
            fragmentTransaction.commit();
        }
    }
}

ListViewFragment.java

import android.app.Fragment;
import android.app.ListFragment;
import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;

/**
 * Created by Sarbast on 30/06/2016.
 */
public class ListViewFragment extends ListFragment {


    OnitemClickedListener mCallBack;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.listview_fragment, container, false);
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        String[] data = getResources().getStringArray(R.array.heros);
        setListAdapter(new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, data));
    }

    @Override
    public void onStart() {
        super.onStart();

        //Highlight the item when using two-pane layout
        Fragment fragmentManager = getFragmentManager()
                .findFragmentById(R.id.ListView_ItemFragmentID);
        ListView v = getListView();

        if (fragmentManager != null && v != null){
            v.setChoiceMode(ListView.CHOICE_MODE_SINGLE);
        }
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);

        try{
            mCallBack = (OnitemClickedListener) context;
        }catch (ClassCastException e){
            throw new ClassCastException(e.toString() + "Must implement OnItemClickedInstance");
        }
    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        super.onListItemClick(l, v, position, id);

        mCallBack.onItemClick(position);
    }

    public interface OnitemClickedListener {
        public void onItemClick(int position);
    }
}

itemInfo_Fragment.java

public class ItemInfo_Fragment extends Fragment {

    TextView mTextView;
    String[] mStrings;
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.iteminfo_fragment, container, false);

        mTextView = (TextView) v.findViewById(R.id.showInfo_ID);
        mStrings = getResources().getStringArray(R.array.heros_info);

        Bundle args = getArguments();
        if (args != null) {
            int index = args.getInt("position");
            mTextView.setText(mStrings[index]);
        }
        return v;
    }

    public void updateInfo(int position){
        mTextView.setText(mStrings[position]);
    }
}

錯誤：

E / AndroidRuntime：致命例外：主進程：com.sarbast.android.listfragmenttest，PID：26930 java.lang.NullPointerException：嘗試調用接口方法'void com.sarbast.android.listfragmenttest.ListViewFragment $ OnitemClickedListener.onItemClick（int） '在com。******。android.listfragmenttest.ListViewFragment.onListItemClick（ListViewFragment.java:63）在android.app.ListFragment $ 2.onItemClick（ListFragment.java:160）上的空對象引用上.AdapterView.performItemClick（AdapterView.java:340）位於android.widget.AbsListView.performItemClick（AbsListView.java:1532）
在android.widget.AbsListView $ PerformClick.run（AbsListView.java:3700）在android.widget.AbsListView $ 3.run（AbsListView.java:5686）在android.os.Handler.handleCallback（Handler.java:739）在android .os.Handler.dispatchMessage（Handler.java:95）位於android.os.Looper.loop（Looper.java:145）位於android.app.ActivityThread.main（ActivityThread.java:6939）在java.lang.reflect。 com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:1404）處java.lang.reflect.Method.invoke（Method.java:372）處的Method.invoke（Method.java:372）。 internal.os.ZygoteInit.main（ZygoteInit.java:1199）

Answer 1

您的回調接口永遠不會初始化，

所以調用方法

@Override
public void onListItemClick(ListView l, View v, int position, long id) {
      super.onListItemClick(l, v, position, id);
      mCallBack.onItemClick(position);
}

拋出一個空指針異常

您需要在觸發回調之前檢查null

if(mCallBack!=null){
     mCallBack.onItemClick(position);
}

或定義一個setter方法，使您可以正確初始化接口。

Answer 2

值得慶幸的是，我找到了答案，因為我使用的API低於23（在我的平板電腦上為22），並且不使用支持庫，所以從未調用過onAttach(Context context)所以我不得不使用onAttach(Activity activity)