java - 使用 fragment 事务的 NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 6 年前。

我从 android 开始，对多个 fragment 的实现感到疯狂。我的 MainActivity 中有两个 fragment :QuestionDetail 和 QuestionListFragment。当我使用 fragment 事务替换 QuestionDetail fragment 时，当有人单击 QuestionListFragment 选项之一时，我收到 NullPointerException 错误。

我几个小时以来一直在尝试解决这个问题。非常感谢任何帮助。

问题详情

public class QuestionListFragment extends Fragment{

private RecyclerView recView;
public static QuestionAdapter adapter;
private QuestionListener listener;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_question_list, container, false);
    recView = (RecyclerView) v.findViewById(R.id.recView);
    return v;
}

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

    LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity());
    layoutManager.setOrientation(LinearLayoutManager.VERTICAL);
    recView.setLayoutManager(layoutManager);

    adapter = new QuestionAdapter(QuestionCatalog.getQuestionCatalog().getQuestionList().getList());
    adapter.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick(View v) {
            if (listener!=null) {
                listener.onQuestionListener(adapter.getItem(recView.getChildAdapterPosition(v)));
            }
        }
    });
    recView.setAdapter(adapter);
}

public interface QuestionListener {
    void onQuestionListener(Question q);
}

public void setQuestionListener(QuestionListener listener) {
    this.listener=listener;
}

问题详情

public class QuestionDetail extends Fragment {
// We use an ID to know which Question we're using
private long questionId;

public QuestionDetail() {
    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    return inflater.inflate(R.layout.fragment_question_detail, container, false);
}

@Override
public void onStart() {
    super.onStart();
    //Fragments don’t include findViewById(). To get a reference to a view, we use getView()
    View view = getView();
    if (view != null) {
        TextView title = (TextView) view.findViewById(R.id.textQuestion);
        Question question = QuestionCatalog.getQuestionCatalog().getQuestionList().getList().get((int) questionId);
        title.setText(question.getName());
        TextView kind = (TextView) view.findViewById(R.id.textKind);
        kind.setText(question.getKind());
    }
}

public void showDetails(String texto) {
    TextView txtName = (TextView)getView().findViewById(R.id.textQuestion);
    TextView txtKind = (TextView)getView().findViewById(R.id.textKind);
    txtName.setText(texto);
    txtKind.setText(texto);
    }
}

主要 Activity

public class MainActivity extends AppCompatActivity implements QuestionListFragment.QuestionListener {

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

    QuestionListFragment frg =(QuestionListFragment)getFragmentManager().findFragmentById(R.id.listFragment);
    frg.setQuestionListener(this);
}

@Override
public void onQuestionListener(Question q) {
    Toast.makeText(this, "num: " + q.getName(),
            Toast.LENGTH_LONG).show();
    FragmentTransaction t = getSupportFragmentManager().beginTransaction();
    QuestionDetail f1 = new QuestionDetail();
    f1.showDetails(q.getName());
    t.replace(R.id.detailFragment,f1);
    t.addToBackStack(null);
    t.commit();
}

堆栈跟踪

08-04 09:54:11.914 22277-22277/com.hfad.predictandwin E/AndroidRuntime: FATAL EXCEPTION: main
                                                                    java.lang.NullPointerException
                                                                        at com.hfad.predictandwin.Fragments.QuestionDetail.showDetails(QuestionDetail.java:50)
                                                                        at com.hfad.predictandwin.MainActivity.onQuestionListener(MainActivity.java:34)
                                                                        at com.hfad.predictandwin.Fragments.QuestionListFragment$1.onClick(QuestionListFragment.java:55)
                                                                        at com.hfad.predictandwin.QuestionAdapter.onClick(QuestionAdapter.java:61)
                                                                        at android.view.View.performClick(View.java:4475)
                                                                        at android.view.View$PerformClick.run(View.java:18786)
                                                                        at android.os.Handler.handleCallback(Handler.java:730)
                                                                        at android.os.Handler.dispatchMessage(Handler.java:92)
                                                                        at android.os.Looper.loop(Looper.java:176)
                                                                        at android.app.ActivityThread.main(ActivityThread.java:5419)
                                                                        at java.lang.reflect.Method.invokeNative(Native Method)
                                                                        at java.lang.reflect.Method.invoke(Method.java:525)
                                                                        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)
                                                                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)
                                                                        at dalvik.system.NativeStart.main(Native Method)

Answer 1

首先，不要在 onStart() 中使用 getView() - 此时您不应该获得 View 。其次，您不应该从其他地方直接调用 Fragment 实例上的方法，在您的例子中为 f1.showDetails(q.getName());。再次，Fragment 没有 View。请参阅Best practice for instantiating a new Android Fragment了解如何在将 Fragment 加载到 FragmentManager 之前将参数传递到 Fragment。

<小时/>

下一步 - 您应该使用 onCreateView 内部的 findViewById 初始化 View ，就像您对其他 Fragment 类所做的那样。例如，

private TextView txtName, txtKind;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View rootView = inflater.inflate(R.layout.fragment_question_detail, container, false);

    txtName = (TextView) rootView.findViewById(R.id.textQuestion);
    txtKind = (TextView) rootView.findViewById(R.id.textKind);

    // TODO: Read the link how to use these
    Bundle arguments = getArguments();

    Question question = QuestionCatalog.getQuestionCatalog()
                .getQuestionList().getList().get((int) questionId);
    showQuestion(question);

    return view;
}

private void showQuestion(Question q) {
    txtName.setText(q.getName());
    txtKind.setText(q.getKind());
}