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java - (Android Studio) 不能在 Fragment 中使用 context/getActivity()

问题描述

首先，我是绝对的初学者，如果这是一个愚蠢的错误，我很抱歉。但我总是收到这个错误

java.lang.NullPointerException：尝试在 com.example.loginregister.ui.home.HomeFragment 的空对象引用上调用虚拟方法 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' $1.onResponse(HomeFragment.java:104)

每当我尝试运行我的应用程序时。我已经将 getActivity() 更改为 HomeFragment.this 或 getApplicationActivity() 但仍然出错，我被困了很长时间，因为我不太了解它是如何工作的，我希望有人能向我解释如何在片段中调用上下文，这里是我的 java

public class HomeFragment extends Fragment {

    private RecyclerView rvData;
    private RecyclerView.Adapter adData;
    private RecyclerView.LayoutManager lmData;
    private List<DataModel> listData = new ArrayList<>();


    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {


        rvData = getActivity().findViewById(R.id.rv_user);
        lmData = new LinearLayoutManager(getActivity(),LinearLayoutManager.VERTICAL,false);
        rvData.setLayoutManager(lmData);
        retrieveData();

        return inflater.inflate(R.layout.fragment_home, container, false);
    }

    public void retrieveData(){

        APIRequestData ardData = RetroServer.connectRetrofit().create(APIRequestData.class);
        Call<ResponseModel> showData = ardData.ardRetrieveData();

        showData.enqueue(new Callback<ResponseModel>() {
            @Override
            public void onResponse(Call<ResponseModel> call, Response<ResponseModel> response) {
                int code = response.body().getCode();
                String message = response.body().getMessage();

                Toast.makeText(getActivity(), "Code: "+code+"| Message: "+message, Toast.LENGTH_SHORT).show();
                listData = response.body().getData();

                adData = new AdapterData(getActivity(),listData);
                rvData.setLayoutManager(lmData);
                adData.notifyDataSetChanged();
            }

            @Override
            public void onFailure(Call<ResponseModel> call, Throwable t) {
                Toast.makeText(getActivity(), "Fail to Retrieve Data", Toast.LENGTH_SHORT).show();
            }
        });

    }

这是我的 fragment_home.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ui.home.HomeFragment">


    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/rv_user"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:listitem="@layout/card_item"
        />


</androidx.constraintlayout.widget.ConstraintLayout>

标签： javaandroidxmlandroid-recyclerview

解决方案

使用此代码

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_home, container, false);
    rvData = view.findViewById(R.id.rv_user);
    return view;
}

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