Java 在片段中的recyclerView上使用findViewById时获取NullPointerException
正在尝试使用5个片段构建应用程序。第一个包含recyclerView，每当我切换选项卡时，视图都会被丢弃，每当我在充气机充气时返回视图时，应用程序都会尝试获取recyclerView的id，该id返回空指针异常。这个问题只会变得越来越复杂，我不认为用正确的方法来做这件事，因为我会犯错误 如果您有任何简单的应用程序示例，其中包含具有recyclerViews的片段，请删除下面的链接，提前感谢您 MainActivity.javaJava 在片段中的recyclerView上使用findViewById时获取NullPointerException,java,android,android-recyclerview,Java,Android,Android Recyclerview,正在尝试使用5个片段构建应用程序。第一个包含recyclerView，每当我切换选项卡时，视图都会被丢弃，每当我在充气机充气时返回视图时，应用程序都会尝试获取recyclerView的id，该id返回空指针异常。这个问题只会变得越来越复杂，我不认为用正确的方法来做这件事，因为我会犯错误 如果您有任何简单的应用程序示例，其中包含具有recyclerViews的片段，请删除下面的链接，提前感谢您 MainActivity.java class MainActivity extends AppComp
class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";
private FragmentHome fragmentHome = new FragmentHome(this);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
fragmentHome.initGetData();
BottomNavigationView bottomNavigationView = findViewById(R.id.bottomNavBar);
getSupportFragmentManager().beginTransaction().replace(R.id.container, fragmentHome).commit();
fragmentHome.initRecyclerView();
bottomNavigationView.setOnNavigationItemSelectedListener(navListener);
bottomNavigationView.setOnNavigationItemReselectedListener(new BottomNavigationView.OnNavigationItemReselectedListener() {
@Override
public void onNavigationItemReselected(@NonNull MenuItem menuItem) {
}
});
}
private BottomNavigationView.OnNavigationItemSelectedListener navListener = new BottomNavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
switch (menuItem.getItemId()) {
case R.id.home: {
getSupportFragmentManager().beginTransaction().replace(R.id.container, fragmentHome).commit();
fragmentHome.initRecyclerView();
break;
}
}
return true;
}
};
}
public class FragmentHome extends Fragment {
private String BASE_URL = "https://www.reddit.com";
// INIT RETROFIT AND CALL ONCE AND FOR ALL
Retrofit retrofit = new Retrofit.Builder()
.baseUrl(BASE_URL)
.addConverterFactory(GsonConverterFactory.create())
.build();
RedditAPI redditAPI = retrofit.create(RedditAPI.class);
private static final String TAG = "FragmentHome";
private RecyclerView feedRecyclerView;
private List<ChildrenData> childrenData;
private String after;
private String previousAfter = "";
private Boolean isScrolling = false;
private int currentItems;
private int totalItems;
private int scrolledOutItems;
private Context context;
private View inflatingView;
public FragmentHome() {
}
public FragmentHome(Context context) {
this.context = context; //I also get some attribute warning
here not important right now though.
}
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
inflatingView = inflater.inflate(R.layout.fragment_home, container, false);
initRecyclerView();
return inflatingView;
}
public void initGetData() {
Call<FeedData> feedDataCall = redditAPI.getFeed();
feedDataCall.enqueue(new Callback<FeedData>() {
@Override
public void onResponse(Call<FeedData> call, final Response<FeedData> response) {
/* Log.d(TAG, "onResponse: " + response.body().getData().getChildren().get(0).getDataModel().getTitle());
Log.d(TAG, "onResponse: " + response.body().getData().getChildren());*/
childrenData = response.body().getData().getChildren();
Log.d(TAG, "onResponse: " + childrenData);
after = response.body().getData().getAfter();
Log.d(TAG, "onResponse: " + after);
}
@Override
public void onFailure(Call<FeedData> call, Throwable t) {
Log.d(TAG, "onFailure: " + t.getMessage());
}
});
}
public void initRecyclerView() {
feedRecyclerView = inflatingView.findViewById(R.id.recyclerView);
final FeedRecyclerViewAdapter feedRecyclerViewAdapter = new FeedRecyclerViewAdapter(childrenData, context);
feedRecyclerView.setAdapter(feedRecyclerViewAdapter);
final LinearLayoutManager linearLayoutManager = new LinearLayoutManager(context);
feedRecyclerView.setLayoutManager(linearLayoutManager);
feedRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
@Override
public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {
super.onScrollStateChanged(recyclerView, newState);
isScrolling = true;
}
@Override
public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
super.onScrolled(recyclerView, dx, dy);
currentItems = linearLayoutManager.getChildCount();
totalItems = linearLayoutManager.getItemCount();
scrolledOutItems = linearLayoutManager.findFirstVisibleItemPosition();
if (isScrolling && (totalItems - 1 == currentItems + scrolledOutItems) && (!previousAfter.equals(after))) {
isScrolling = false;
fetchNewData(after, feedRecyclerViewAdapter);
}
}
});
}
public void fetchNewData(final String after_use, final FeedRecyclerViewAdapter feedRecyclerViewAdapter) {
Call<FeedData> feedDataCall_withAfter = redditAPI.getFeed_withAfter(after_use);
previousAfter = after_use;
feedDataCall_withAfter.clone().enqueue(new Callback<FeedData>() {
@Override
public void onResponse(Call<FeedData> call, Response<FeedData> response) {
Log.d(TAG, "onResponse: " + response.body().toString());
childrenData.addAll(response.body().getData().getChildren());
after = response.body().getData().getAfter();
feedRecyclerViewAdapter.notifyDataSetChanged();
}
@Override
public void onFailure(Call<FeedData> call, Throwable t) {
}
});
Log.d(TAG, "fetchNewData: " + childrenData.size());
}
}
class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";
private FragmentHome fragmentHome = new FragmentHome(this);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
fragmentHome.initGetData();
BottomNavigationView bottomNavigationView = findViewById(R.id.bottomNavBar);
getSupportFragmentManager().beginTransaction().replace(R.id.container, fragmentHome).commit();
fragmentHome.initRecyclerView();
bottomNavigationView.setOnNavigationItemSelectedListener(navListener);
bottomNavigationView.setOnNavigationItemReselectedListener(new BottomNavigationView.OnNavigationItemReselectedListener() {
@Override
public void onNavigationItemReselected(@NonNull MenuItem menuItem) {
}
});
}
private BottomNavigationView.OnNavigationItemSelectedListener navListener = new BottomNavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
switch (menuItem.getItemId()) {
case R.id.home: {
getSupportFragmentManager().beginTransaction().replace(R.id.container, fragmentHome).commit();
fragmentHome.initRecyclerView();
break;
}
}
return true;
}
};
}
public class FragmentHome extends Fragment {
private String BASE_URL = "https://www.reddit.com";
// INIT RETROFIT AND CALL ONCE AND FOR ALL
Retrofit retrofit = new Retrofit.Builder()
.baseUrl(BASE_URL)
.addConverterFactory(GsonConverterFactory.create())
.build();
RedditAPI redditAPI = retrofit.create(RedditAPI.class);
private static final String TAG = "FragmentHome";
private RecyclerView feedRecyclerView;
private List<ChildrenData> childrenData;
private String after;
private String previousAfter = "";
private Boolean isScrolling = false;
private int currentItems;
private int totalItems;
private int scrolledOutItems;
private Context context;
private View inflatingView;
public FragmentHome() {
}
public FragmentHome(Context context) {
this.context = context; //I also get some attribute warning
here not important right now though.
}
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
inflatingView = inflater.inflate(R.layout.fragment_home, container, false);
initRecyclerView();
return inflatingView;
}
public void initGetData() {
Call<FeedData> feedDataCall = redditAPI.getFeed();
feedDataCall.enqueue(new Callback<FeedData>() {
@Override
public void onResponse(Call<FeedData> call, final Response<FeedData> response) {
/* Log.d(TAG, "onResponse: " + response.body().getData().getChildren().get(0).getDataModel().getTitle());
Log.d(TAG, "onResponse: " + response.body().getData().getChildren());*/
childrenData = response.body().getData().getChildren();
Log.d(TAG, "onResponse: " + childrenData);
after = response.body().getData().getAfter();
Log.d(TAG, "onResponse: " + after);
}
@Override
public void onFailure(Call<FeedData> call, Throwable t) {
Log.d(TAG, "onFailure: " + t.getMessage());
}
});
}
public void initRecyclerView() {
feedRecyclerView = inflatingView.findViewById(R.id.recyclerView);
final FeedRecyclerViewAdapter feedRecyclerViewAdapter = new FeedRecyclerViewAdapter(childrenData, context);
feedRecyclerView.setAdapter(feedRecyclerViewAdapter);
final LinearLayoutManager linearLayoutManager = new LinearLayoutManager(context);
feedRecyclerView.setLayoutManager(linearLayoutManager);
feedRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
@Override
public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {
super.onScrollStateChanged(recyclerView, newState);
isScrolling = true;
}
@Override
public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
super.onScrolled(recyclerView, dx, dy);
currentItems = linearLayoutManager.getChildCount();
totalItems = linearLayoutManager.getItemCount();
scrolledOutItems = linearLayoutManager.findFirstVisibleItemPosition();
if (isScrolling && (totalItems - 1 == currentItems + scrolledOutItems) && (!previousAfter.equals(after))) {
isScrolling = false;
fetchNewData(after, feedRecyclerViewAdapter);
}
}
});
}
public void fetchNewData(final String after_use, final FeedRecyclerViewAdapter feedRecyclerViewAdapter) {
Call<FeedData> feedDataCall_withAfter = redditAPI.getFeed_withAfter(after_use);
previousAfter = after_use;
feedDataCall_withAfter.clone().enqueue(new Callback<FeedData>() {
@Override
public void onResponse(Call<FeedData> call, Response<FeedData> response) {
Log.d(TAG, "onResponse: " + response.body().toString());
childrenData.addAll(response.body().getData().getChildren());
after = response.body().getData().getAfter();
feedRecyclerViewAdapter.notifyDataSetChanged();
}
@Override
public void onFailure(Call<FeedData> call, Throwable t) {
}
});
Log.d(TAG, "fetchNewData: " + childrenData.size());
}
}
公共类FragmentHome扩展了Fragment{
专用字符串BASE_URL=”https://www.reddit.com";
//初始化改装并一次性呼叫
改装改装=新改装.Builder（）
.baseUrl（基本URL）
.addConverterFactory（GsonConverterFactory.create（））
.build（）；
RedditAPI RedditAPI=reformation.create（RedditAPI.class）；
私有静态最终字符串标记=“FragmentHome”；
私人RecyclerView feedRecyclerView；
私有列表childrenData；
私有字符串之后；
私有字符串previousAfter=“”；
私有布尔isScrolling=false；
私人物品；
私人物品；
私有int-scrolledOutItems；
私人语境；
私有视图膨胀视图；
公共家庭（）{
}
公共碎片主页（上下文）{
this.context=context；//我还得到一些属性警告
不过现在这里并不重要。
}
@可空
@凌驾
创建视图时的公共视图（@NonNull LayoutInflater inflater、@Nullable ViewGroup container、@Nullable Bundle savedInstanceState）{
充气视图=充气机。充气（R.layout.fragment\u home，container，false）；
initRecyclerView（）；
返回膨胀视图；
}
public void initGetData（）{
调用feedDataCall=redditAPI.getFeed（）；
enqueue（新的回调函数（）{
@凌驾
公共void onResponse（呼叫、最终响应）{
/*Log.d（标记“onResponse:+response.body（）.getData（）.getChildren（）.get（0.getDataModel（）.getTitle（））；
Log.d（标记“onResponse:+response.body（）.getData（）.getChildren（））*/
childrenData=response.body（）.getData（）.getChildren（）；
Log.d（标记“onResponse:+childrenData”）；
after=response.body（）.getData（）.getAfter（）；
Log.d（标签“onResponse:”+之后）；
}
@凌驾
失败时公共无效（调用调用，可丢弃的t）{
Log.d（标记“onFailure:+t.getMessage（））；
}
});
}
public void initRecyclerView（）{
feedRecyclerView=膨胀视图.findViewById（R.id.recyclerView）；
最终FeedRecycleServiceAdapter FeedRecycleServiceAdapter=新的FeedRecycleServiceAdapter（childrenData，上下文）；
FeedRecycleView.setAdapter（FeedRecycleWebAdapter）；
最终LinearLayoutManager LinearLayoutManager=新的LinearLayoutManager（上下文）；
feedRecyclerView.setLayoutManager（linearLayoutManager）；
feedRecyclerView.addOnScrollListener（新的RecyclerView.OnScrollListener（）{
@凌驾
CrollStateChanged上的公共void（@NonNull RecyclerView RecyclerView，int newState）{
super.onScrollStateChanged（recyclerView、newState）；
isScrolling=true；
}
@凌驾
已克隆的公共void（@NonNull RecyclerView RecyclerView，int-dx，int-dy）{
super.onScrolled（recyclerView、dx、dy）；
currentItems=linearLayoutManager.getChildCount（）；
totalItems=linearLayoutManager.getItemCount（）；
scrolledOutItems=linearLayoutManager.findFirstVisibleItemPosition（）；
如果（isScrolling&&（totalItems-1==currentItems+scrolledOutItems）&（！previousAfter.equals（after）））{
IsCrolling=假；
fetchNewData（FeedRecycleServiceAdapter之后）；
}
}
});
}
public void fetchNewData（使用后的最终字符串，final feedRecycleServiceAdapter feedRecycleServiceAdapter）{
调用feedDataCall_withAfter=redditAPI.getFeed_withAfter（使用后）；
previousAfter=使用后；
feedDataCall_withAfter.clone（）.enqueue（新回调（）命令）{
@凌驾
公共void onResponse（调用、响应）{
Log.d（标记“onResponse:+response.body（）.toString（））；
addAll（response.body（）.getData（）.getChildren（））；
after=response.body（）.getData（）.getAfter（）；
FeedRecycleServiceAdapter.notifyDataSetChanged（）；
}
@凌驾
失败时公共无效（调用调用，可丢弃的t）{
}
});
Log.d（标记“fetchNewData:+childrenData.size（））；
}
}