实现 RecyclerView 时出现 NullPointerException
NullPointerException while implementing RecyclerView
我正在尝试实现 RecyclerView 以显示 Fragment 中的项目列表。
片段代码：
import android.app.Activity;
import android.os.Bundle;
import android.support.v4.app.LoaderManager;
import android.support.v4.content.Loader;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.lang.reflect.Type;
import java.util.List;
public class SearchFragment extends BaseFragment implements LoaderManager.LoaderCallbacks<StringBuilder> {
private RecyclerView mRecyclerView;
private SearchAdapter mAdapter;
private RecyclerView.LayoutManager mLayoutManager;
private String searchQuery;
private User user;
private OnFragmentInteractionListener mListener;
public SearchFragment() {
}
public static SearchFragment newInstance() {
SearchFragment fragment = new SearchFragment();
return fragment;
}
public void setSearchQuery(String searchQuery) {
this.searchQuery = searchQuery;
}
public void search() {
try {
Bundle bundle = new Bundle();
bundle.putString(API.DISH_SEARCH_KEY, searchQuery);
getLoaderManager().restartLoader(LoaderConstants.DISH_SEARCH_LOADER_ID, bundle, SearchFragment.this).forceLoad();
} catch (IllegalStateException e) {
// thrown when Fragment not attached to Activity
}
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments() != null) {
user = new Gson().fromJson(getArguments().getString(API.USER_KEY), User.class);
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_search, container, false);
mRecyclerView = (RecyclerView) view.findViewById(R.id.my_recycler_view);
mLayoutManager = new LinearLayoutManager(getActivity());
mRecyclerView.setLayoutManager(mLayoutManager);
search();
return view;
}
@Override
public Loader<StringBuilder> onCreateLoader(int id, Bundle bundle) {
String searchQuery = bundle.getString(API.DISH_SEARCH_KEY);
String url = String.format(API.DISH_SEARCH_URL, searchQuery);
bundle.putString(API.URL_KEY, url);
bundle.putString(API.TYPE_KEY, API.TYPE_GET);
return new CravyAsyncLoader(getActivity(), bundle);
}
@Override
public void onLoadFinished(Loader<StringBuilder> loader, StringBuilder data) {
if (Util.isEmptyJsonResponse(data)) {
Toast.makeText(getActivity(), "No dishes found!", Toast.LENGTH_LONG).show();
return;
}
List<Dish> dishes;
try {
// Convert JSONArray response to List<Dish>
Type listType = new TypeToken<List<Dish>>() {
}.getType();
dishes = (List<Dish>) new Gson().fromJson(data.toString(), listType);
} catch (Exception ex) {
Toast.makeText(getActivity(), "Something went wrong. Try again.", Toast.LENGTH_LONG).show();
return;
}
if (dishes != null) {
mAdapter = new SearchAdapter(dishes, SearchFragment.this);
mRecyclerView.setAdapter(mAdapter);
}
}
@Override
public void onLoaderReset(Loader<StringBuilder> loader) {
}
}
适配器：
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;
import android.widget.TextView;
import com.google.gson.Gson;
import java.util.List;
public class SearchAdapter extends RecyclerView.Adapter<SearchAdapter.ViewHolder> {
private List<Dish> mDishes;
private Fragment fragment;
public SearchAdapter(List<Dish> dishes, Fragment fragment) {
mDishes = dishes;
this.fragment = fragment;
}
// Provide a reference to the views for each data item
// Complex data items may need more than one view per item, and
// you provide access to all the views for a data item in a view holder
public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
// each data item is just a string in this case
public LinearLayout mView;
private Fragment fragment;
public ViewHolder(LinearLayout itemView, Fragment fragment) {
super(itemView);
this.fragment = fragment;
itemView.setClickable(true);
itemView.setOnClickListener(this);
}
@Override
public void onClick(View v) {
TextView textView = (TextView) v.findViewById(R.id.dish_id);
String id = textView.getText().toString();
Dish dish = getDishById(id);
Bundle bundle = new Bundle();
bundle.putString(API.DISH_KEY, new Gson().toJson(dish));
// Create new fragment and transaction
DishDetailsFragment dishDetailsFragment = DishDetailsFragment.newInstance(bundle);
FragmentTransaction transaction = fragment.getFragmentManager().beginTransaction();
// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.content, dishDetailsFragment);
transaction.addToBackStack(null);
// Commit the transaction
transaction.commit();
}
}
// Create new views (invoked by the layout manager)
@Override
public SearchAdapter.ViewHolder onCreateViewHolder(ViewGroup parent,
int viewType) {
LinearLayout view = (LinearLayout) LayoutInflater.from(parent.getContext())
.inflate(R.layout.search_list_item, parent, false);
return new ViewHolder(view, fragment);
}
// Replace the contents of a view (invoked by the layout manager)
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
// - get element from your dataset at this position
// - replace the contents of the view with that element
((TextView) holder.mView.findViewById(R.id.dish_id)).setText(mDishes.get(position).getId());
((TextView) holder.mView.findViewById(R.id.dish_name)).setText(mDishes.get(position).getName());
((TextView) holder.mView.findViewById(R.id.restaurant_name)).setText(mDishes.get(position).getRestaurantId());
try {
((TextView) holder.mView.findViewById(R.id.price)).setText(String.valueOf(mDishes.get(position).getPrice().getValue()));
} catch (Exception ex) {
((TextView) holder.mView.findViewById(R.id.price)).setText("No Price Available");
}
}
// Return the size of your dataset (invoked by the layout manager)
@Override
public int getItemCount() {
return mDishes.size();
}
public Dish getDishById(String id) {
for (Dish dish : mDishes) {
if (dish.getId().equals(id))
return dish;
}
return null;
}
}
我在 Adapter 行
中遇到错误
((TextView) holder.mView.findViewById(R.id.dish_id)).setText(mDishes.get(position).getId());
错误：
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.LinearLayout.findViewById(int)' on a null object reference
at com.fonduetech.cravy.adapter.SearchAdapter.onBindViewHolder(SearchAdapter.java:87)
at com.fonduetech.cravy.adapter.SearchAdapter.onBindViewHolder(SearchAdapter.java:21)
at android.support.v7.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:4138)
at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:3448)
at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:3340)
at android.support.v7.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:1810)
at android.support.v7.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1306)
at android.support.v7.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1269)
at android.support.v7.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:523)
at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:1988)
at android.support.v7.widget.RecyclerView.onLayout(RecyclerView.java:2237)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.support.design.widget.CoordinatorLayout.layoutChild(CoordinatorLayout.java:1000)
at android.support.design.widget.CoordinatorLayout.onLayoutChild(CoordinatorLayout.java:710)
at android.support.design.widget.ViewOffsetBehavior.onLayoutChild(ViewOffsetBehavior.java:42)
at android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onLayoutChild(AppBarLayout.java:982)
at android.support.design.widget.CoordinatorLayout.onLayout(CoordinatorLayout.java:723)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.support.v4.widget.DrawerLayout.onLayout(DrawerLayout.java:907)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1466)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1466)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2086)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1843)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1061)
at android.view.ViewR
我尝试调试应用程序。它将 mView 显示为 null。为什么？
public ViewHolder(LinearLayout itemView, Fragment fragment) {
super(itemView);
this.fragment = fragment;
this.mView = itemView; //this line is missing
itemView.setClickable(true);
itemView.setOnClickListener(this);
}
你用过 Android Studio 吗？？
如果是，只需将其包含在 build.grade 中：
compile 'com.android.support:cardview-v7:21.0.+'
compile 'com.android.support:recyclerview-v7:21.0.+'
然后只需同步成绩或只需按顶部的 gradle 按钮即可。我想这会给你一些结果
如果您的 RecyclerView 适配器没有数据意味着您没有调用 setAdapter 那么它是 return 错误。
因此默认情况下，如果您有数据，则必须将 RecyclerView 设置为不可见 属性，然后先设置数据，然后设置 RecyclerView 的可见 属性。
我正在尝试实现 RecyclerView 以显示 Fragment 中的项目列表。
片段代码：
import android.app.Activity;
import android.os.Bundle;
import android.support.v4.app.LoaderManager;
import android.support.v4.content.Loader;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.lang.reflect.Type;
import java.util.List;
public class SearchFragment extends BaseFragment implements LoaderManager.LoaderCallbacks<StringBuilder> {
private RecyclerView mRecyclerView;
private SearchAdapter mAdapter;
private RecyclerView.LayoutManager mLayoutManager;
private String searchQuery;
private User user;
private OnFragmentInteractionListener mListener;
public SearchFragment() {
}
public static SearchFragment newInstance() {
SearchFragment fragment = new SearchFragment();
return fragment;
}
public void setSearchQuery(String searchQuery) {
this.searchQuery = searchQuery;
}
public void search() {
try {
Bundle bundle = new Bundle();
bundle.putString(API.DISH_SEARCH_KEY, searchQuery);
getLoaderManager().restartLoader(LoaderConstants.DISH_SEARCH_LOADER_ID, bundle, SearchFragment.this).forceLoad();
} catch (IllegalStateException e) {
// thrown when Fragment not attached to Activity
}
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments() != null) {
user = new Gson().fromJson(getArguments().getString(API.USER_KEY), User.class);
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_search, container, false);
mRecyclerView = (RecyclerView) view.findViewById(R.id.my_recycler_view);
mLayoutManager = new LinearLayoutManager(getActivity());
mRecyclerView.setLayoutManager(mLayoutManager);
search();
return view;
}
@Override
public Loader<StringBuilder> onCreateLoader(int id, Bundle bundle) {
String searchQuery = bundle.getString(API.DISH_SEARCH_KEY);
String url = String.format(API.DISH_SEARCH_URL, searchQuery);
bundle.putString(API.URL_KEY, url);
bundle.putString(API.TYPE_KEY, API.TYPE_GET);
return new CravyAsyncLoader(getActivity(), bundle);
}
@Override
public void onLoadFinished(Loader<StringBuilder> loader, StringBuilder data) {
if (Util.isEmptyJsonResponse(data)) {
Toast.makeText(getActivity(), "No dishes found!", Toast.LENGTH_LONG).show();
return;
}
List<Dish> dishes;
try {
// Convert JSONArray response to List<Dish>
Type listType = new TypeToken<List<Dish>>() {
}.getType();
dishes = (List<Dish>) new Gson().fromJson(data.toString(), listType);
} catch (Exception ex) {
Toast.makeText(getActivity(), "Something went wrong. Try again.", Toast.LENGTH_LONG).show();
return;
}
if (dishes != null) {
mAdapter = new SearchAdapter(dishes, SearchFragment.this);
mRecyclerView.setAdapter(mAdapter);
}
}
@Override
public void onLoaderReset(Loader<StringBuilder> loader) {
}
}
适配器：
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;
import android.widget.TextView;
import com.google.gson.Gson;
import java.util.List;
public class SearchAdapter extends RecyclerView.Adapter<SearchAdapter.ViewHolder> {
private List<Dish> mDishes;
private Fragment fragment;
public SearchAdapter(List<Dish> dishes, Fragment fragment) {
mDishes = dishes;
this.fragment = fragment;
}
// Provide a reference to the views for each data item
// Complex data items may need more than one view per item, and
// you provide access to all the views for a data item in a view holder
public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
// each data item is just a string in this case
public LinearLayout mView;
private Fragment fragment;
public ViewHolder(LinearLayout itemView, Fragment fragment) {
super(itemView);
this.fragment = fragment;
itemView.setClickable(true);
itemView.setOnClickListener(this);
}
@Override
public void onClick(View v) {
TextView textView = (TextView) v.findViewById(R.id.dish_id);
String id = textView.getText().toString();
Dish dish = getDishById(id);
Bundle bundle = new Bundle();
bundle.putString(API.DISH_KEY, new Gson().toJson(dish));
// Create new fragment and transaction
DishDetailsFragment dishDetailsFragment = DishDetailsFragment.newInstance(bundle);
FragmentTransaction transaction = fragment.getFragmentManager().beginTransaction();
// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.content, dishDetailsFragment);
transaction.addToBackStack(null);
// Commit the transaction
transaction.commit();
}
}
// Create new views (invoked by the layout manager)
@Override
public SearchAdapter.ViewHolder onCreateViewHolder(ViewGroup parent,
int viewType) {
LinearLayout view = (LinearLayout) LayoutInflater.from(parent.getContext())
.inflate(R.layout.search_list_item, parent, false);
return new ViewHolder(view, fragment);
}
// Replace the contents of a view (invoked by the layout manager)
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
// - get element from your dataset at this position
// - replace the contents of the view with that element
((TextView) holder.mView.findViewById(R.id.dish_id)).setText(mDishes.get(position).getId());
((TextView) holder.mView.findViewById(R.id.dish_name)).setText(mDishes.get(position).getName());
((TextView) holder.mView.findViewById(R.id.restaurant_name)).setText(mDishes.get(position).getRestaurantId());
try {
((TextView) holder.mView.findViewById(R.id.price)).setText(String.valueOf(mDishes.get(position).getPrice().getValue()));
} catch (Exception ex) {
((TextView) holder.mView.findViewById(R.id.price)).setText("No Price Available");
}
}
// Return the size of your dataset (invoked by the layout manager)
@Override
public int getItemCount() {
return mDishes.size();
}
public Dish getDishById(String id) {
for (Dish dish : mDishes) {
if (dish.getId().equals(id))
return dish;
}
return null;
}
}
我在 Adapter 行
((TextView) holder.mView.findViewById(R.id.dish_id)).setText(mDishes.get(position).getId());
错误：
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.LinearLayout.findViewById(int)' on a null object reference
at com.fonduetech.cravy.adapter.SearchAdapter.onBindViewHolder(SearchAdapter.java:87)
at com.fonduetech.cravy.adapter.SearchAdapter.onBindViewHolder(SearchAdapter.java:21)
at android.support.v7.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:4138)
at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:3448)
at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:3340)
at android.support.v7.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:1810)
at android.support.v7.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1306)
at android.support.v7.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1269)
at android.support.v7.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:523)
at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:1988)
at android.support.v7.widget.RecyclerView.onLayout(RecyclerView.java:2237)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.support.design.widget.CoordinatorLayout.layoutChild(CoordinatorLayout.java:1000)
at android.support.design.widget.CoordinatorLayout.onLayoutChild(CoordinatorLayout.java:710)
at android.support.design.widget.ViewOffsetBehavior.onLayoutChild(ViewOffsetBehavior.java:42)
at android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onLayoutChild(AppBarLayout.java:982)
at android.support.design.widget.CoordinatorLayout.onLayout(CoordinatorLayout.java:723)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.support.v4.widget.DrawerLayout.onLayout(DrawerLayout.java:907)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1466)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1466)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579)
at android.widget.FrameLayout.onLayout(FrameLayout.java:514)
at android.view.View.layout(View.java:15671)
at android.view.ViewGroup.layout(ViewGroup.java:5038)
at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2086)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1843)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1061)
at android.view.ViewR
我尝试调试应用程序。它将 mView 显示为 null。为什么？
public ViewHolder(LinearLayout itemView, Fragment fragment) {
super(itemView);
this.fragment = fragment;
this.mView = itemView; //this line is missing
itemView.setClickable(true);
itemView.setOnClickListener(this);
}
你用过 Android Studio 吗？？
如果是，只需将其包含在 build.grade 中：
compile 'com.android.support:cardview-v7:21.0.+'
compile 'com.android.support:recyclerview-v7:21.0.+'
然后只需同步成绩或只需按顶部的 gradle 按钮即可。我想这会给你一些结果
如果您的 RecyclerView 适配器没有数据意味着您没有调用 setAdapter 那么它是 return 错误。
因此默认情况下，如果您有数据，则必须将 RecyclerView 设置为不可见 属性，然后先设置数据，然后设置 RecyclerView 的可见 属性。