您好我是Android的新手，并且遇到了Recycler View和片段的问题。 有3个片段可以滑动，并且有一个带有适配器的回收器视图。我从http://developer.android.com/training/implementing-navigation/lateral.html获得了一个示例代码，但我在List对象中得到了空指针异常。提前致谢。我的代码如下所示：
public class TripHistoryActivity extends FragmentActivity implements ActionBar.TabListener{
private static final String TAG = "TripHistoryActivity";
private CommonDBHelper commonDBHelper;
private List<Trip> allTrips;
private ArrayList<Trip> currentTrips;
private ArrayList<Trip> pastTrips;
private ArrayList<Trip> futureTrips;
AppSectionsPagerAdapter mAppSectionsPagerAdapter;
ViewPager mViewPager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_trip_history);
commonDBHelper = new CommonDBHelper(this);
TripDBHelper tripDBHelper = new TripDBHelper(commonDBHelper.getWritableDatabase());
allTrips = tripDBHelper.getAllTrips();
currentTrips = new ArrayList<>();
pastTrips = new ArrayList<>();
futureTrips = new ArrayList<>();
Calendar calendar = Calendar.getInstance();
int currentDay = calendar.get(calendar.DAY_OF_MONTH);
int currentMonth = calendar.get(calendar.MONTH);
int currentYear = calendar.get(calendar.YEAR);
for (Trip trip : allTrips) {
String date = trip.getTime().substring(0, trip.getTime().indexOf(" "));
String[] dates = date.split("/");
if (currentYear < Integer.valueOf(dates[2])) {
futureTrips.add(trip);
}
else if (currentYear > Integer.valueOf(dates[2])
|| currentMonth > Integer.valueOf(dates[0])
|| currentDay > Integer.valueOf(dates[1])) {
pastTrips.add(trip);
}
else {
currentTrips.add(trip);
}
}
mAppSectionsPagerAdapter = new AppSectionsPagerAdapter(getSupportFragmentManager());
final ActionBar actionBar = getActionBar();
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mAppSectionsPagerAdapter);
mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
@Override
public void onPageSelected(int position) {
// When swiping between different app sections, select the corresponding tab.
// We can also use ActionBar.Tab#select() to do this if we have a reference to the
// Tab.
actionBar.setSelectedNavigationItem(position);
}
});
for (int i = 0; i < mAppSectionsPagerAdapter.getCount(); i++) {
// Create a tab with text corresponding to the page title defined by the adapter.
// Also specify this Activity object, which implements the TabListener interface, as the
// listener for when this tab is selected.
actionBar.addTab(
actionBar.newTab()
.setText(mAppSectionsPagerAdapter.getPageTitle(i))
.setTabListener(this));
}
}
@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
// When the given tab is selected, switch to the corresponding page in the ViewPager.
mViewPager.setCurrentItem(tab.getPosition());
}
@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
/**
* A {@link FragmentPagerAdapter} that returns a fragment corresponding to one of the primary
* sections of the app.
*/
public class AppSectionsPagerAdapter extends FragmentPagerAdapter {
public AppSectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public android.support.v4.app.Fragment getItem(int i) {
switch (i) {
case 0:
android.support.v4.app.Fragment currentTripsFragment = new CurrentTripsFragment();
Bundle currentTripsBundle = new Bundle();
currentTripsBundle.putParcelableArrayList("currentTrips", currentTrips);
currentTripsFragment.setArguments(currentTripsBundle);
return currentTripsFragment;
case 1:
android.support.v4.app.Fragment pastTripsFragment = new PastTripsFragment();
Bundle pastTripsBundle = new Bundle();
pastTripsBundle.putParcelableArrayList("pastTrips", pastTrips);
pastTripsFragment.setArguments(pastTripsBundle);
return pastTripsFragment;
default:
android.support.v4.app.Fragment futureTripsFragment = new FutureTripsFragment();
Bundle futureTripsBundle = new Bundle();
futureTripsBundle.putParcelableArrayList("futureTrips", futureTrips);
futureTripsFragment.setArguments(futureTripsBundle);
return futureTripsFragment;
}
}
@Override
public int getCount() {
return 3;
}
@Override
public CharSequence getPageTitle(int position) {
switch (position) {
case 0:
return "Current Trips";
case 1:
return "Past Trips";
case 2:
return "Future Trips";
default:
return "Fault";
}
}
}
/**
* A fragment that launches other parts of the demo application.
*/
public static class CurrentTripsFragment extends android.support.v4.app.Fragment {
private RecyclerView mRecyclerView;
private Context context;
List<Trip> currentTrips;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_current_trips, container, false);
context = rootView.getContext();
mRecyclerView = (RecyclerView) rootView.findViewById(R.id.recycler_view);
currentTrips = getArguments().getParcelableArrayList("currentTrips");
Log.d(TripHistoryActivity.TAG, String.valueOf(currentTrips == null));
RecyclerView.LayoutManager manager = new LinearLayoutManager(context);
mRecyclerView.setLayoutManager(manager);
mRecyclerView.setItemAnimator(new DefaultItemAnimator());
CardViewAdapter cardViewAdapter = new CardViewAdapter(currentTrips);
mRecyclerView.setAdapter(cardViewAdapter);
cardViewAdapter.setOnItemClick(new CardViewAdapter.OnItemClick() {
@Override
public void onItemClick(int position) {
handleItemClick(position);
}
});
return rootView;
}
public void handleItemClick (int position) {
Toast.makeText(context, "Clicked " + position, Toast.LENGTH_SHORT).show();
Intent intent = new Intent(context, ViewTripActivity.class);
intent.putExtra("trip", currentTrips.get(position));
startActivity(intent);
}
}
还有2个其他片段与此片段相同，所以我省略了它们。
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import com.nyu.cs9033.eta.R;
import com.nyu.cs9033.eta.models.Trip;
import java.util.List;
/**
* Created by Ray on 2016/4/2.
*/
public class CardViewAdapter extends RecyclerView.Adapter<CardViewAdapter.CardViewHolder> {
private final List<Trip> trips;
public CardViewAdapter (List<Trip> trips) {
this.trips = trips;
}
public interface OnItemClick {
public void onItemClick(int position);
}
private OnItemClick onItemClick;
public void setOnItemClick (OnItemClick onItemClick) {
this.onItemClick = onItemClick;
}
@Override
public CardViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.history_list_item, parent, false);
return new CardViewHolder(view);
}
@Override
public void onBindViewHolder(final CardViewHolder holder, final int position) {
holder.name.setText(trips.get(position).getName());
holder.location.setText(trips.get(position).getLocation());
holder.time.setText(trips.get(position).getTime());
if (onItemClick != null) {
holder.itemView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
onItemClick.onItemClick(position);
}
});
}
}
@Override
public int getItemCount() {
return trips.size();
}
class CardViewHolder extends RecyclerView.ViewHolder {
private TextView name;
private TextView location;
private TextView time;
public CardViewHolder (View view) {
super(view);
name = (TextView) view.findViewById(R.id.card_trip_name);
location = (TextView) view.findViewById(R.id.card_trip_location);
time = (TextView) view.findViewById(R.id.card_trip_time);
}
}
}
这就是logcat所说的：
FATAL EXCEPTION: mainProcess: com.nyu.cs9033.eta, PID: 14018
java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()'
on a null object reference
at com.nyu.cs9033.eta.adapters.CardViewAdapter.getItemCount(CardViewAdapter.java: 57)
at android.support.v7.widget.RecyclerView.onMeasure(RecyclerView.java: 2357)
at android.view.View.measure(View.java: 18796)
at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java: 715)
at android.widget.RelativeLayout.onMeasure(RelativeLayout.java: 461)
at android.view.View.measure(View.java: 18796)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java: 5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java: 194)
at android.view.View.measure(View.java: 18796)
at android.support.v4.view.ViewPager.onMeasure(ViewPager.java: 1488)
at android.view.View.measure(View.java: 18796)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java: 5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java: 194)
at android.view.View.measure(View.java: 18796)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java: 5951)
at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java: 446)
at android.view.View.measure(View.java: 18796)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java: 5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java: 194)
at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java: 2643)
at android.view.View.measure(View.java: 18796)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java: 2108)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java: 1224)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java: 1460)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java: 1115)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java: 6023)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java: 858)
at android.view.Choreographer.doCallbacks(Choreographer.java: 670)
at android.view.Choreographer.doFrame(Choreographer.java: 606)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java: 844)
at android.os.Handler.handleCallback(Handler.java: 739)
at android.os.Handler.dispatchMessage(Handler.java: 95)
at android.os.Looper.loop(Looper.java: 148)
at android.app.ActivityThread.main(ActivityThread.java: 5422)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java: 726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java: 616)
答案 0 :(得分：1)
您应该从onCreate()而不是onCreateView()获取捆绑包。在Fragment类中尝试以下更改：
public static class CurrentTripsFragment extends android.support.v4.app.Fragment {
private RecyclerView mRecyclerView;
private Context context;
List<Trip> currentTrips;
@Override
public void onCreate(Bundle savedInstanceState){
super().onCreate(savedInstanceState);
if(getArguments()!= null){
currentTrips = getArguments().getParcelableArrayList("currentTrips");
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_current_trips, container, false);
context = rootView.getContext();
mRecyclerView = (RecyclerView) rootView.findViewById(R.id.recycler_view);
Log.d(TripHistoryActivity.TAG, String.valueOf(currentTrips == null));
RecyclerView.LayoutManager manager = new LinearLayoutManager(context);
mRecyclerView.setLayoutManager(manager);
mRecyclerView.setItemAnimator(new DefaultItemAnimator());
CardViewAdapter cardViewAdapter = new CardViewAdapter(currentTrips);
mRecyclerView.setAdapter(cardViewAdapter);
cardViewAdapter.setOnItemClick(new CardViewAdapter.OnItemClick() {
@Override
public void onItemClick(int position) {
handleItemClick(position);
}
});
return rootView;
}
public void handleItemClick (int position) {
Toast.makeText(context, "Clicked " + position, Toast.LENGTH_SHORT).show();
Intent intent = new Intent(context, ViewTripActivity.class);
intent.putExtra("trip", currentTrips.get(position));
startActivity(intent);
}
}
答案 1 :(得分：0)
您在列表中遇到空指针异常，用于在回收站视图中显示列表项...请检查您是否在列表中获取数据...问题似乎在这里：< / p>
CardViewAdapter cardViewAdapter = new CardViewAdapter(currentTrips);
在这里：
currentTrips = getArguments().getParcelableArrayList("currentTrips");