我在Android中创建了一个浮动操作按钮或FAB，可以处理重要操作。在我们的应用程序中，我们有一个包含3个菜单项的浮动操作菜单，每个菜单项用于按名称，日期和评级排序电影结果我使用java比较器并覆盖compareTo方法来比较Lhs和Rhs电影对象并返回比较结果。
但是，当我按下其中一个菜单时，例如：按名称排序电影结果，总是出现错误java.lang.NullPointerException
我的代码存在什么问题？
此代码中的java.lang.NullPointerException：
Fragment mFragment = (Fragment) mAdapter.instantiateItem(viewPager, viewPager.getCurrentItem());
和Logcat：
04-16 02:10:22.137 3410-3410/com.qferiz.qferiztrafficinfo E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.NullPointerException
at com.qferiz.qferiztrafficinfo.activities.MainActivity.onClick(MainActivity.java:229)
at android.view.View.performClick(View.java:4336)
at android.view.View$PerformClick.run(View.java:17853)
at android.os.Handler.handleCallback(Handler.java:800)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:194)
at android.app.ActivityThread.main(ActivityThread.java:5469)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:525)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:857)
我已在顶级公共类MainActivity上声明：
private ViewPagerAdapter mAdapter;
我的代码出了什么问题？ 请帮帮我.. :(
这是我在MainActivity.java中的代码：
package com.qferiz.qferiztrafficinfo.activities;
import android.content.Intent;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ImageView;
import android.widget.Toast;
import com.oguzdev.circularfloatingactionmenu.library.FloatingActionButton;
import com.oguzdev.circularfloatingactionmenu.library.FloatingActionMenu;
import com.oguzdev.circularfloatingactionmenu.library.SubActionButton;
import com.qferiz.qferiztrafficinfo.extras.SortListener;
import com.qferiz.qferiztrafficinfo.fragments.FragmentBoxOffice;
import com.qferiz.qferiztrafficinfo.fragments.FragmentSearch;
import com.qferiz.qferiztrafficinfo.fragments.FragmentUpcoming;
import com.qferiz.qferiztrafficinfo.fragments.MyFragment;
import com.qferiz.qferiztrafficinfo.fragments.NavigationDrawerFragment;
import com.qferiz.qferiztrafficinfo.R;
import com.qferiz.qferiztrafficinfo.logging.L;
import it.neokree.materialtabs.MaterialTab;
import it.neokree.materialtabs.MaterialTabHost;
import it.neokree.materialtabs.MaterialTabListener;
public class MainActivity extends ActionBarActivity implements MaterialTabListener, View.OnClickListener {
private Toolbar toolbar;
private ViewPager viewPager;
private MaterialTabHost tabHost;
public static final int MOVIES_SEARCH_RESULT = 0;
public static final int MOVIES_HITS = 1;
public static final int MOVIES_UPCOMING = 2;
//tag associated with the FAB menu button that sorts by name
private static final String TAG_SORT_NAME = "sortName";
//tag associated with the FAB menu button that sorts by date
private static final String TAG_SORT_DATE = "sortDate";
//tag associated with the FAB menu button that sorts by ratings
private static final String TAG_SORT_RATINGS = "sortRatings";
private ViewPagerAdapter mAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
toolbar = (Toolbar) findViewById(R.id.app_bar);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayShowHomeEnabled(true);
NavigationDrawerFragment drawerFragment = (NavigationDrawerFragment)
getSupportFragmentManager().findFragmentById(R.id.fragment_navigation_drawer);
drawerFragment.setUp(R.id.fragment_navigation_drawer, (DrawerLayout) findViewById(R.id.drawer_layout), toolbar);
tabHost = (MaterialTabHost) findViewById(R.id.materialTabHost);
viewPager = (ViewPager) findViewById(R.id.viewPager);
//mPager.setAdapter(new MyPagerAdapter(getSupportFragmentManager()));
ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(adapter);
viewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
@Override
public void onPageSelected(int position) {
super.onPageSelected(position);
tabHost.setSelectedNavigationItem(position);
}
});
for (int i = 0; i < adapter.getCount(); i++) {
// TAB TEXT
//tabHost.addTab(tabHost.newTab().setText(adapter.getPageTitle(i)).setTabListener(this));
// TAB ICON
tabHost.addTab(tabHost.newTab().setIcon(adapter.getIcon(i)).setTabListener(this));
}
// CircularFloatingActionMenu Button
setupFAB();
}
private void setupFAB() {
// CircularFloatingActionMenu Button
//define the icon for the main floating action button
ImageView iconActionButton = new ImageView(this);
iconActionButton.setImageResource(R.drawable.ic_action_new);
// Create a button to attach the menu:
//set the appropriate background for the main floating action button along with its icon
FloatingActionButton actionButton = new FloatingActionButton.Builder(this)
.setContentView(iconActionButton)
.setBackgroundDrawable(R.drawable.selector_button_red)
.build();
// Create menu items:
//define the icons for the sub action buttons
ImageView iconSortName = new ImageView(this);
iconSortName.setImageResource(R.drawable.ic_notifications_white_36dp);
ImageView iconSortDate = new ImageView(this);
iconSortDate.setImageResource(R.drawable.ic_map_white_36dp);
ImageView iconSortRatings = new ImageView(this);
iconSortRatings.setImageResource(R.drawable.ic_directions_white_36dp);
//set the background for all the sub buttons
SubActionButton.Builder itemBuilder = new SubActionButton.Builder(this);
itemBuilder.setBackgroundDrawable(getResources().getDrawable(R.drawable.selector_sub_button_gray));
//build the sub buttons
SubActionButton buttonSortName = itemBuilder.setContentView(iconSortName).build();
SubActionButton buttonSortDate = itemBuilder.setContentView(iconSortDate).build();
SubActionButton buttonSortRatings = itemBuilder.setContentView(iconSortRatings).build();
//to determine which button was clicked, set Tags on each button
buttonSortName.setTag(TAG_SORT_NAME);
buttonSortDate.setTag(TAG_SORT_DATE);
buttonSortRatings.setTag(TAG_SORT_RATINGS);
buttonSortName.setOnClickListener(this);
buttonSortDate.setOnClickListener(this);
buttonSortRatings.setOnClickListener(this);
//add the sub buttons to the main floating action button
FloatingActionMenu actionMenu = new FloatingActionMenu.Builder(this)
.addSubActionView(buttonSortName)
.addSubActionView(buttonSortDate)
.addSubActionView(buttonSortRatings)
.attachTo(actionButton)
.build();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
Toast.makeText(this, "Anda telah meng-Klik " + item.getTitle(), Toast.LENGTH_SHORT).show();
return true;
}
if (id == R.id.navigate) {
startActivity(new Intent(this, SubActivity.class));
}
if (id == R.id.tabWithLibrary) {
startActivity(new Intent(this, ActivityUsingTabLibrary.class));
}
if (id == R.id.vectorTestActivity) {
startActivity(new Intent(this, VectorTestActivity.class));
}
return super.onOptionsItemSelected(item);
}
@Override
public void onTabSelected(MaterialTab materialTab) {
viewPager.setCurrentItem(materialTab.getPosition());
}
@Override
public void onTabReselected(MaterialTab materialTab) {
}
@Override
public void onTabUnselected(MaterialTab materialTab) {
}
@Override
public void onClick(View view) {
Fragment mFragment = (Fragment) mAdapter.instantiateItem(viewPager, viewPager.getCurrentItem());
if (mFragment != null){
if (mFragment instanceof SortListener){
if (view.getTag().equals(TAG_SORT_NAME)){
//L.t(this,"Submenu SortName Clicked");
((SortListener) mFragment).onSortByName();
}
if (view.getTag().equals(TAG_SORT_DATE)){
//L.t(this,"Submenu SortDate Clicked");
((SortListener) mFragment).onSortByDate();
}
if (view.getTag().equals(TAG_SORT_RATINGS)){
//L.t(this,"Submenu SortRatings Clicked");
((SortListener) mFragment).onSortByRating();
}
}
}
else {
L.t(this, "Error Fragment Null");
}
}
private class ViewPagerAdapter extends FragmentStatePagerAdapter {
int icons[] = {R.drawable.ic_notifications_white_36dp,
R.drawable.ic_map_white_36dp,
R.drawable.ic_directions_white_36dp};
FragmentManager fragmentManager;
public ViewPagerAdapter(FragmentManager fm) {
super(fm);
fragmentManager = fm;
}
@Override
public Fragment getItem(int num) {
//return MyFragment.getInstance(num);
Fragment fragment = null;
switch (num){
case MOVIES_SEARCH_RESULT:
fragment = FragmentSearch.newInstance("", "");
break;
case MOVIES_HITS:
fragment = FragmentBoxOffice.newInstance("", "");
break;
case MOVIES_UPCOMING:
fragment = FragmentUpcoming.newInstance("", "");
break;
}
return fragment;
}
@Override
public int getCount() {
return 3; // Jumlah TAB
}
@Override
public CharSequence getPageTitle(int position) {
return getResources().getStringArray(R.array.tabs)[position];
}
private Drawable getIcon(int position) {
return getResources().getDrawable(icons[position]);
}
}
}
Code MovieSorter.java：
package com.qferiz.qferiztrafficinfo.extras;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
/**
* Created by Qferiz on 15/04/2015.
*/
public class MovieSorter {
public void sortMoviesByName(ArrayList<Movie> movies){
Collections.sort(movies, new Comparator<Movie>() {
@Override
public int compare(Movie lhs, Movie rhs) {
return lhs.getTitle().compareTo(rhs.getTitle());
}
});
}
public void sortMoviesByDate(ArrayList<Movie> movies){
Collections.sort(movies, new Comparator<Movie>() {
@Override
public int compare(Movie lhs, Movie rhs) {
return lhs.getReleaseDateTheater().compareTo(rhs.getReleaseDateTheater());
}
});
}
public void sortMoviesByRating(ArrayList<Movie> movies){
Collections.sort(movies, new Comparator<Movie>() {
@Override
public int compare(Movie lhs, Movie rhs) {
//return lhs.getReleaseDateTheater().compareTo(rhs.getReleaseDateTheater());
int ratingLhs = lhs.getAudienceScore();
int ratingRhs = rhs.getAudienceScore();
if (ratingLhs < ratingRhs){
return 1;
}
else if (ratingLhs > ratingRhs){
return -1;
}
else {
return 0;
}
}
});
}
}
FragmentBoxOffice.java的一些代码，方法：
@Override
public void onSortByName() {
//L.t(getActivity(),"Sort name Box Office");
movieSorter.sortMoviesByName(listMovies);
adapterBoxOffice.notifyDataSetChanged();
}
@Override
public void onSortByDate() {
//L.t(getActivity(),"Sort Date Box Office");
movieSorter.sortMoviesByDate(listMovies);
adapterBoxOffice.notifyDataSetChanged();
}
@Override
public void onSortByRating() {
//L.t(getActivity(),"Sort Rating Box Office");
movieSorter.sortMoviesByRating(listMovies);
adapterBoxOffice.notifyDataSetChanged();
}
这是FragmentBoxOffice.java中的完整代码：
package com.qferiz.qferiztrafficinfo.fragments;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import com.android.volley.AuthFailureError;
import com.android.volley.NetworkError;
import com.android.volley.NoConnectionError;
import com.android.volley.ParseError;
import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.ServerError;
import com.android.volley.TimeoutError;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.ImageLoader;
import com.android.volley.toolbox.JsonObjectRequest;
import com.qferiz.qferiztrafficinfo.R;
import com.qferiz.qferiztrafficinfo.adapters.AdapterBoxOffice;
import com.qferiz.qferiztrafficinfo.extras.Constants;
import com.qferiz.qferiztrafficinfo.extras.Movie;
import com.qferiz.qferiztrafficinfo.extras.MovieSorter;
import com.qferiz.qferiztrafficinfo.extras.MyApplication;
import com.qferiz.qferiztrafficinfo.extras.SortListener;
import com.qferiz.qferiztrafficinfo.logging.L;
import com.qferiz.qferiztrafficinfo.network.VolleySingleton;
import static com.qferiz.qferiztrafficinfo.extras.UrlEndpoints.*;
import static com.qferiz.qferiztrafficinfo.extras.Keys.EndpointBoxOffice.*;
import static com.qferiz.qferiztrafficinfo.extras.Constants.*;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
/**
* A simple {@link Fragment} subclass.
* Use the {@link FragmentBoxOffice#newInstance} factory method to
* create an instance of this fragment.
*/
public class FragmentBoxOffice extends Fragment implements SortListener {
// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";
public static final String URL_ROTTEN_TOMATOES_BOX_OFFICE =
"http://api.rottentomatoes.com/api/public/v1.0/lists/movies/box_office.json";
// TODO: Rename and change types of parameters
private String mParam1;
private String mParam2;
private VolleySingleton volleySingleton;
private ImageLoader imageLoader;
private RequestQueue requestQueue;
private ArrayList<Movie> listMovies = new ArrayList<>();
private DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
private RecyclerView listMovieHits;
private AdapterBoxOffice adapterBoxOffice;
private TextView textVolleyError;
private MovieSorter movieSorter;
/**
* Use this factory method to create a new instance of
* this fragment using the provided parameters.
*
* @param param1 Parameter 1.
* @param param2 Parameter 2.
* @return A new instance of fragment FragmentBoxOffice.
*/
// TODO: Rename and change types and number of parameters
public static FragmentBoxOffice newInstance(String param1, String param2) {
FragmentBoxOffice fragment = new FragmentBoxOffice();
Bundle args = new Bundle();
args.putString(ARG_PARAM1, param1);
args.putString(ARG_PARAM2, param2);
fragment.setArguments(args);
return fragment;
}
public static String getRequestUrl(int limit){
//return URL_ROTTEN_TOMATOES_BOX_OFFICE+"?apikey="+ MyApplication.API_KEY_ROTTEN_TOMATOES+"&limit="+limit;
return URL_BOX_OFFICE
+ URL_CHAR_QUESTION
+ URL_PARAM_API_KEY
+ MyApplication.API_KEY_ROTTEN_TOMATOES
+ URL_CHAR_AMEPERSAND
+ URL_PARAM_LIMIT + limit;
}
public FragmentBoxOffice() {
// Required empty public constructor
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments() != null) {
mParam1 = getArguments().getString(ARG_PARAM1);
mParam2 = getArguments().getString(ARG_PARAM2);
}
volleySingleton = VolleySingleton.getInstance();
requestQueue = volleySingleton.getRequestQueue();
sendJsonRequest();
}
private void sendJsonRequest() {
JsonObjectRequest request = new JsonObjectRequest(Request.Method.GET,
getRequestUrl(30),
(String)null,
new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
//L.t(getActivity(), response.toString());
textVolleyError.setVisibility(View.GONE);
listMovies = parseJSONResponse(response);
adapterBoxOffice.setMovieList(listMovies);
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
//L.t(getActivity(), error.getMessage() + "");
handleVolleyError(error);
}
});
requestQueue.add(request);
}
private void handleVolleyError(VolleyError error) {
textVolleyError.setVisibility(View.VISIBLE);
if (error instanceof TimeoutError || error instanceof NoConnectionError){
textVolleyError.setText(R.string.error_timeout);
} else if (error instanceof AuthFailureError){
//TODO:
textVolleyError.setText(R.string.error_auth_failure);
} else if (error instanceof ServerError){
//TODO:
textVolleyError.setText(R.string.error_auth_failure);
} else if (error instanceof NetworkError){
//TODO:
textVolleyError.setText(R.string.error_network);
} else if (error instanceof ParseError){
//TODO:
textVolleyError.setText(R.string.error_parser);
}
}
private ArrayList<Movie> parseJSONResponse(JSONObject response) {
ArrayList<Movie> listMovies = new ArrayList<>();
if (response != null && response.length() > 0){
try {
JSONArray arrayMovies = response.getJSONArray(KEY_MOVIES);
for (int i = 0; i < arrayMovies.length(); i++){
long id = -1;
String title = Constants.NA;
String releaseDates = Constants.NA;
int audienceScore = -1;
String synopsis = Constants.NA;
String urlThumbnail = Constants.NA;
JSONObject currentMovies = arrayMovies.getJSONObject(i);
// get the id of the current movie
if (currentMovies.has(KEY_ID) && !currentMovies.isNull(KEY_ID)){
id = currentMovies.getLong(KEY_ID);
}
// get the title of the current movie
if (currentMovies.has(KEY_TITLE) && !currentMovies.isNull(KEY_TITLE)) {
title = currentMovies.getString(KEY_TITLE);
}
// get the release date of the current movie
if (currentMovies.has(KEY_RELEASE_DATES) && !currentMovies.isNull(KEY_RELEASE_DATES)) {
JSONObject objectReleaseDates = currentMovies.getJSONObject(KEY_RELEASE_DATES);
if (objectReleaseDates !=null && objectReleaseDates.has(KEY_THEATER)
&& !objectReleaseDates.isNull(KEY_THEATER)) {
releaseDates = objectReleaseDates.getString(KEY_THEATER);
}
}
// get the audience score for the current movie
JSONObject objectRatings = currentMovies.getJSONObject(KEY_RATINGS);
if (objectRatings.has(KEY_AUDIENCE_SCORE) && !currentMovies.isNull(KEY_RATINGS)){
if (objectRatings != null && objectRatings.has(KEY_AUDIENCE_SCORE)
&& !objectRatings.isNull(KEY_AUDIENCE_SCORE)){
audienceScore = objectRatings.getInt(KEY_AUDIENCE_SCORE);
}
}
// get the synopsis for the current movie
if (currentMovies.has(KEY_SYNOPSIS) && !currentMovies.isNull(KEY_SYNOPSIS)) {
synopsis = currentMovies.getString(KEY_SYNOPSIS);
}
// get the posters & thumbnail for the current movie
if (currentMovies.has(KEY_POSTERS) && !currentMovies.isNull(KEY_POSTERS)) {
JSONObject objectPosters = currentMovies.getJSONObject(KEY_POSTERS);
if (objectPosters != null && objectPosters.has(KEY_THUMBNAIL)
&& !objectPosters.isNull(KEY_THUMBNAIL)) {
urlThumbnail = objectPosters.getString(KEY_THUMBNAIL);
}
}
Movie movie = new Movie();
movie.setId(id);
movie.setTitle(title);
Date date = null;
try {
date = dateFormat.parse(releaseDates);
}catch (ParseException e){
}
movie.setReleaseDateTheater(date);
movie.setAudienceScore(audienceScore);
movie.setSynopsis(synopsis);
movie.setUrlThumbnail(urlThumbnail);
if (id != -1 && !title.equals(Constants.NA)){
listMovies.add(movie);
}
}
//L.T(getActivity(), listMovies.toString());
} catch (JSONException e){
}
//L.t(getActivity(), listMovies.size() + " rows fetched");
}
L.t(getActivity(), listMovies.size() + " rows fetched");
return listMovies;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_box_office, container, false);
textVolleyError = (TextView) view.findViewById(R.id.textVolleyError);
listMovieHits = (RecyclerView) view.findViewById(R.id.listMoviesHits);
listMovieHits.setLayoutManager(new LinearLayoutManager(getActivity()));
adapterBoxOffice = new AdapterBoxOffice(getActivity());
listMovieHits.setAdapter(adapterBoxOffice);
sendJsonRequest();
return view;
//return inflater.inflate(R.layout.fragment_box_office, container, false);
}
@Override
public void onSortByName() {
//L.t(getActivity(),"Sort name Box Office");
movieSorter.sortMoviesByName(listMovies);
adapterBoxOffice.notifyDataSetChanged();
}
@Override
public void onSortByDate() {
//L.t(getActivity(),"Sort Date Box Office");
movieSorter.sortMoviesByDate(listMovies);
adapterBoxOffice.notifyDataSetChanged();
}
@Override
public void onSortByRating() {
//L.t(getActivity(),"Sort Rating Box Office");
movieSorter.sortMoviesByRating(listMovies);
adapterBoxOffice.notifyDataSetChanged();
}
}
提前致谢..
答案 0 :(得分：0)
在MainActivity.java第229行中更改：
Fragment mFragment = (Fragment) mAdapter.instantiateItem(viewPager, viewPager.getCurrentItem());
要：
Fragment mFragment = (Fragment) adapter.instantiateItem(viewPager, viewPager.getCurrentItem());
它应该可以正常工作。