如果这个问题很明显，您好，但是自从两天后，我得到了空指针异常
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myapplicationteeeeeeeeeest, PID: 13439
java.lang.NullPointerException: Attempt to invoke virtual method 'void java.util.ArrayList.add(int, java.lang.Object)' on a null object reference
at com.example.myapplicationteeeeeeeeeest.ui.FSettings.SettingsFragment$4.onDataChange(SettingsFragment.java:336)
at com.google.firebase.database.Query$1.onDataChange(com.google.firebase:firebase-database@@19.0.0:179)
at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@19.0.0:75)
at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@19.0.0:63)
at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@19.0.0:55)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
I/Process: Sending signal. PID: 13439 SIG: 9
Disconnected from the target VM, address: 'localhost:8617', transport: 'socket'
我知道Firebase是异步的，返回值是null，但是它随机出现。 这是它所使用的类。
package com.example.myapplicationteeeeeeeeeest.ui.FSettings;
import ...
public class SettingsFragment extends Fragment {
private SettingsViewModel sendViewModel;
private Recipe recipe_1,recipe_2,recipe_3,recipe_4,recipe_5,recipe_6,recipe_7,recipe_8,recipe_9,recipe_10,recipe_11,recipe_12,recipe_13,recipe_14,recipe_15,recipe_16,recipe_17,recipe_18,recipe_19,recipe_20;
private ArrayList<String> ingridients1,ingridients2,ingridients3,ingridients4,ingridients5,ingridients6,ingridients7,ingridients8,ingridients9,ingridients10,ingridients11,ingridients12,ingridients13,ingridients14,ingridients15,ingridients16,ingridients17,ingridients18,ingridients19,ingridients20;
private View root;
private String[] resultAmount,resultIngredients;
ExpandableListView expandableListView;
ExpandableListAdapter expandableListAdapter;
List<String> expandableListTitle;
HashMap<String, List<String>> expandableListDetail;
public View onCreateView(@NonNull LayoutInflater inflater,
ViewGroup container, Bundle savedInstanceState) {
sendViewModel =
ViewModelProviders.of( this ).get( SettingsViewModel.class );
View root = inflater.inflate( R.layout.fragment_settings, container, false );
expandableListView = (ExpandableListView) root.findViewById(R.id.expandableListView);
expandableListDetail = ExpandableListDataPump.getData();
expandableListTitle = new ArrayList<String>(expandableListDetail.keySet());
expandableListAdapter = new CustomExpandableListAdapter(getActivity(), expandableListTitle, expandableListDetail);
expandableListView.setAdapter(expandableListAdapter);
expandableListView.setOnGroupExpandListener(new ExpandableListView.OnGroupExpandListener() {
@Override
public void onGroupExpand(int groupPosition) {
Toast.makeText(getActivity().getApplicationContext(),
expandableListTitle.get(groupPosition) + " List Expanded.",
Toast.LENGTH_SHORT).show();
}
});
expandableListView.setOnGroupCollapseListener(new ExpandableListView.OnGroupCollapseListener() {
@Override
public void onGroupCollapse(int groupPosition) {
Toast.makeText(getActivity().getApplicationContext(),
expandableListTitle.get(groupPosition) + " List Collapsed.",
Toast.LENGTH_SHORT).show();
}
});
expandableListView.setOnChildClickListener(new ExpandableListView.OnChildClickListener() {
@Override
public boolean onChildClick(ExpandableListView parent, View v,
int groupPosition, int childPosition, long id) {
Toast.makeText(
getActivity().getApplicationContext(),
expandableListTitle.get(groupPosition)
+ " -> "
+ expandableListDetail.get(
expandableListTitle.get(groupPosition)).get(
childPosition), Toast.LENGTH_SHORT
).show();
return false;
}
});
obtainFirebase();
return root;
}
private void initIngredients() {
ingridients1 = new ArrayList<>( );
ingridients2 = new ArrayList<>( );
ingridients3 = new ArrayList<>( );
ingridients4 = new ArrayList<>( );
ingridients5 = new ArrayList<>( );
ingridients6 = new ArrayList<>( );
ingridients7 = new ArrayList<>( );
ingridients8 = new ArrayList<>( );
ingridients9 = new ArrayList<>( );
ingridients10 = new ArrayList<>( );
ingridients11 = new ArrayList<>( );
ingridients12 = new ArrayList<>( );
ingridients13 = new ArrayList<>( );
ingridients14 = new ArrayList<>( );
ingridients15 = new ArrayList<>( );
ingridients16 = new ArrayList<>( );
ingridients17 = new ArrayList<>( );
ingridients18 = new ArrayList<>( );
ingridients19 = new ArrayList<>( );
ingridients20 = new ArrayList<>( );
resultAmount = new String[15];
resultIngredients = new String[15];
}
public void obtainFirebase() {
FirebaseDatabase.getInstance().getReference().child( "recipes" )
.addListenerForSingleValueEvent( new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
if( snapshot.getKey().equals("recipe_2")) {
//do your stuff
recipe_2 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_2.getAmountOfIngredients().values().toArray(),recipe_2.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_2.getIngredients().values().toArray(),recipe_2.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients2.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_3")) {
//do your stuff
recipe_3 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_3.getAmountOfIngredients().values().toArray(),recipe_3.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_3.getIngredients().values().toArray(),recipe_3.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients3.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_4")) {
//do your stuff
recipe_4 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_4.getAmountOfIngredients().values().toArray(),recipe_4.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_4.getIngredients().values().toArray(),recipe_4.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients4.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_5")) {
//do your stuff
recipe_5 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_5.getAmountOfIngredients().values().toArray(),recipe_5.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_5.getIngredients().values().toArray(),recipe_5.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients5.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_6")) {
//do your stuff
recipe_6 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_6.getAmountOfIngredients().values().toArray(),recipe_6.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_6.getIngredients().values().toArray(),recipe_6.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients6.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_7")) {
//do your stuff
recipe_7 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_7.getAmountOfIngredients().values().toArray(),recipe_7.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_7.getIngredients().values().toArray(),recipe_7.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients7.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_8")) {
//do your stuff
recipe_8 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_8.getAmountOfIngredients().values().toArray(),recipe_8.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_8.getIngredients().values().toArray(),recipe_8.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients8.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
//Same for recipes 9-14 just too many chars for so
}
if( snapshot.getKey().equals("recipe_15")) {
//do your stuff
recipe_15 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_15.getAmountOfIngredients().values().toArray(),recipe_15.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_15.getIngredients().values().toArray(),recipe_15.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients15.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_16")) {
//do your stuff
recipe_16 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_16.getAmountOfIngredients().values().toArray(),recipe_16.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_16.getIngredients().values().toArray(),recipe_16.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients16.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_17")) {
//do your stuff
recipe_17 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_17.getAmountOfIngredients().values().toArray(),recipe_17.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_17.getIngredients().values().toArray(),recipe_17.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients17.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_18")) {
//do your stuff
recipe_18 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_18.getAmountOfIngredients().values().toArray(),recipe_18.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_18.getIngredients().values().toArray(),recipe_18.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients18.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_19")) {
//do your stuff
recipe_19 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_19.getAmountOfIngredients().values().toArray(),recipe_19.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_19.getIngredients().values().toArray(),recipe_19.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients19.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
if( snapshot.getKey().equals("recipe_20")) {
//do your stuff
recipe_20 = snapshot.getValue( Recipe.class );
ArrayList<String> amountIngridients = new ArrayList<>( );
ArrayList<String> ingridients = new ArrayList<>( );
String[] resultAmount = new String[15];
resultAmount = Arrays.copyOf(recipe_20.getAmountOfIngredients().values().toArray(),recipe_20.getAmountOfIngredients().values().toArray().length,String[].class );
Collections.addAll( amountIngridients,resultAmount );
String[] resultIngredients = new String[15];
resultIngredients = Arrays.copyOf(recipe_20.getIngredients().values().toArray(),recipe_20.getIngredients().values().toArray().length,String[].class );
Collections.addAll( ingridients,resultAmount );
for (int i = 0; i < resultAmount.length;i++) {
ingridients20.add( i,resultAmount[i]+resultIngredients[i]);
}
Arrays.fill( resultAmount,null );
Arrays.fill( resultIngredients,null );
}
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
} );
}
}
如果有人有什么主意，为什么会发生这种情况应该很棒，我们多年来一直在研究这个问题，而且似乎无法解决。预先感谢！
答案 0 :(得分：1)
看起来您的代码从不调用initIngredients()来创建数据库回调所需的所有ArrayList对象。它们最初都是空的，尝试向其中之一添加内容会导致该错误。