问题描述:

I'm trying to retreive an object from my firebase databse by its id. But my app crashes with a null pointer exception. Please help me out. I'm new to Firebase.

Also I'd like to know how I can find out if the search is unsuccessful.

Here's my code for data retreival. The Query class belongs to com.google.firebase and not to com.firebase.client.

 @Override

public void onClick(View view) {

key = scanContent;

Query q = dbRef.child("idno").equalTo(key);

if (key != null) {

q.addListenerForSingleValueEvent(new com.google.firebase.database.ValueEventListener() {

@Override

public void onDataChange(com.google.firebase.database.DataSnapshot dataSnapshot) {

Item i = dataSnapshot.getValue(Item.class);

id.setText(scanContent);

nam.setText(i.getName());

brand.setText(i.getBrand());

cost.setText(i.getCost());

sell.setText("Add to list of sold items?");

}

@Override

public void onCancelled(DatabaseError databaseError) {

AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());

builder.setMessage(databaseError.getMessage())

.setTitle("Error!")

.setPositiveButton(android.R.string.ok, null);

AlertDialog dialog = builder.create();

dialog.show();

}

});

} else {

search.setError("Enter a valid value");

}

}

Here are the contents of my logcat.

09-24 13:32:26.332 9228-9228/com.strokx.user.stockmanager W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0x40c03a68)

09-24 13:32:26.352 9228-9228/com.strokx.user.stockmanager E/UncaughtException: java.lang.NullPointerException

at com.strokx.user.stockmanager.SearchFragment$2$1.onDataChange(SearchFragment.java:106)

at com.google.firebase.database.Query$1.onDataChange(Unknown Source)

at com.google.android.gms.internal.zzaie.zza(Unknown Source)

at com.google.android.gms.internal.zzaje.zzcta(Unknown Source)

at com.google.android.gms.internal.zzajh$1.run(Unknown Source)

at android.os.Handler.handleCallback(Handler.java:605)

at android.os.Handler.dispatchMessage(Handler.java:92)

at android.os.Looper.loop(Looper.java:137)

at android.app.ActivityThread.main(ActivityThread.java:4517)

at java.lang.reflect.Method.invokeNative(Native Method)

at java.lang.reflect.Method.invoke(Method.java:511)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:993)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:760)

at dalvik.system.NativeStart.main(Native Method)

09-24 13:32:26.572 9228-9228/com.strokx.user.stockmanager D/FirebaseCrashApiImpl: throwable java.lang.NullPointerException

09-24 13:32:26.672 9228-9228/com.strokx.user.stockmanager E/AndroidRuntime: FATAL EXCEPTION: main

java.lang.NullPointerException

at com.strokx.user.stockmanager.SearchFragment$2$1.onDataChange(SearchFragment.java:106)

at com.google.firebase.database.Query$1.onDataChange(Unknown Source)

at com.google.android.gms.internal.zzaie.zza(Unknown Source)

at com.google.android.gms.internal.zzaje.zzcta(Unknown Source)

at com.google.android.gms.internal.zzajh$1.run(Unknown Source)

at android.os.Handler.handleCallback(Handler.java:605)

at android.os.Handler.dispatchMessage(Handler.java:92)

at android.os.Looper.loop(Looper.java:137)

at android.app.ActivityThread.main(ActivityThread.java:4517)

at java.lang.reflect.Method.invokeNative(Native Method)

at java.lang.reflect.Method.invoke(Method.java:511)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:993)

网友答案:

I think you need a default empty constructor for your Item class.

网友答案:

When you execute a query against the Firebase Database, the result is going to be a list of items. Even when there is only a single match for your query, the result will be a list of one. So your code needs to handle the list, by looping over dataSnapshot.getChildren():

Query q = dbRef.child("idno").equalTo(key);
if (key != null) {
    q.addListenerForSingleValueEvent(new com.google.firebase.database.ValueEventListener() {
        @Override
        public void onDataChange(com.google.firebase.database.DataSnapshot dataSnapshot) {
            (for DataSnapshot itemSnapshot: dataSnapshot.getChildren()) {
                Item i = dataSnapshot.getValue(Item.class);
                ...

We recently added an example of how to do this to the Firebase documentation on filtering data with queries.

网友答案:

Fixed it. My query was wrong. Although fixing the query still wouldn't make the listview display the retrieved result. I used the firebase ui list adapter to fix that. I've posted the correct code below, if it helps anyone. Thanks everyone.

 dbRef= FirebaseDatabase.getInstance().getReferenceFromUrl("https://stockmanager-142503.firebaseio.com/Items");


        imgbtn.setOnClickListener(new View.OnClickListener() {
                                      @Override
                                      public void onClick(View view) {
                                          key = scanContent;
                                          Query q = dbRef.orderByChild("idno").equalTo(key);
                                          FirebaseListAdapter<Item> adapter =new FirebaseListAdapter<Item>(getActivity(),Item.class,R.layout.rowitem,q) {
                                              @Override
                                              protected void populateView(View v, Item item, int position) {
                                                  TextView text1=(TextView)v.findViewById(R.id.text1);
                                                  TextView text2=(TextView)v.findViewById(R.id.text2);
                                                  TextView text3=(TextView)v.findViewById(R.id.text3);
                                                  TextView text4=(TextView)v.findViewById(R.id.text4);
                                                  TextView text5=(TextView)v.findViewById(R.id.text5);
                                                  text1.setText(item.getIdno());
                                                  text2.setText(item.getName());
                                                  text3.setText(item.getBrand());
                                                  text4.setText(item.getCost());
                                                  text5.setText(item.getDate());
                                                  solditem=item;
                                              }
                                          };
                                          sell.setText("Add this result to Sold Items List?");
                                      sList.setAdapter(adapter);
                                      }
                                  });
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