我有我的基本Firebase应用程序，它涉及从我设计的嵌入式系统发送数据，嵌入式工作正常，但我无法将数据显示为数据库中的子条目。
应用程序将连接并写入Firebase，即JSON
{
"users" : {
"-Kb6KXHoEjDtrT5KFAkk" : {
"title" : "Hello"
},
"-Kb6KcJRJchZ6FdYQwE3" : {
"title" : "Hello"
}
}
}
信息不会显示在列表视图中，我的应用程序不需要对象，只需要在数据库中显示子条目。
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Initialize Firebase Auth and Database Reference
mFirebaseAuth = FirebaseAuth.getInstance();
mFirebaseUser = mFirebaseAuth.getCurrentUser();
mDatabase = FirebaseDatabase.getInstance().getReference();
if (mFirebaseUser == null) {
// Not logged in, launch the Log In activity
loadLogInView();
} else {
mUserId = mFirebaseUser.getUid();
// Set up ListView
final ListView listView = (ListView) findViewById(R.id.listview);
final ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1);
listView.setAdapter(adapter);
Item item = new Item("Hello"); // testing that i can write values
mDatabase.child("users").push().setValue(item); // Values can be written
// Use Firebase to populate the list.
mDatabase.child("users").addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
adapter.add((String) dataSnapshot.child("users").getValue());
}
@Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
adapter.add((String) dataSnapshot.child("users").getValue());
}
@Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
adapter.remove((String) dataSnapshot.child("users").getValue());
}
@Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
Firebase权限是公开读写
{
"rules": {
".read": true,
".write": true,
}
}
我得到的错误是一个空对象引用
FATAL EXCEPTION: main
Process: com.m.j.project.josephmay, PID: 6843
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:401)
at android.widget.ArrayAdapter.getView(ArrayAdapter.java:369)
at android.widget.AbsListView.obtainView(AbsListView.java:2929)
at android.widget.ListView.makeAndAddView(ListView.java:1945)
at android.widget.ListView.fillDown(ListView.java:719)
at android.widget.ListView.fillFromTop(ListView.java:785)
at android.widget.ListView.layoutChildren(ListView.java:1721)
at android.widget.AbsListView.onLayout(AbsListView.java:2723)
at android.view.View.layout(View.java:17945)
at android.view.ViewGroup.layout(ViewGroup.java:5812)
at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1080)
at android.view.View.layout(View.java:17945)
at android.view.ViewGroup.layout(ViewGroup.java:5812)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:344)
at android.widget.FrameLayout.onLayout(FrameLayout.java:281)
at android.view.View.layout(View.java:17945)
at android.view.ViewGroup.layout(ViewGroup.java:5812)
at android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:433)
at android.view.View.layout(View.java:17945)
at android.view.ViewGroup.layout(ViewGroup.java:5812)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:344)
at android.widget.FrameLayout.onLayout(FrameLayout.java:281)
at android.view.View.layout(View.java:17945)
at android.view.ViewGroup.layout(ViewGroup.java:5812)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1742)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1585)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1494)
at android.view.View.layout(View.java:17945)
at android.view.ViewGroup.layout(ViewGroup.java:5812)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:344)
at android.widget.FrameLayout.onLayout(FrameLayout.java:281)
at com.android.internal.policy.PhoneWindow$DecorView.onLayout(PhoneWindow.java:3178)
at android.view.View.layout(View.java:17945)
at android.view.ViewGroup.layout(ViewGroup.java:5812)
at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2716)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2417)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1487)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7450)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:920)
at android.view.Choreographer.doCallbacks(Choreographer.java:695)
at android.view.Choreographer.doFrame(Choreographer.java:631)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:906)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:158)
at android.app.ActivityThread.main(ActivityThread.java:7229)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)