What happens if a firebase url doesnot exist and we try to add a listener to it?

Question

I am developing an application in which I ask the user to enter the city and state name and return certain location in that place. But the problem is that if the specified s

Answer 1

The methods in a ChildEventListener are called when a child is added, changed, removed or moved. On an empty location, none of these events happen, so none of the methods are called.

A ValueEventListener will be called with an empty value if no value exists at a location. So if you also call addListenerForSingleValueEvent(), you can detect that the location has no value:

Firebase cityRef = firebase.child("markers").child(state).child(city);
cityRef.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot snapshot) {
        if (snapshot.getValue() == null) {
            System.out.println("No data exists for "+city);
        }
    }
    @Override
    public void onCancelled(FirebaseError firebaseError) {
    }
});

If you want to handle the children in the same call, you can just loop over them in onDataChange():

public void onDataChange(DataSnapshot snapshot) {
    if (snapshot.getValue() == null) {
        System.out.println("No data exists for "+city);
    }
    else {
        for (DataSnapshot citySnapshot: snapshot.getChildren()) {
            // Do something with citySnapshot.getValue()
        }
    }
}