Can't read Firebase Database

This is my code for reading Firebase data:

final String[] booknum = {"0"};
databaseReference.child("All BID").addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot snapshot) {
booknum[0] =Long.toString(snapshot.getChildrenCount());
Toast.makeText(getApplicationContext(), booknum[0],Toast.LENGTH_LONG).show();

}
@Override public void onCancelled(DatabaseError databaseError) {
Log.w(TAG, "loadPost:onCancelled", databaseError.toException());
}
});
Toast.makeText(getApplicationContext(), booknum[0],Toast.LENGTH_LONG).show();

When I am executing this, the first toast(inside ValueEventListener) prints the right answer (eg '8'). But the toast outside always prints 0 no matter what.

Please Help!

The ValueEventListener is asynchronous. The lines of code inside of the listener, including the assignment of booknum[0], are not guaranteed to execute before the lines of code written outside of the listener. If you depend on the new value of booknum[0] for some operation, consider moving that operation inside of onDataChange() to ensure it uses the new value.