mFirebaseInstance.getReference(&quot;app_title&quot;).addValueEventListener(new ValueEventListener() {     @Override     public void onDataChange(DataSnapshot dataSnapshot) {         Log.e(TAG, &quot;App title updated&quot;);          String appTitle = dataSnapshot.getValue(String.class);         long s = dataSnapshot.child(&quot;daysOnWhichPresent&quot;).getValue(Long.class);          // update toolbar title          textGrid.setText(appTitle);     }      @Override     public void onCancelled(DatabaseError error) {         // Failed to read value         Log.e(TAG, &quot;Failed to read app title value.&quot;, error.toException());     } });   java.lang.NullPointerException: Attempt to invoke virtual method 'long java.lang.Long.longValue()' on a null object reference at omari.hamza.storyviewdemo.MainActivity$15.onDataChange(MainActivity.java:673) at com.google.firebase.database.core.ValueEventRegistration.fireEvent(ValueEventRegistration.java:75) at com.google.firebase.database.core.view.DataEvent.fire(DataEvent.java:63) at com.google.firebase.database.core.view.EventRaiser$1.run(EventRaiser.java:55)
What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?  What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?
Do you gain current HP when you level up? Typo in First Posts queue If you go to the First Posts review queue, and if there's an answer in the queue and if you hover with your mouse over "No Action Needed", then you get a tooltip "this answers seems to be valid":  But it should be "this answer seems to be valid". Where did the "play on realms" option go? My Minecraft app now only has the option of "play" and there is no "play on realms" to be found!  I downloaded - and paid for - the multiplayer app, but there doesn't seem to be a way to get to any realms on that app either. I'm learning about private/public IPs, port forwarding and NAT, and still fail to see the answer to a simple question:  Suppose two users on the same network are communicating with a single server outside their network (say, me &amp; my wife send http requests to yahoo.com).  The server sees both users as a single public ip, and often - as in this case - communicating on the same port.  How does the router route the packets according to the two different connections? Is there any routing info inside the packet beyond ip/port?  Is there some 'session table' maintained somewhere? By whom and what exactly does it include? How to prove for each positive integer $n$, the sum of the first $n$ odd positive integers is $n^2$? I'm new to induction so please bear with me. How can I prove using induction that, for each positive integer $n$, the sum of the first $n$ odd positive integers is $n^2$?  I think $9$ can be an example since the sum of the first $9$ positive odd numbers is $1,3,5,7,9,11,13,15,17 = 81 = 9^2$, but where do I go from here. Problem. Use Tutte's 1-factor theorem to prove that every 3-regular graph with no bridges has a 1-factor.  I honestly have no idea how to approach this problem. I have drawn a couple of graphs to gain some insight to no avail. Any hints or tips will be greatly appreciated. Exponent and abelian groups Math-like chaining of the comparison operator - as in, "if ( (5<j<=1) )" No output from goroutine Get total height of webView's content using Javascript Chain length. how long or how many links I measured from the middle of each sprocket and it's 17 inches. Not sure what length of chain I should get Why is super.super.method(); not allowed in Java? I read  and thought that would easily be solved (not that it isn't solvable without) if one could write:   @Override public String toString() {     return super.super.toString(); }   I'm not sure if it is useful in many cases, but I wonder why it isn't and if something like this exists in other languages.  What do you guys think?  EDIT: To clarify: yes I know, that's impossible in Java and I don't really miss it. This is nothing I expected to work and was surprised getting a compiler error. I just had the idea and like to discuss it. "reflect" vs "reflection on" or "reflection of" The bride's elegant dress reflected her good taste. The bride's elegant dress is a reflection on her good taste. The bride's elegant dress is a reflection of her good taste.  Would you tell me if they mean the same thing? If they would mean the same thing, what about the following question--link--? or I mean how could we distinguish such a distinction between these functions? Similarly, what about these:  A student's grades reflected her teacher A student's grades are a reflection on her teacher A student's grade are a reflection of her teacher  When, where or in which situation don't they mean the same? How can we distinguish such a distinction? User is suspended network-wide, except not really A user has been active on one of the sites recently, even though they're concurrent suspended network-wide on the rest of the network. How is this possible, and is it possible to evade a network suspension on only one site? Speed of light in a gravitational field? Let $K=F[x]/(p(x))$, where $F$ is a field, $p(x)\in F[x]$ is an irreducible polynomial.  What are all the maximal ideals in $K[y]$?  Attempt: I have an attempt, but not entirely sure it is correct.  Since $(p(x))$ is a maximal ideal in $F[x]$, this means $K$ is actually a field.  Thus $K[y]$ is a PID.  So maximal ideals in $K[y]$ are $(g(y))$ for irreducible polynomials $g(y)\in K[y]$? How to install Python 3.6? I'd like to install the latest Python, which is 3.6 at the time of this post. However, the repository is saying that Python 3.4.2 is the newest version.  I've tried:   $ sudo apt-get update $ sudo apt-get install python3  python3 is already the newest version.  $ python -V  Python 3.4.2   To upgrade to Python 3.6 on my Windows workstation, I simply downloaded an exe, clicked "next" a few times, and it's done. What's the proper and officially accepted procedure to install Python 3.6 on Debian Jessie? "This question has been asked at Stack Overflow" vs. "on Stack Overflow" How should I phrase it:     This question has been asked at Stack&nbsp;Overflow.   Or,     This question has been asked on Stack&nbsp;Overflow. Finding the inverse of the arc length function Finite Sets, Equal Cardinality, Injective $\iff$ Surjective.