null pointer exception while searching linked list
What is a NullPointerException, and how do I fix it?
I accidentally closed the window for the verfication code I wanted to create an account for Ubuntu One, but I got impatient when the email with the verification code didn't arrive... I thought I might have typed the wrong address, closed the window and tried again. it told me the address already existed and I just got the code. But since I closed the window I missed it. Is there any way to recover the code? I'm not quite sure how the total attack bonus interacts with BAB.   Can you make more attacks than shown by your BAB (e.g. +11/+6/+1 BAB allows you to make 3 attacks) by having a high enough DEX, STR or other attack bonus modifiers? Assuming I have +11/+6/+1 BAB, and a spell gives me +6 attack bonus, would that make me able to attack as if I had +17/+12/+7/+2 BAB, giving me an extra attack?)   Can you make multiple attacks even if you have one of your stats drained, making your attack bonus negative, or less than 5?      It says in the SRD that the formula for attack bonus in melee is this:       Base attack bonus + Strength modifier + size modifier   I've also been told that you can make multiple attacks in one turn, but each attack gains a -5 penalty to attack bonus. Give a combinatorial proof to show the following for all integers $n \geq 2$. $$2^{n-2} n (n -1) = \sum\limits_{k=2}^n k (k - 1) \binom{n}{k}.$$  I'm completely stumped. I just have no idea how to do this. What I've tried so far has been simplifying the right hand side slightly to $\sum\limits_{k=2}^n \binom{n}{k - 2}$, and then proving that this somehow is equal to some manipulation of the left hand side. I can't seem to find any breakthroughs that way though.  If anyone could help out or point me in the right direction that would be great.  EDIT: I've actually just realized my simplification of the right hand side is wrong. Now I'm even more stuck. Let $q \in \mathbb C$, $|q|=1$ and $q^n \neq 1, \forall n \in \mathbb N$. Show that $\{q^n:  n \in \mathbb N\}$ is dense in $S^1$.  My attempt: As $(q^n)$ is limited, there is subsequence $(q^{n_j})$ convergent. So given $\epsilon &gt; 0, \exists j_0 \in \mathbb N, \quad |q^{n_j} - q^{n_k}| &lt; \epsilon, \quad \forall j,k \geq j_0$. Supose that $n_j &gt; n_k$, then  $|q^{n_j} - q^{n_k}| = |q^{n_k}||q^{n_j - n_k} - 1| = |q^{n_j - n_k} - 1| &lt; \epsilon, \quad \forall j, k \geq j_0$. So, there is a subsequence of $(q^n)$ that converges to $1$.  Using this fact, how can I ensure that for all $w \in S^1$ there is a subsequence of $(q^n)$ that converges to  $w$? Conveying "under the radar" in a less colloquial way Scala final vs val for concurrency visibility I see questions come up quite often that have this underlying issue, but they're all caught up in the particulars of a given feature or tool. Here's an attempt to create a canonical answer we can refer users to when this comes up - with lots of animated examples! :)    Let's say we're making a first-person camera. The basic idea is it should yaw to look left &amp; right, and pitch to look up &amp; down. So we write a bit of code like this (using Unity as an example):  void Update() {     float speed = lookSpeed * Time.deltaTime;      // Yaw around the y axis using the player's horizontal input.             transform.Rotate(0f, Input.GetAxis("Horizontal") * speed, 0f);      // Pitch around the x axis using the player's vertical input.     transform.Rotate(-Input.GetAxis("Vertical") * speed,  0f, 0f); }   or maybe  // Construct a quaternion or a matrix representing incremental camera rotation. Quaternion rotation = Quaternion.Euler(                         -Input.GetAxis("Vertical") * speed,                          Input.GetAxis("Horizontal") * speed,                          0);  // Fold this change into the camera's current rotation. transform.rotation *= rotation;   And it mostly works, but over time the view starts to get crooked. The camera seems to be turning on its roll axis (z) even though we only told it to rotate on the x and y!    This can also happen if we're trying to manipulate an object in front of the camera - say it's a globe we want to turn to look around:    The same problem - after a while the North pole starts to wander away to the left or right. We're giving input on two axes but we're getting this confusing rotation on a third. And it happens whether we apply all our rotations around the object's local axes or the world's global axes.  In many engines you'll also see this in the inspector - rotate the object in the world, and suddenly numbers change on an axis we didn't even touch!    So, is this an engine bug? How do we tell the program we don't want it adding extra rotation?  Does it have something to do with Euler angles? Should I use Quaternions or Rotation Matrices or Basis Vectors instead? MySQL AUTO_INCREMENT does not ROLLBACK Intuition behind the definition of a measurable set I've installed Ubuntu 14.04 and I'm experiencing a fuzzy/blurry screen and bad support for multiple screens.  I realized that Ubuntu installs nouveau as default, but I need to install nvidia drivers, so I tried this:   sudo apt-add-repository ppa:ubuntu-x-swat/x-updates apt-get update apt-get install nvidia-current apt-get upgrade       Now that nvidia is installed, you need to blacklist the nouveau driver   so it don’t pop out when you will reboot. Create or edit the following   file:  nano /etc/modprobe.d/blacklist-nouveau.conf       …and add these lines at the end:  blacklist nouveau blacklist lbm-nouveau options nouveau modeset=0 alias nouveau off alias lbm-nouveau off    I checked via the software &amp; updates if nvidia driver got installed successfully, and it says the driver installed is nvidia 304.117  I then tried to run:  lshw -c video | grep 'configuration'   and the result is that the driver is still nouveau:  configuration: driver=nouveau latency=0   So how do I install and use the nvidia drivers? How To Make Clickable Text In Minecraft I am trying to make clickable text in Minecraft in the chat box. It was used in the adventure map, "UP".   The idea is that you are given an option in the text to click, "Yes", or "No". Each word will give an output of a command.  How is this achieved? How many connected components does $\mathrm{GL}_n(\mathbb R)$ have? I've noticed that $\mathrm{GL}_n(\mathbb R)$ is not a connected space, because if it were $\det(\mathrm{GL}_n(\mathbb R))$ (where $\det$ is the function ascribing to each $n\times n$ matrix its determinant) would be a connected space too, since $\det$ is a continuous function. But $\det(\mathrm{GL}_n(\mathbb R))=\mathbb R\setminus\{0\},$ so not connected.  I started thinking if I could prove that $\det^{-1}((-\infty,0))$ and $\det^{-1}((0,\infty))$ are connected. But I don't know how to prove that. I'm reading my notes from the topology course I took last year and I see nothing about proving connectedness... How can I verify the following statement?     The image of a morphism $f : A \longrightarrow B$ is defined as $\text{Im}(f) = \text{Ke}r(\text{coke}r f)$ whenever it exists (e.g., in every abelian category). The morphism $f : A \longrightarrow B$ factors   uniquely through $\text{Im} f \longrightarrow B$ whenever $\text{Im} f$ exists, and $A \longrightarrow \text{Im} f$ is an epimorphism and a cokernel of ker $f \longrightarrow A$ in every abelian category. Can Wubi use the ISO I already have? The Windows Installer for Ubuntu desktop instructions say that an ISO image for Ubuntu is downloaded during the installation process.  But what if I already have an ISO image for Ubuntu. Can I make the Windows Installer use that, so it doesn't have to download an ISO image again?  12.04 LTS and 12.10 are the versions of Ubuntu I'm interested in installing, in case that matters. Is it possible to know from apt if the package was installed explicitly or as a dependency of another package? Cooking more than one dish that require different temperatures I'm cooking two different dishes in the oven at the same time that each require different temperatures. One is a roasted vegetable dish that says to cook at 475 for 35-40 minutes and the other is parmesan crusted pork chops at 350 for 40-45 minutes. What do you suggest I do? I'm a quarter Draconian, a quarter Dragonborn and half Half-Dragon? What are the differences between "Dragonborn", "Half-Dragon", and "Draconian" races in D&amp;D?  In the Hoard of the Dragon Queen adventure, for example there is  a Half-Dragon npc. I had assumed that when writing the adventure they were only allowed to use known material from Basic, and so renamed the Dragonborn into "Half-Dragon" for the sake of compatibility etc. However, the dragon breath rules are not at all similar for the half-dragon and the dragonborn, and I've also noticed wiki article that imply that they are different races entirely. (Thus introducing me to the term Draconian)  Are there clear differences between these three races, (in the same way there are differences between Goblins and Hobgoblins, or Dragonborn and Lizardfolk and Kobolds,) or are they really just three different setting names for the same basic race?  Answers can either be D&amp;D 5e specific, or be based on the lore from all editions of Dungeons and Dragons. (I'm assuming here that the answer will be the same regardless)  Here are two images, the first one is a "half-dragon" from the front of the Hoard of the Dragon queen module, and the second one is a "dragonborn" from the 5e player's Handbook.  I've included these images to help explain my confusion. How did the AT-ATs get to the surface of Hoth? Can't install wine on Ubuntu 14.04 I cannot install Wine on Ubuntu 14.04.  When I try to install it on the Ubuntu Software Center I always get a message that says I have unmet dependencies. I tried installing Wine through the terminal and I get this:  Reading package lists... Done Building dependency tree        Reading state information... Done Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming. The following information may help to resolve the situation:  The following packages have unmet dependencies:  wine : Depends: wine1.6 but it is not going to be installed E: Unable to correct problems, you have held broken packages. $a+b+c =0$; $a^2+b^2+c^2=1$. Prove that: $a^2 b^2 c^2 \le \frac{1}{54}$ If a,b,c are real numbers satisfying $a+b+c =0; a^2+b^2+c^2=1$.  Prove that $a^2 b^2 c^2 \le \frac{1}{54}$.