NullPointerException problems
What is a NullPointerException, and how do I fix it?
Which tools and technologies are used to build the Stack Exchange Network? Hydrostatic pressure - doesn't density vary with depth? Our class is learning about hydrostatic water pressure and we have been told that we can calculate the force of the liquid on an object at any depth using "the density x 9.8 x the depth". However, as the depth increases, wouldn't the density of the liquid increase because of the weight of the liquid above it compressing it? So should't there be something in the equation to account for the varying density? To me, "density x 9.8 x depth" seems like it is saying that the density will be constant... Why does reputation start at 1, and have a lower bound of 1? If a user is the type of person to consecutively get downvoted to potential negative reputation, so be it. What's the reasoning to maintain a minimum of one?  One reason that may justify this is of trying to not deter new users from the site who haven't read the FAQ and unknowingly and wrongly ask or answer a question. But most of us acknowledge this and not downvote in such a scenario.  So what's the reasoning behind this lower bound of 1? In my native language, we can say:     I have dog   Because I don't want to say a dog (one dog, how many dogs) or the dog (that dog, the listener don't care which dog).  p.s. after 3 years later, I have to say, why I ask this question, is I still cannot grasp how to use the right 'article word'. sometimes, a noun will use an article, sometimes, a noun could not(or omit) the article word before it. there aren't a formula for this. So, I have to remember all the time! I feel so frustrate I have not done much proofs before this and need some guidance. I know that for a simple graph such as this :  node - node - node -node   Removing the first and last vertex will not disconnect the graph. So far, I can think of two cases: Where there are leaves, or nodes with degree of 1. Where there are no leaves, which means all nodes have degrees 2 or more.   In the first case, removing the leaf will not disconnect the graph. In the second case, removing any should not disconnect the graph. Problem is, I don't know how to formulate this into a good proof. Will I still receive OTA updates after rooting my Nexus 5? I just wanna ask that after rooting Nexus 5 will I be receiving OTA updates by Google ? Or I'll have to install a custom ROM ? Where to ask "subjective and argumentative" questions on programming? Recently  was "closed as subjective and argumentative".  I wonder, where would you recommend to ask such questions? Why doesn't a bike/bicycle fall if going with a high speed? Why does a bike/bicycle fall when its speed is very low or close to zero and is balanced when going with a high speed? How to tell what method was used to log in (Password vs. Keyfile) from the bashrc calculation of $\int_{0}^{1}\tan^{-1}(1-x+x^2)dx$ Compute the definite integral   $$ \int_{0}^{1}\tan^{-1}(1-x+x^2)\,dx $$   Failed Attempt:  Let $1-x+x^2=t$. Then  $$ \begin{align} (2x-1)\,dx &amp;= dt\\ dx &amp;= \frac{1}{(2x-1)}dt \end{align} $$  Changing the limits of integration, we get  $$\int_{1}^{1}\tan^{-1}(t)\cdot \frac{1}{(2x-1)}dt = \int_{1}^{1}\tan^{-1}(t)\cdot f(t)dt = 0 $$  where $f(t)=\frac{1}{(2x-1)}$.  Is it true that $\int_{a}^{a}f(x)dx = 0$? If not, then where have I made a mistake in my attempted solution? What can I do when getting “This question body does not meet our quality standards”? While trying to ask a question, I got the error message     This question body does not meet our quality standards. Please make sure that it completely describes your problem - including what you have already tried - and is written using proper grammar.    Why am I getting this message? What can I do to get the system to accept my question? Can you be more specific? What are good resources on how to ask high-quality questions?      For more information, see  in the . I need to graph a rectangle on the Cartesian coordinate system. Is there an equation for a rectangle? I can't find it anywhere. iCloud has sucked up all my laptop's files - can you transfer files directly from iCloud to Google Drive? I'm trying to disconnect my macbook from iCloud, as it's been creating some problems.  I was told by an Apple Support employee that doing so is only recommended once you've backed up everything from your Mac somewhere, to ensure you don't lose any files - so my goal is to upload everything to Google Drive before messing with my laptop's iCloud settings. The problem is, almost none of the projects that I want to back up to Google Drive are truly on my laptop - instead I mostly just have folders full of file names, which are placeholders for the real files that have been sucked into iCloud. This means I have to re-download everything to my computer before uploading it all to Google Drive. Another crucial piece of information is that I only have 1 GB of storage left on my macbook, which prevents me from re-downloading any complete projects/folders at a time. So unfortunately my current method is this: downloading 1-5 files at a time, uploading these few things to Google Drive, deleting the copies from my laptop and trash, repeat (thousands of times). So, I was wondering if anyone could offer any alternative methods for uploading stuff to Google Drive in this situation - for example, is there a way to transfer files directly from iCloud to Google Drive without having to download them to my laptop first? I should probably mention that I've scoured my laptop for duplicated files and applications I no longer use, make sure to empty my trash regularly, etc (I also asked Apple Support to help me make sure I wasn't missing out on any other ways to maximize storage space). The most obvious culprit of my lack of space is the massive 150 GB iCloud folder on my laptop, which appears to contain duplicates of everything on my computer. I can't seem to do anything about it until I've disconnected from iCloud itself (its reasons for existence are unclear to me in the first place...but that's beside the point lol). When should I start the DLC? I'm late to the Borderlands 2 party, but I picked up the Season Pass for a steal.  I'm still only in the teens as far as my level is concerned.  However, there are now 3 DLC packs out and I've got them all installed.  Sometimes it's tempting to go away from the main quest and just jump into something new and potentially interesting.  In BL1, I played through the DLC between playthroughs of the game, treating the add-ons like continuations of the game post-final-boss.  I'm not sure if this is a good idea in BL2 (or even if it was smart in BL1, honestly...)  Is there a recommended level for starting the DLCs?  Will I encounter enemies that are too hard if I go early, or too easy if I go late?  Are there some weapons or quest rewards that I really can't get the most of until after I'm a certain level? Problem on Linear Diophantine Equation over 3 variables How to solve $ax+by+cz=d$ over integers where $a,b,c,d$ are integers? In 10.04, I had a lot of keyboard shortcuts defined using Super/Mod4 and one single other key,   Super+O ran Opera Super+W opened Nautilus pointing to my Work folder, etc.   In 11.04, these do not seem to work -- only Super+R works to run the terminal, and Super+Shift+W successfully runs Nautilus.  Is there some way I can get these to function again?  Adding them in Keyboard Shortcuts does not work, and neither does adding commands in CompizConfig Settings Manager. virtualbox crash on kernel 4.13.0-26 Ubuntu 16.04 LTS Virtualbox 5.0.40 ubuntu r 115130  Today after upgrading to kernel 4.13.0-26 guest vbox machines started to crash whole ubuntu. just hard poweroff is working.  If I startup with kernel 4.10.0-42 everything works fine.  what may be the problem ? I would like to prove the following inequality, for $n=0,1,2,...$,   $$ \binom{2n}{n}\leq 4^n.$$ I already proved it by induction, and I'm looking for another proof. How can I reverse (flip horizontal) a table? What is a bus error? What does the "bus error" message mean, and how does it differ from a segfault?