Attempt to invoke interface method on a null object reference while implement interface
What is a NullPointerException, and how do I fix it?
Potential difference across a zero resistance wire How do I get attention for one of my own questions without a good answer? Bottom to top explanation of the Mahalanobis distance? save as a video as .mp4 I asked about it on  and they told me to post it here.  Probably need to add the :active to the :hover meta class or something like that.  Example:      Hammer Time Understanding NSString comparison Send file using POST from a Python script Is there a way to send a file using POST from a Python script? Why can't the Others go around the Wall at sea? As explained in a  earlier , the Wall does not extend all the way to the sea on the East, and the castle there is essentially undefended anyway. Thus, wildlings -- and Stannis' army -- were able to go around it undetected.  We know that the Others and the wights they raise can't cross through the Wall, and I think we can assume that it's built to stand up to any attacks they make trying to bring it down. But there does not appear to be any reason they can't just go around like anyone else.  Possibly related: in the TV Show, we actually see the attempted evacuation of Hardhome, and the Night King allows Jon Snow and his human army and civilians to flee back to their boats to safety, even after raising a massive army of wights. You could interpret this as merely a psychological ploy, but it may be a hint that the dead can't cross the water either.  Has there been any indication (books or show) that the Others/White Walkers and their army of wights are blocked by open water, or are somehow tied to the land of the Frozen North, or that there's any other obstacle keeping them from just sailing/swimming around the Wall? Is it possible to legally visit uninhabited islands in the Pacific for a couple of days/weeks like Robinson Crusoe Cleric build to maximize melee damage and spell healing In a reverse split, what happens to odd lots? Let us say for example that I have 27 shares of stock X (there really is an X but its identity is not relevant).  In this example, X does a 5:1 reverse split. How many shares do I end up with. If I have fractional shares, what can I do with them? Some questions about Hilbert matrix This Exercise $12$ page $27$ from Hoffman and Kunze's book Linear Algebra.     The result o Example $16$ suggests that perhaps the matrix      $$A = \begin{bmatrix} 1 &amp; \frac{1}{2} &amp; \ldots &amp; \frac{1}{n}\\  \frac{1}{2} &amp; \frac{1}{3} &amp; \ldots &amp; \frac{1}{n+1}\\ \vdots &amp; \vdots &amp;  &amp; \vdots \\ \frac{1}{n} &amp;\frac{1}{n+1} &amp;\ldots &amp;\frac{1}{2n-1} \end{bmatrix}$$      is invertible and $A^{-1}$ has integer entries. Can you prove that?   I will copy their words before that example:      Some people find it less awkward to carry along two sequences of   matrices, one describing the reduction of $A$ to the identity and the   other recording the effect of the same sequence of operations starting   from the identity.   In this , Adrián Barquero says:      If your linear algebra textbook is Kenneth Hoffmann and Ray Kunze's   Linear Algebra book then I was in the same situation you're right now   a few years ago, but I was adviced at the moment that this particular   exercise wasn't as easy as one might expect at first.      The matrix you have is called the Hilbert matrix and the question you   have was already asked a couple of times in math overflow  and   . They have excellent answers so I will just point you to them.   My question: Is it possible answer the question made by the authors without using high techniques?  I am not allowed to use determinants here.  PS: Those answers in MO don't answer the question above. I apologise if this question is unappropriated or if I was unable to understand their answers. My understanding is that a question is historically locked because the information in it is too valuable for the question to be deleted.   But if the "valuable information" becomes obsolete, what happens then?  We cannot modify the question content, or the answers for a historically locked question, right? How to install latest HPLIP on my Ubuntu to support my HP printer and/or scanner? Let's assume that one has some modern HP printer and/or scanner which is not supported by HPLIP package from official repository. Currently Ubuntu versions have :  trusty (14.04LTS): 3.14.3-0ubuntu3.4 xenial (16.04LTS): 3.16.3+repack0-1 bionic (18.04LTS): 3.17.10+repack0-5 focal (20.04LTS): 3.20.3+dfsg0-2 groovy (20.10): 3.20.5+dfsg0-3build1 hirsute (21.04): 3.21.2+dfsg1-2   What can one do if printer is supported in newer version of HPLIP (checked this in )? In high Reynolds numbers we have turbulent flow. This is because the inertial forces are much greater than the viscous forces. I understand inertial forces to be actually the fictional forces due to the momentum of the moving fluid. And viscous to be the forces that drag each layers together. But why when inertial forces are too high than the viscous forces, there is turbulence? Why don't the molecules just continue to move laminarly due to their momentum? I can assume the viscous forces are not very strong in high speeds (in high inertial forces) to keep them together to form a laminar flow? But again, why they result in irrational movement after the effect of viscous forces is not anymore considerable? As a mechanical analogue, if I have balls attached together and make them travel linearly, and remove their between them attachment, they can continue to travel linearly. But what happens in fluid molecules that turbulence is created? Am I required to pay UK/ USA Visa fees if I have a German residence permit? Here is the situation. I am non-european but staying in Germany with a Residence permit. I can visit all Schengen listed countries with no issue. Assuming that I would want to travel to a non-Schengen country e.g. UK or USA, can I use my residence permit to travel there or am I required to apply as a citizen of my country? Assuming Germans are not required to pay Visa fees to that country, will I have to pay since I only have a residence permit? Third case, Europeans can fly directly to the airport of some countries and apply for a visa from there. In some other countries, you are required to book a Visa appointment at the embassy, which may be granted or not. Does that apply for one with a residence permit? At the start of A New Hope are Yoda and Obi-Wan really the last remaining Jedi? I have never read any Star Wars books and have no knowledge outside of the Star Wars films, but after seeing Episode III I am left wondering if there are any other Jedi that escape death and remain in hiding like Yoda did. I mean with regard to the "official" events in the Star Wars universe. I am trying to install Linux headers for Kali Linux on my machine and I have tried every possible solution on the internet but it always show "Unable to locate packages "  root@kali:/usr/sbin# apt-get install linux-headers-4.6.0-kali1-amd64 Reading package lists... Done Building dependency tree        Reading state information... Done E: Unable to locate package linux-headers-4.6.0-kali1-amd64 E: Couldn't find any package by glob 'linux-headers-4.6.0-kali1-amd64' E: Couldn't find any package by regex 'linux-headers-4.6.0-kali1-amd64'   Here is my sources.list file :  # Regular Repositories deb http://http.kali.org/kali sana main non-free contrib deb http://security.kali.org/kali-security sana/updates main contrib non-free # Source repositories deb-src http://http.kali.org/kali sana main non-free contrib deb-src http://security.kali.org/kali-security sana/updates main contrib non-free   Uname -a output :   root@kali:/usr/sbin# uname -a Linux kali 4.6.0-kali1-amd64 #1 SMP Debian 4.6.4-1kali1 (2016-07-21) x86_64 GNU/Linux Display command in xterm titlebar My Bash Prompt is currently setting the xterm titlebar using the following sequence:  PS1='\033]0;\u@\h&gt;\w\007'   Is there an easy way to display the current command in the titlebar. For example, if I am tailing a file using tail -f foo.log, I want my titlebar to say tail -f foo.log. Google Maps API map doesn't appear