Java Servlet runs with status code HTTP/500
What is a NullPointerException, and how do I fix it?
I set up a micro instance (Ubuntu, 32 bit) and assigned an elastic IP (23.23.245.104)  Public DNS: ec2-23-23-245-104.compute-1.amazonaws.com  I successfully ssh'ed to the machine and installed nginx.  Then I edited the server_name attribute in nginx's config file (/etc/nginx/sites-available/default)  server_name ec2-23-23-245-104.compute-1.amazonaws.com;   Restarted nginx and hit  but got a "could not connect" error.  What did I forget to do?  Thanks On November 11, my American girlfriend flew over to the UK to visit me. She was detained and questioned, and refused entry under the belief she would not leave within those six months. This was due to her having no return flight booked, as we were going to wait until we were together in the UK to book a return flight for either herself, or both of us back to the USA once we decided what exact dates I would be able to travel. Lesson learned. A hard lesson, but we learned!  Anyway, they said although she'll flag up in the future, it won't be a straight ban from entering and she could even try to come back the next day if she so pleased, although it would be unwise if her circumstances had not changed.  Fast forward to December 18. She got herself a job in the States to prove she intends to leave at the end of her stay, and has since applied for a UK Standard Visitor Visa. She posted her documents off to New York earlier today.  Will they send her a letter back refusing to issue her a visa since, as an American citizen, she does not even need one? She wanted to get one to be safe as I told her not to fly to the UK even with return flights booked if she doesn't have a visa, as she risks being denied once again after a long flight. It's hard to get this information on the Internet, and some stories I read said that they were sent a letter back from the UK Embassy denying their visa simply because they don't need one in the first place - even after that person was denied entry.  We're in a bit of a pickle granted, so any help would be greatly appreciated.   Cheers. php - insert a variable in an echo string Distant aliens receive signal with instructions on how to construct humans, long after Earth is extinct How do I resolve these unmet dependencies? Given a subset $A$ of $\mathbb{R}^{n}$, a point $x \in \mathbb{R}^{n}$ is said to be in the boundary of A if and only if for every open rectangle $B\subseteq\mathbb{R}^{n}$ with $x\in B$, $B$ contains both a point of $A$ and a point of $\mathbb{R}^{n}\setminus A$.  My question is from Spivak's Calculus on Manifolds: Construct a set $A \subseteq [0,1]\times [0,1]$ such that $A$ contains at most one point on each horizontal and vertical line but has boundary equal to $[0,1]\times[0,1]$. Markov's paper on insolubility of the homeorphy problem Can you use twinned spell on the same target? So me and my DM have been arguing on how Twinned spell works. Twinned spell:  When you cast a spell that targets only one creature and  doesn't have a range of self, you can spend a number of  sorcery points equal to the spell's level to target a second  creature in range with the same spell (1 sorcery point if  the spell is a cantrip).  I argued that &quot;a second creature&quot; can be a the same target as it is not worded as &quot;another creature&quot; For this example counter spell with twinned spell.  A wizard casts fireball on me I use CounterSpell#1 with Twinned Spell Wizard casts CounterSpell on my CounterSpell#1 I use him as the same target for my twinned Spell CounterSpell#2  Result countered Is this possible? There are similar questions for  and , but I'm after a Windows solution here.  The problem is as follows: I want to write several (js) script files in a folder, and have a  program monitor that folder for file changes and new files being added, and run a command whenever that happens (to compile them all into one single file).  The solution has to:   Monitor both file changes and new files being added, in a folder. Run a command only if there is any change.   It would be best if it either is a built-in solution (like a JScript or VBscript snippet), or something that does not require installation. Linux/Bash: Create relative soft links to files in directory tree? Problem: Let $f$ be defined for all real $x$, and suppose that   $$|f(x)-f(y)|\le (x-y)^2$$  for all real $x$ and $y$. Prove $f$ is constant.  Source: W. Rudin, Principles of Mathematical Analysis, Chapter 5, exercise 1. I just came across this code and a few Google searches turn up no explanation of this mysterious (to me) syntax.  Hashtable^ tempHash = gcnew Hashtable(iterators_);  IDictionaryEnumerator^ enumerator = tempHash-&gt;GetEnumerator();   What the heck does the caret mean? (The gcnew is also new to me, and I asked about that .) Supremum and Infimum Let $A$ be a nonempty set of real numbers which is bounded below. Let $-A$ be the set of all numbers $-x$, where $x \in A$. Prove that $\inf A = -\sup(-A)$.   We know that $-A$ is bounded above. Hence $\sup(-A)$ exists. So to proceed, one would show that $\inf A \leq -\sup(-A)$ and $\inf A \geq -\sup(-A)$? Or $-\inf A &gt; \sup(-A)$ and $-\inf A &lt; \sup(-A)$.  I get the sense that one can use the following fact as well: (i) Let $\alpha = \sup S$. If $\beta &lt; \alpha$ then $\beta$ is not an upper bound for $S$. Problem with TP-Link TL-WN823N Recently I installed Ubuntu 16.04 but cant get the usb wifi adapter to work properly , can anyone help me?  When wifi adpater is plugged , the lsusb output:  Bus 001 Device 003: ID 2357:0109 Bus 001 Device 002: ID 0e0f:000b VMware, Inc.  Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub  Bus 002 Device 004: ID 0e0f:0008 VMware, Inc.  Bus 002 Device 003: ID 0e0f:0002 VMware, Inc. Virtual USB Hub Bus 002 Device 002: ID 0e0f:0003 VMware, Inc. Virtual Mouse Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub   When wifi adapter is unplugged , the lsusb output:  Bus 001 Device 002: ID 0e0f:000b VMware, Inc.  Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub Bus 002 Device 004: ID 0e0f:0008 VMware, Inc.  Bus 002 Device 003: ID 0e0f:0002 VMware, Inc. Virtual USB Hub Bus 002 Device 002: ID 0e0f:0003 VMware, Inc. Virtual Mouse Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub Please help solve 'oracle-java7-installer' missing. Error below  dpkg: warning: files list file for package !['oracle-java7-installer' missing;][1]  assuming package has no files currently installed (Reading database ... 302933 files and directories currently installed.) Removing linux-image-extra-3.8.0-19-generic ... /var/lib/dpkg/info/linux-image-extra-3.8.0-19-generic.postrm: 6: /var/lib/dpkg/info/linux-image-extra-3.8.0-19-generic.postrm: Syntax error: ")"  unexpected dpkg: error processing linux-image-extra-3.8.0-19-generic (--remove):  subprocess installed post-removal script returned error exit status 2  Errors were encountered while processing:  linux-image-extra-3.8.0-19-generic  E: Sub-process /usr/bin/dpkg returned an error code (1) A package failed to install.  Trying to recover: Novel about alien conquerers who conduct games to select servants I read this maybe 10-15 years ago.   Aliens have conquered earth. Possibly quite some time before the time then novel is set in. They have built their own cities. They conduct periodic athletic competitions for humans similar to the current olympics. The winners are take into the city to act as servants to the masters. The protagonist is part of a human resistance movement who is competing in the games. The plan is to infiltrate the city, gather intel and escape. He wins, get in, finally has to kill his master and escape down a waste disposal system which ends up outside the city.   Anybody knows which one I am talking about? Get selected value in dropdown list using JavaScript A non-diagonal $4\times4$ matrix such that $A^3=A^2=0$ and its rank is $2$ Let $A$ be a $4\times 4$ matrix over $\mathbb C$ such that $\operatorname{rank} A=2$ and $A^3=A^2\neq 0$  Suppose $A$ is not diagonalizable.  Then which one is true   $A^2=A\neq 0$ $\exists$ a vector $v$ such that $Av\neq 0$ but $A^2v= 0$ the characteristic polynomial of $A$ is $x^4-x^3$   My try:  If $A^2=A$ then minimal polynomial of $A$ has two distinct roots and hence $A$ is diagonalisable hence false. Since $A^3=A^2\neq 0$ so the characteristic roots of $A$ are $0,0,1$ Since rank of $A$ is $2$ $1$ must appear twice so the characteristic polynomial of $A=x^4-(0+0+1+1)x^3+(0.0.1+0.1.1+1.1.0+1.0.1)x^2+(0.0+0.1+1.1+...)x+(0.0.1.1)$ hence 3 is false  2 I am not sure Please find the error in my solution and don't refer it to some previous solution Select translation: where? Explicit example of a compact manifold of dimension $>2$ with strictly negative sectional curvature I am looking for examples of compact manifold of dimension $&gt;2$ with strictly negative sectional curvature (for dimension 2 it is well-known). Can anybody please help?