java.lang.NullPointerException on doFilter
What is a NullPointerException, and how do I fix it?
I need to find limit points of the set $\{  \frac{1}{n} +\frac{1}{m} \mid n, m = 1,2,3,\dots \}$.  My try :   If both $m$ and $n$ tend to very large values say $\infty$ then the value of $\{ \frac{1}{n} +\frac{1}{m} \}$ tends to $0$, and if only one of $m$ or $n$ tends to very large values, then the set $\{ \frac{1}{n} \mid n=1,2,3,\dots\} $ acts as limit points.   So is it true that a set of limit points is $\{0\}$ $\cup$  $\{\frac{1}{k} \mid k=1,2,3,\dots \}$?  How should I write this proof rigorously? Proving that the tangent to a convex function is always below the function iOS crash log catch, debug info.. Catch and send via email to the Dev team Take $\mathbb{R}$ as a vector space over $\mathbb{Q}$, then a basis for $\mathbb{R}$, then $|B|=2^{\aleph_0}$ I am trying to prove that Given $\mathbb{R}$ as a vector space over $\mathbb{Q}$, and a basis for $\mathbb{R}$, then $|B|=2^{\aleph_0}$.  Proof: Suppse that $|B|&lt;2^{\aleph_0}$, then $\mathbb R = span\{ b_i ; b_i \in B \}$ So $ \mathbb R = \{ \Sigma_{i=1}^{\infty}\alpha_i b_i ; \alpha_i \in \mathbb Q, b_i \in B, i \in \mathbb N \} = \bigcup_{n=1}^{\infty}\{\Sigma_{i=1}^n \alpha_ib_i\}$. Which is a contradiction since a countable union of countable union of countable sets is countable and $\mathbb R$ is not.  What do you think?  Thank you, Shir NSInvocation for Dummies? How exactly does NSInvocation work? Is there a good introduction?  I’m specifically having issues understanding how the following code (from Cocoa Programming for Mac OS X, 3rd Edition) works, but then also be able to apply the concepts independently of the tutorial sample. The code:  - (void)insertObject:(Person *)p inEmployeesAtIndex:(int)index {     NSLog(@"adding %@ to %@", p, employees);     // Add inverse of this operation to undo stack     NSUndoManager *undo = [self undoManager];     [[undo prepareWithInvocationTarget:self] removeObjectFromEmployeesAtIndex:index];     if (![undo isUndoing])         [undo setActionName:@"Insert Person"];      // Finally, add person to the array     [employees insertObject:p atIndex:index]; }  - (void)removeObjectFromEmployeesAtIndex:(int)index {     Person *p = [employees objectAtIndex:index];     NSLog(@"removing %@ from %@", p, employees);     // Add inverse of this operation to undo stack     NSUndoManager *undo = [self undoManager];     [[undo prepareWithInvocationTarget:self] insertObject:p                                        inEmployeesAtIndex:index];     if (![undo isUndoing])         [undo setActionName:@"Delete Person"];      // Finally, remove person from array     [employees removeObjectAtIndex:index]; }   I get what it’s trying to do. (BTW, employees is an NSArray of a custom Person class.)  Being a .NET guy, I try to associate unfamiliar Obj-C and Cocoa concepts to roughly analogous .NET concepts. Is this similar to .NET’s delegate concept, but untyped?   This isn’t 100% clear from the book, so I’m looking for something supplemental from real Cocoa/Obj-C experts, again with the goal that I understand the fundamental concept beneath the simple(-ish) example. I'm really looking to be able to independently apply the knowledge -- up until chapter 9, I was having no difficulty doing that. But now ...    Thanks in advance! Alphabetically display the items in itemize I am wondering whether it is possible to display the items alphabetically regardless of the order we type on the backend? For example I am using itemize as follows:  \begin{itemize}     \item ISDYNSTP:  Is dynamic time step used?     \item ISCDCA:     \item MVAR     \item IS2TL_ \end{itemize}   But I want to display the items in the alphabetical order. I don't know whether it is possible or not. I have like hundreds of itemize items. Actually I am trying to create a glossary for the different parameters. Windows 7 won't boot after installing Ubuntu 11 In the example below, an error is caused by \begin{align} and \end{align} but I do not see why:  \documentclass{book} \usepackage{amsmath} \newenvironment{subalign}{\begin{subequations}\begin{align}}{\end{align}\end{subequations}} \begin{document} \begin{subalign}      a&amp;=b\\     &amp;=b \end{subalign} \end{document} Matrices that commute with all matrices Adverb between conjugated verb and infinitive? Here's a module with castellated pads attached to an eval board:  (source: ) Looking closely at the wires interfacing with the castellated pads, they're clearly soldered to the eval board. However, they don't look like they're soldered to the module, but rather they're being pressed against the pads with the tension in the wire. It also looks like the free end of the loops are protruding down into a channel cut in the PCB. Do these kind of connectors/metal loops have a name? Verify $\int\sec x\ dx=\frac12 \ln \left\lvert\frac{1+\sin x}{1-\sin x}\right\rvert + C$ Which Ubuntu repositories are totally safe and free from malware? I just recently bought an arduino and requires gcc-avr/avrdude to compile the software. I installed avr for another microprocessor component a while ago but is obviously an outdated version (gcc version 3.3 20030512 (prerelease)) so I went ahead to update these but it didn't work.   (Please note that before hand I also broke my aptdaemon through an incomplete wine installation (couldn't get passed font installation agreement) but I fixed that via a re-installation then accepting the agreement.)   I am trying to update these by running  but the dependencies it requires cannot be installed due to avr... terminal reports   Reading package lists... Done Building dependency tree        Reading state information... Done build-essential is already the newest version. libncurses5-dev is already the newest version. libncurses5-dev set to manually installed. libusb-0.1-4 is already the newest version. libx11-dev is already the newest version. You might want to run 'apt-get -f install' to correct these: The following packages have unmet dependencies: arduino-core : Depends: gcc-avr but it is not going to be installed avr-libc : Depends: gcc-avr (&gt;= 1:4.3.4) but it is not going to be installed Depends: binutils-avr (&gt;= 2.20) but it is not going to be installed binutils-dev : Depends: binutils (= 2.21.0.20110327-2ubuntu3) but 2.21.0.20110327-2ubuntu2 is to be installed libcwidget-dev : Depends: libcwidget3 (= 0.5.16-3ubuntu2) but it is not going to be installed Depends: libsigc++-2.0-dev but it is not going to be installed Depends: libncursesw5-dev but it is not going to be installed libmpfr-dev : Depends: libgmp3-dev (&gt;= 4.2.dfsg-1) but it is not going to be installed python-dev : Depends: python (= 2.7.1-0ubuntu5.1) but 2.7.1-0ubuntu5 is to be installed Depends: python2.7-dev (&gt;= 2.7.1-1~) but it is not going to be installed tk8.4-dev : Depends: tk8.4 (= 8.4.19-4) but it is not going to be installed E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).   But running   apt-get -f install   results in   (Reading database ... 163021 files and directories currently installed.) Unpacking binutils-avr (from .../binutils-avr_2.20.1-1ubuntu2_i386.deb) ... dpkg: error processing /var/cache/apt/archives/binutils-avr_2.20.1-1ubuntu2_i386.deb (--unpack): trying to overwrite '/usr/bin/avr-size', which is also in package avr-binutils 2.13.90.030512-2 dpkg-deb: error: subprocess paste was killed by signal (Broken pipe) Unpacking gcc-avr (from .../gcc-avr_1%3a4.3.5-1_i386.deb) ... dpkg: error processing /var/cache/apt/archives/gcc-avr_1%3a4.3.5-1_i386.deb (--unpack): trying to overwrite '/usr/bin/avr-g++', which is also in package avr-gcc-c++ 3.2.90.20030512-2 dpkg-deb: error: subprocess paste was killed by signal (Broken pipe) Errors were encountered while processing: /var/cache/apt/archives/binutils-avr_2.20.1-1ubuntu2_i386.deb /var/cache/apt/archives/gcc-avr_1%3a4.3.5-1_i386.deb E: Sub-process /usr/bin/dpkg returned an error code (1)   I have tried running update manager and updating my system via it but all I get is an error message  The following packages have unmet dependencies: arduino-core: Depends: gcc-avr but it is not installed avr-libc: Depends: gcc-avr (&gt;= 1:4.3.4) but it is not installed Depends: binutils-avr (&gt;= 2.20) but it is not installed   which then tells me to try running apt-get -f install which just results the same as last time.  So how can I fix my system, I really need the new avr, please :) BTW, my system is Ubuntu 11.04 How can I easily encrypt a file? Is there any simple (IE: right click in Nautilus) way to password protect a particular folder/file in Ubuntu? I've got a few files containing sensitive info and I'd much prefer that if/when I leave my computer alone, they aren't accidentally accessed by someone else.  The secruty does not have to be extremely tight. My only concern is that when family/friends come over, I don't really like the idea of them looking at my bank details, accounts or, you guessed it, porn collection.  A simple, effective way to let me put my machine in the hands of someone else knowing that said machine can not cause me embarresment is the sole reason why I'd like to see this in Ubuntu. What is duplicate content and how can I avoid being penalized for it on my site? I've been told not to use for...in with arrays in JavaScript. Why not? There are 8 crew members, The leading member wants that only a crew of 5 people or more could open a safe he bought, To each member he gave equal amount of keys, and locked the safe with several locks.  What's the minimal number of locks he should put:   at least 5 crew members would be required to open the safe? Any team of 5 crew members could open the safe, but none of the 4 team crew members.   1,2 stands individually of course. How do I increase the render distance? Many anime do not follow the manga history and are made shorter. However there are a few that do follow it or try to finish the history with the manga like the big three (Naruto, Bleach, One Piece), Death Note, and FMA Brotherhood. But others finish the anime without the real ending like FMA, Ouran High School, Pandora Hearts, Tsubasa Reservoir Chronicle. Why is this so? Is it because of the high cost? Or the lack of popularity?