my error is:   Exception in thread "main" java.lang.NullPointerException     at ClienteDB.incluir(ClienteDB.java:30)     at ClienteInterface.main(ClienteInterface.java:16)   I just checked some other questions, but didn't found why this is happening. Usually people say that have some variable with value null. In a first moment, the variable will be null, but i didn't do the assignment with null.  Line 30 is: c[npp].setCpf(ccpf); That is one method of my program, but the only one that is not working.  Thanks in advance for the help!  import javax.swing.JOptionPane;  public class ClienteDB { Cliente c[]= new Cliente[11];  public void incluir() {     int flag = 0;     String np = JOptionPane             .showInputDialog("Entre a posição que você deseja incluir um usuário (1 a 10):");     int npp = Integer.parseInt(np);     if (c[npp] != null) {         JOptionPane.showMessageDialog(null,                 "Já possui um cliente neste campo.");     } else {         String ccpf = JOptionPane.showInputDialog("Qual o cpf do cliente "                 + npp + "?");         for (int np2 = 1; np2 &lt; 11; np2++) {             if ((c[np2] != null) &amp;&amp; (c[np2].getCpf().equals(ccpf))) {                 JOptionPane.showMessageDialog(null,                         "Existe um usuário com este CPF.\n");                 flag = 1;                 break;             } else {                 c[npp].setCpf(ccpf);                 break;             }         }         if (flag != 1) {             String cnome = JOptionPane                     .showInputDialog("Qual o nome do cliente " + npp + "?");             c[npp].setNome(cnome);             String cend = JOptionPane                     .showInputDialog("Qual o endereço do cliente " + npp                             + "?");             c[npp].setEndereco(cend);             String ctel = JOptionPane                     .showInputDialog("Qual o telefone do cliente " + npp                             + "?");             c[npp].setTelefone(ctel);         }     } }   Thats my main:      import javax.swing.JOptionPane;  public class ClienteInterface { public static void main (String args[]) {   ClienteDB cc = new ClienteDB();     int funcao;     do{         String fc=JOptionPane.showInputDialog("Bem vindo!\nQual função você deseja?\n1-Incluir Cliente\n2-Consultar Cliente\n3-Alterar Cliente\n4-Excluir Cliente\n5-Listar clientes\n9-Fim");         funcao=Integer.parseInt(fc);       switch (funcao)      {         case 1:         cc.incluir();         break;          case 2:         cc.consultar();         break;          case 3:         cc.alterar();         break;          case 4:         cc.excluir();         break;          case 5:         cc.listar();         break;          case 9:         break;      }                     }while (funcao!=9); }   }
What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?  What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?
How to add percent sign to NSString Cannot Delete Item "Could Not Find This Item" issue A friend sent a long a file (a .rar) he wanted me to check out for him before he installed it.  I downloaded it and unrared it with no problems, but it was full of .exe's instead of the intended contents (fonts) so I advised him to delete it immediately and not use.  I then proceeded to do the same, but the folder simply will not delete. Oddly the files went fine, and I never ran anything, but this is what I'm seeing:   Could not find this item  This is no longer located in C:\Users\This_User\Desktop. verify the item's location and try again.   I've tried the following things with no help:   Using "Unlocker" to Unlock and delete Using move on reboot and rebooting Using PendMoves (from sysinternals) and rebooting Elevating a cmd line, doing a dir /x to get the short name of the folder, and then del 'shortna~1' Moving the folder to a new folder and then trying to delete the parent folder   I'm on Windows 7 RTM, very fresh install. Any thoughts?  Update: Just to confirm, I've run Hijack this and half a dozen other malware detectors, and everything came back clean (no extra processes, no other obvious badness). Rebooting in safe mode didn't help either. Irrationality of $\sum_{p\in\mathbb{P}} \frac{1}{2^{p}}$ Let $\mathbb{P}$ be the set of prime numbers, and consider $m=\displaystyle\sum_{p\in\mathbb{P}} \frac{1}{2^{p}}$. Is $m$ irrational?  In the following  , the author recalls several sufficient criteria for irrationality. When applying some of this criteria to $m$ I arrive that the condition of some of the criteria are not satisfied by means of Bertrand´s Postulate.  I found a similar result by Sándor:  Let $\lbrace a_m\rbrace$, $m\geq 1$ be a sequence of positive integers such that $$\text{lim sup}\frac{a_{m+1}}{a_1a_2\cdots a_m}=\infty \;\;\;\;\text{and}\;\;\;\;\;\;\text{lim inf}\frac{a_{m+1}}{a_m}&gt;1$$ Then the series $\displaystyle\sum_{m} \frac{1}{a_m}$ is an irrational number.  I have yet to prove that if $f$ is a continuous function then $\text{lim sup} \;f(a_m)=f(\text{lim sup}\, a_m)$. Assuming this;  $$\text{lim inf}\; P_{m+1}-P_{m}&gt;1$$ where $P_m$ is the $m$-th prime.  But I have trouble with $$\text{lim sup}\; P_{m+1}-\sum_{j=1}^{m}P_{j}$$ my guess is that $$\text{lim sup}\; P_{m+1}-\sum_{j=1}^{m}P_{j}\neq\infty$$ so $$\text{lim sup}\frac{2^{P_{m+1}}}{2^{P_1}\cdots 2^{P_m}}\neq\infty$$   and this theorem will result useless to tell if $m$ is irrational.   How can I prove (or disprove) that $m$ is irrational (are there any other simpler criteria)? How can I use Dirichlet criterion  or Hurwitz criterion? What is its ?    Any help is highly appreciated.      Edit: That $m$ is irrational is clear by the criteria provided in the comments, namely that if $x$ is rational then the binary representation of $x$ is periodic. The number $m$ in binary expansion has $1$ in the $P$-th position and zero elsewhere. As there are arbitrarily large gaps between primes; then the binary representation fails to be periodic.  So the question that remains unsolved is:   What is its irrationality measure? Universe being flat and why we can't see or access the space "behind" our universe plane? I'm a layman, but i watched some intereting videos about big bang on youtube[michio kaku, hawking this kind of things, not some crackpots :)]   I described everything on my picture:     So is there any possibility to see that inner side, for example, by creating wormhole, or entering new dimension? Or i get something wrong, on the way, and my model is totally wrong?  And another question. I heard that even if you would fly a ship across the universe you would eventually end in the place where you started, because we live in a 3d space that is defined on a surface of big sphere. But why a big sphere? Universe might be a big mobius strip, and the same principle of ending at start will apply. Is it possible?  Thanks in advance, and please don't bash me for silly questions. Im just curious, but i don't have any degree in physics. Im also not an english native speaker. A word for "every two days" I am looking for the name of a sci-fi cartoon. Key plot points  There were Space ships flown by some American voiced crew. They were some kind of rescue squad or maybe space police. In one episode they went to an orange tree farm in a canyon which used mirrors on the canyon walls to direct sunlight to the oranges. They had a force field that protected the canyon but some bad guys came and shot the mirrors. The heroes came back and saved the day. Another episode was set in an underground ancient tomb of some kind that was covered by a sandstorm once they escaped.  Chronology and other relevant information It had kind of 80s or 90s animation. I don't think I ever saw it on TV. I only ever watched the same 3 episodes on VHS. The error: "mount: only root can do that" while mounting a partition Is it possible to add non-number characters on the left of a code block?  Specifically, I want to add ... to correspond with some source code (see image).    I am currently using commands to stop / start the line numbers. Here is the code:  \documentclass[12pt]{article}  \usepackage[a4paper, margin = 20mm, top = 30mm, bottom = 25mm]{geometry} \usepackage{xcolor, soul} \usepackage{listings}  \definecolor{codebackground}{rgb}{0.95, 0.95, 0.92}  \lstset {     frame            = tb, % hrule above and below     keepspaces       = true,     columns          = flexible,     basicstyle       = {\normalsize\ttfamily},     escapeinside     = {(*@}{@*)}, % for escaping     backgroundcolor  = \color{codebackground},     showstringspaces = false,     language         = C,     keywordstyle     = \color{blue},     stringstyle      = \color{red},     commentstyle     = \color{teal},     numbers          = left, % {none, left, right}     firstnumber      = 1,     numberstyle      = \scriptsize\color{black},     numbersep        = 5pt }  \let\origthelstnumber\thelstnumber \makeatletter % for stopping line numbers \newcommand*\stopnumber {     \lst@AddToHook{OnNewLine}     {         \let\thelstnumber\relax         \advance\c@lstnumber-\@ne\relax     } } % for starting line numbers \newcommand*\startnumber[1] {     \setcounter{lstnumber}{\numexpr#1-1\relax}     \lst@AddToHook{OnNewLine}     {         \let\thelstnumber\origthelstnumber         \refstepcounter{lstnumber}     } } \makeatother  \begin{document} \begin{lstlisting}[firstnumber = 456] int third_function(const int c) {     /* error checking */     if(c &lt; 0)     {         return;     } (*@ \stopnumber @*)      ...     (*@ \startnumber{512} @*)     /* other stuff */     int d = 4;     char* word = "third function"; (*@ \stopnumber @*)      ...     (*@ \startnumber{678} @*)     return (c * d); } \end{lstlisting} \end{document} I'm planning to visit a summer school in the Schengen zone. What kind of visa should I apply for? I will be attending a summer school which will last 10 days.  I want to know: for applying for a Schengen visa, is the purpose of my trip considered as touristic or study? What are the options for managing multiple Ubuntu installations together and keeping them in same state Squeezing all the SEO out of a URL as possible I am working on an ecommerce site, I told our SEO consultant that I plan to make the URL scheme: /products/&lt;id&gt;/&lt;name&gt;. This is similar to Stackoverflow's URLs which are /questions/&lt;id&gt;/&lt;title&gt;.  He asked me if I could change the URL scheme to /p/&lt;id&gt;/&lt;name&gt; instead. I know why he wants this change, the word "products" isn't needed to find the correct product, and it doesn't offer any SEO, so shortening it to just p would make the relevant keywords in the &lt;name&gt; weigh more.  His main priority is maximizing SEO, but the part that I don't think he is considering is how this effects the semantics of the site. Also having the word "products" looks like it has meaning and a reason for being there, just having a p looks chaotic and ugly to me. I also don't think it makes that much of a difference does it? Stackoverflow doesn't use /q/&lt;id&gt;/&lt;title&gt; and they do just fine, I do realize that theres many factors at play here though, not just the URL.  So I want some outside opinions on which is the better way and why? I am looking for an English translation of Markov's 1958 paper, On insolubility of the homeorphy problem, which I remember coming across on a website for a computational topology course (taught by Afra Zomorodian). literally as a hyperbole Possible Duplicate:          I have seen a lot of backlash in internet media against people using the word literally to mean something not literal. Something like "he was literally as big as a house" to mean someone was very big I think this comic sums up the anti literal movement well   Is this actually grammatically incorrect or can the word "literally" be a hyperbole strengthening a metaphor? Gradient of $a^T X b$ with respect to $X$ How can I find the gradient of the term $a^TXb$ where $X$ is a $n \times m$ matrix, and $a$ and $b$ are column vectors.  Since the gradient is with respect to a matrix, it should be a matrix.  But I do not have a clue on how to derive this gradient.   Any help ? What screenshot tools are available? I have noticed that many people use images in their Ask Ubuntu answers. What software are people using? I have a Toshiba z830 with ubuntu 12.04. I could use without problems the DSL connection (I have no router, only a modem) at home until a couple of days ago:   It stopped to work when while connected with the DSL connection via CiscoVPN (to my workplace), I had to leave the laptop unattended for a while and I guess the laptop tried to go to suspend. When I came back there were many messages on a black screen (sorry, I cannot attach an image because of my low reputation) and the laptop was not responding. I had to reset the it.   After rebooting it, the internet connection didn't work as did before (no browsing, no ssh, no skype, etc) although it says it is connected. I only can browse, etc when I connect to CiscoVPN to the same place where I was connected when it crashed.  When I am physically where I connected via VPN (at work) I can use the wireless, which I am not able to use now anymore since the incident described above.  Some additional information:  martillu@ubuntu:~$ cat /etc/resolv.conf   domain km.icrr.u-tokyo.ac.jp   nameserver 10.240.12.134   nameserver 10.240.12.135    martillu@ubuntu:~$ route -n   Kernel IP routing table   Destination Gateway Genmask Flags Metric Ref Use Iface   0.0.0.0 61.127.116.199 0.0.0.0 UG 0 0 0 ppp0   61.127.116.199 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0   169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 ppp0    martillu@ubuntu:~$ ifconfig   eth0 Link encap:Ethernet HWaddr e8:e0:b7:2f:bc:5a   inet6 addr: fe80::eae0:b7ff:fe2f:bc5a/64 Scope:Link   UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1   RX packets:17620 errors:0 dropped:1 overruns:0 frame:0   TX packets:13168 errors:0 dropped:0 overruns:0 carrier:0   collisions:0 txqueuelen:1000   RX bytes:19452232 (19.4 MB) TX bytes:2568218 (2.5 MB)   Interrupt:20 Memory:c0700000-c0720000    lo Link encap:Local Loopback   inet addr:127.0.0.1 Mask:255.0.0.0   inet6 addr: ::1/128 Scope:Host   UP LOOPBACK RUNNING MTU:16436 Metric:1   RX packets:134 errors:0 dropped:0 overruns:0 frame:0   TX packets:134 errors:0 dropped:0 overruns:0 carrier:0   collisions:0 txqueuelen:0   RX bytes:36316 (36.3 KB) TX bytes:36316 (36.3 KB)    ppp0 Link encapoint-to-Point Protocol   inet addr:219.167.252.226 P-t-P:61.127.116.199 Mask:255.255.255.255   UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1454 Metric:1   RX packets:17608 errors:0 dropped:0 overruns:0 frame:0   TX packets:13129 errors:0 dropped:0 overruns:0 carrier:0   collisions:0 txqueuelen:3   RX bytes:18993590 (18.9 MB) TX bytes:2221195 (2.2 MB)    wlan0 Link encap:Ethernet HWaddr 9c:b7:0d:d9:21:f3   UP BROADCAST MULTICAST MTU:1500 Metric:1   RX packets:0 errors:0 dropped:0 overruns:0 frame:0   TX packets:0 errors:0 dropped:0 overruns:0 carrier:0   collisions:0 txqueuelen:1000   RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)    I think that: domain km.icrr.u-tokyo.ac.jp in the resolv.conf may be suspicious, since if I am not connected via CiscoVPN I do not know why it should appear "km.icrr.u-tokyo.ac.jp" (this is where I connect via VPN). In  thread, the question was to find a $f: \mathbb{R} \to \mathbb{R}$ such that  $$f(f(x)) = f(x) + x$$  (which was revealed in the comments to be solved by $f(x) = \varphi x$ where $\varphi$ is the  $\frac{1+\sqrt{5}}{2}$).    Having read about  shortly before though, I came up with this train of thought:  $$f(f(x)) = f(x) + x$$ $$\Leftrightarrow f^2 = f^1 + f^0$$ $$f^2 - f - f^0 = 0$$  where $f^n$ denotes the $n$'th iterate of $f$.  Now I solved the resulting quadratic equation much as I did with plain numbers  $$f = \frac{1}{2} \pm \sqrt{\frac{1}{4} + 1}$$ $$f = \frac{1 \pm \sqrt{1+4}}{2} = \frac{1 \pm \sqrt{5}}{2}\cdot f^0$$  And finally the solution $$f(x) = \frac{1 \pm \sqrt{5}}{2} x .$$  Now my question is: *Is it somehow allowed to work with functions in that way?** I know that in the above, there are denotational ambiguities as $1$ is actually treated as $f^0 = id$ ... But since the result is correct, there seems to be some correct thing in this approach.  So can I actually solve certain functional equations like this? And if true, how would the correct notation of the above be? Is a transit visa required to pass through Bangkok Airport? Does encrypting my home folder make my computer more secure? Do I have to enter my password more if my home folder is encrypted? What else should I know about encrypting my home folder? Why does N[1.000 01, 10] return 1.00001, but N[1.000 001, 10] returns only 1.?