java.lang.NullPointerException Main.java:6
What is a NullPointerException, and how do I fix it?
Is a constant condition more costly than switching shaders? Ignore folders/files when Directory.GetFiles() is denied access I am trying to display a list of all files found in the selected directory (and optionally any subdirectories). The problem I am having is that when the GetFiles() method comes across a folder that it cannot access, it throws an exception and the process stops.  How do I ignore this exception (and ignore the protected folder/file) and continue adding accessible files to the list?  try {     if (cbSubFolders.Checked == false)     {         string[] files = Directory.GetFiles(folderBrowserDialog1.SelectedPath);         foreach (string fileName in files)             ProcessFile(fileName);     }     else     {         string[] files = Directory.GetFiles(folderBrowserDialog1.SelectedPath, "*.*", SearchOption.AllDirectories);         foreach (string fileName in files)             ProcessFile(fileName);     }     lblNumberOfFilesDisplay.Enabled = true; } catch (UnauthorizedAccessException) { } finally {} I read this book about 20-30 years ago. It's about a group of teenagers who wake up in a huge room where each is on a separate platform. As the book progresses, they go unconscious and the platforms change, move or become connected. The children are punished or rewarded through a variety of methods including food and pain. The goal is to get them to either hurt each other, or see if they band together, I can't quite remember. Please tell me if this sounds familiar. I want to say:  Please, check out our handbook/knowledge base.  ...but it can be interpreted in two ways:  Please, check out our (handbook/knowledge) base. — as if there is a &quot;handbook base&quot; and a knowledge base. Please, check out our handbook/(knowledge base). — the meaning I'm after.  I thought that maybe I should put spaces around the slash to better separate the two choices:  Please, check out our handbook / knowledge base.  ...but I'm not sure that does it. I checked out  on the topic and it didn't answer my question.  Is it possible to convey the right meaning with a slash, or should I just avoid it altogether? Is the density operator a mathematical convenience or a 'fundamental' aspect of quantum mechanics? In quantum mechanics, one makes the distinction between mixed states and pure states.  A classic example of a mixed state is a beam of photons in which 50% have spin in the positive $z$-direction and 50% have spin in the positive $x$-direction.  Note that this is not the same as a beam of photons, 100% of which are in the state $$ \frac{1}{\sqrt{2}}[\lvert z,+\rangle + \lvert x,+\rangle]. $$ It seems, however, that at least in principle, we could describe this beam of particles as a single pure state in a 'very large' Hilbert space, namely the Hilbert space that is the tensor product of all the $\sim 10^{23}$ particles (I think that's the proper order of magnitude at least).  So then, is the density operator a mathematical convenience, or are there other aspects of quantum mechanics that truly require the density operator to be a 'fundamental' object of the theory?  (If what I mean by this is at all unclear, please let me know in the comments, and I will do my best to clarify.) In how many ways $5$ different rings can be worn on $4$ fingers ?   Although these is similar question here   But I want to answer it in different way which is like take $4$ fingers like $a,b,c,d$. Now $a$ can be filled in $5$ ways and so are others. So total ways are $5^4$ but answer is $4^5$. What is wrong in my reasoning? Which cases have I left out? Is there a way to change the restrictions passcode on an iPhone without restoring the phone? I haven't forgotten the passocde; I just want to change it. How to export SVG without lines, polylines and polygons? Suppose that $X_1,X_2,\ldots,X_n$ is a random sample from a distribution with probability function:  $$p_X(x)=\begin{cases} 1/2 &amp;x=-1,1 \\ 0 &amp; \text{otherwise} \end{cases}$$  Now if we define the average   $\bar{X}= \frac{1}{n} \sum X_i$, I need to show that that for an odd $n$ the probability function for $\bar{X}$ is:  $$p_{\bar{X}} (x)= \frac{\binom{n}{\frac{n}{2} (x+1)}}{2^n}$$ for $x=\pm 1/n,\pm 3/n,\ldots,\pm 1$, $0$ otherwise.    Since $P[\sum X_i=k] =P [\bar{X}=k/n ]$, I thought it would be easier to first derive the probability function for the sum. Since they are still $2^n$ n-tuples and each one is equilikely, I need to count the ones whose sum is $k$ for:  $k=\pm 1,\pm 3,\ldots \pm n-2,\pm n$ (since even numbers are not a feasible combination)  The problem here though is that I do not immediately recognize how I could count all these tuples. Could you please help me with this? If it is easier to proceed another way, I am of course all ears. Is there a website that summarizes the luggage and check-in requirements for each airplane company? I don't travel much by plane but I use a wide variety of companies. Since there is not much harmonization, specially with LCC, each one has its own requirements about luggage size, weight, extra luggage price, need for on-line check-in and printed ticket.  Each time I fly I ask my self what are the requirements, and although I can check them online at the official website this can't be quick, sometimes I leave it for last or, with so many, I just confuse airline requirements.  Is there a website or resource that lists this information? This would be very useful to quickly check or just compare requirements. Elements order in a "for (… in …)" loop Does the "for…in" loop in Javascript loop through the hashtables/elements in the order they are declared? Is there a browser which doesn't do it in order? The object I wish to use will be declared once and will never be modified.  Suppose I have:  var myObject = { A: "Hello", B: "World" };   And I further use them in:  for (var item in myObject) alert(item + " : " + myObject[item]);   Can I expect 'A : "Hello"' to always come before 'B : "World"' in most decent browsers? Anbox snap installation fails I'm trying to install Anbox in my new Ubuntu 16.04 using instructions from  . It seemed to work, but I got no icon in my dash, so I suspect that the installation is not complete. With Synaptic  Package Manager I can see that anbox-common &amp; anbox-modules-dkms are installed, but I have no icon in my dash, so I tried running   sudo snap install --edge --devmode anbox   and got this error:  error: cannot install "anbox": Get    https://search.apps.ubuntu.com/api/v1/snaps/details/core?channel=stable&amp;fields=anon_download_url%2Carchitecture%2Cchannel%2Cdownload_sha3_384%2Csummary%2Cdescription%2Cdeltas%2Cbinary_filesize%2Cdownload_url%2Cepoch%2Cicon_url%2Clast_updated%2Cpackage_name%2Cprices%2Cpublisher%2Cratings_average%2Crevision%2Cscreenshot_urls%2Csnap_id%2Csupport_url%2Ccontact%2Ctitle%2Ccontent%2Cversion%2Corigin%2Cdeveloper_id%2Cprivate%2Cconfinement%2Cchannel_maps_list:    dial tcp: lookup search.apps.ubuntu.com on [::1]:53: read udp    [::1]:54255-&gt;[::1]:53: read: connection refused   What am I doing wrong? Why was the connection refused? According to the instructions, it seems this is the correct command to install it.  ps: I'm not that familiar with Linux, I'm more of a Windows guy, trying to learn Linux. Find the last $2$ digits of $$\large7^{7^{7^{10217}}}$$   So far I have: $17\cdot 601=10217$ and $7=7 \pmod{10}$. Any help greatly appreciated. How do you get the footer to stay at the bottom of a Web page? I have a simple 2-column layout with a footer that clears both the right and left div in my markup.  My problem is that I can't get the footer to stay at the bottom of the page in all browsers.  It works if the content pushes the footer down, but that's not always the case. Equifax credit score discrepancy in 1 month, why? I just went to Citi Bank's website (which gives free FICO scores from Equifax for users of some credit cards) and I realized that the score I saw there was a whopping ~50 points higher than the number I saw on Mint.com, despite the fact that they both claim to have received the number from Equifax. The only difference is the fact that Mint checked my score at the end of February 2015, whereas Citi checked it at the end of January 2015.  However, nothing unusual has happened to my credit history in between these two. I can confirm on Mint's credit report that nothing seems wrong, so I'm quite confused:   What can cause such a discrepancy? Whom should I contact to understand what is going on -- Mint, Citi, Equifax, or other? How should I contact them -- file a dispute, call them, or go somewhere in person?   Note that I don't feel I'm "disputing" anything, since I don't know if there is any cause for a dispute, and hence I'm not sure it makes sense to open a dispute with Equifax. I just want to know what is going on and why two third-party agencies claim to have received such wildly different numbers from the same agencies in the span of 1 month. Reclassifying only part of raster keeping rest unchanged? I'm trying to reclassify only a certain part of a raster...not the entire raster.  Here's the problem.  I have a LULC raster of the entire US.  There are some issues with the raster that I want to fix, namely some cells that should be water are classified as wetlands.  These are mainly off the coast, not inland.  However, some cells that are classified as wetlands should really be wetlands (mainly inland) and I don't want these to change.  I plan on using a ZIP Code boundary file to determine which cells are considered "off the coast" and which ones are not.  I have access to ArcGIS 10.1 and R with the appropriate raster packages I need so a solution in either software would be fine.  Can anyone help me figure this out? Combo box in Java - nullpointer Partition 256 GB SSD for dual Boot, Windows 10 + Ubuntu 19.04 I plan on installing Ubuntu 19.04 and give it 100 GB of my 256 GB SSD space and leave the rest for Windows as I would primarily be using Ubuntu.     A. Now, as I plan to install a lot of applications in Ubuntu, should I give more space to the / partition than the home partition? (Or maybe even drop the /home altogether?).    B. I have another 128 GB SD card for files like photos, audio and video files. Please suggest me the optimal partitioning for the use case.    C. Also, I have 8 GB of RAM so what should be the optimal swap partition size?    D. How would the distribution be if I choose "install Ubuntu alongside Windows" option? How does the computer know that the PC was not properly shut down? Actually, this question struck me because of power cuts in my house. When there is a power cut, there is a sudden loss of power from the computer.   How does the computer know that the shutdown was not properly done? Find the probability of an event determined by an inequality on exponential random variables Let X and Y be independent random variables such that $X∼Exp(1)$ and $Y∼Exp(2)$.  Find the probability that $3X+4Y≤5$.  I thought the equation for solving this was the integral from $\int_0^a \int_ 0^{(a-y)} \lambda_1 e^{-\lambda_1 x}\lambda_2 e^{-\lambda_2 y} dx dy$ where $a = 5$, so I first started with the integral from $\int_0^5 \int_0^{(5-y)} 3 \times 1e^{-1x} \times 4 \times 2e^{-2y} dx dy$ and got $23.83828927$ which is much larger than a probability can be. Any help?