Currently I  have a code like this The School array represents the School and each element in the array represents a classroom At the moment I want to store the Class teachers name in the array and if there is no class teacher assigned yet, that particular array element has &quot;empty&quot; import java.util.*; public class Main {     public static void main(String[] args) {         String[] school = new String[8];         initialise(school);     }          private static void initialise( String schoolRef[] ) {         for (int x = 0; x &lt; schoolRef.length; x++ ) schoolRef[x] = &quot;empty&quot;;         System.out.println(Arrays.toString(schoolRef));     } }   Output of this is [empty, empty, empty, empty, empty, empty, empty, empty]  However now I want to create a Class called School and a Class called Classroom I want to use an array of Classroom objects School.java import java.util.Arrays;  public class School {      private Room[] rooms;      public static void initialise(Room[] schoolRef) {         for (int x = 0; x &lt; schoolRef.length; x++) {             schoolRef[x].setName(&quot;empty&quot;);             System.out.println(Arrays.toString(schoolRef));         }     } }  Room.java public class Room {      public String getName() {         return name;     }      public void setName(String name) {         this.name = name;     }      private String name;    }  Main.java public class Main {      public static void main(String[] args) {          Room[] rooms = new Room[8];         School.initialise(rooms);     } }  However when I run the Main method I get a NullPointerException. I want to create an array of Classroom objects instead of writing everything in one Class and I'm not sure what I am doing wrong in my approach. Appreciate any help.Thanks
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?
When entering the US on an international flight, and then on to a connecting domestic flight, what is the exact order of the process that will take place? In other words, what happens first, second, third, etc.? Can't login with Google on recently released iPhones and iPads I just switched from an iPhone 7 to an iPhone XR (both are on latest iOS version available).  When I try to login into the Stack Exchange app with my Google account it says:     Request failed   Could not perform this request with your account. Please try logging   back in. If this persist, let us know on meta.   this happens consistently.  I have 2FA enabled on my Google account if it matters.  Is this a known bug? Time travel short story where dinosaur doesn't taste like chicken I ran across this  and it reminded me of an old short story I read a long time ago.  I read it in a collection of short stories.  It would have been a book because I borrowed it from a school library.   Probably in the 1970s or 1980s.  The book and the story would have been older, though.  Probably from the 1960s or earlier.  In the story, a scientist has a machine that he can use to view the past.  It isn't like a TV or something, though.  It opens a portal into the past that he can look through.  It is very difficult to control, though.  Pointing it at a place and time is likened to working in a hurricane.  The portal jumps and moves about randomly.  The steadier he can hold it, the clearer the past appears.  When it is really steady, he can hear sounds from the past.  It never gets really steady enough to reach through.  They try pushing tools through the portal to pick up things in the past.  The steadier the portal, the easier it is to push tools through.  It never gets steady enough for them to push a tool all the way through, though.  It just gets easier to make progress.  One day, they have the portal open and it is acting really wild.  At some point the workers take a break.  The portal is still open, and while they are loafing the portal settles down by itself and becomes steadier than it has ever been.  So steady that a piece of the past falls through.  The portal scooped a chunk of the ground out of the past.  The chunk of ground contains a nest with dinosaur eggs.  The scientist  rescues the eggs, and they manage to keep them warm enough until they hatch.  The dinosaurs are some kind of harmless things, and become the lab mascots.  Always wandering the grounds, and in and out of the lab.  One day, the have the portal open again and one of the dinosaurs wanders into the lab and walks into some high voltage terminal.  Crash, bang.  Short circuit.  All the equipment to open the portal is destroyed, and the dinosaur is roasted from the flash.  The roasted dinosaur smells good, like chicken, and the scientist's son (who has been telling the story) takes a bite to see what it tastes like.  He says "It resembled chicken the way an asteroid resembles Jupiter."   It tastes fantastic. With all of their work ruined, and the ruins of a fortune in expensive equipment scattered around them, the scientists and the labworkers all chow down on dinosaur.  Half raw, half burned to a crisp from the explosion.  They eat the dinosaur down to the bones.  They start breeding the dinosaurs, and sell them as food.  The guy's father makes a fortune selling dinosaur meat, but remains unhappy.  With money from selling dinosaur meat, he rebuilt his lab but never ever got the portal steady enough to actually visit the past or take samples.  Does anyone know the name of the story and the author, and maybe what collection it might have been in? Why Do Jedi Use One Lightsaber in Combat? While I've seen Ahsoko Tano, in The Clone Wars, fight with two lightsabers (one in each hand), this seems to be extremely rare for Jedi.  We've seen Sith do this frequently, like Asajj Ventress, with two lightsabers.  Darth Maul used a dual lightsaber against Qui-Gon Jinn and Obi-Wan Kenobi and then there's General Grievous, who uses multiple lightsabers with his multiple limbs.  But most Jedi use just one blade, and their lightsaber hilts tend to be simple, just a straight cylinder without much adornment, while Sith sometimes use curved grips or other more exotic stylings.  Is there a reason Jedi stick with the simplest lightsaber designs and why the vast majority (at least in the movies and in The Clone Wars series) use only a single lightsaber? Detach (move) subdirectory into separate Git repository I have a  repository which contains a number of subdirectories. Now I have found that one of the subdirectories is unrelated to the other and should be detached to a separate repository.  How can I do this while keeping the history of the files within the subdirectory?  I guess I could make a clone and remove the unwanted parts of each clone, but I suppose this would give me the complete tree when checking out an older revision etc. This might be acceptable, but I would prefer to be able to pretend that the two repositories doesn't have a shared history.  Just to make it clear, I have the following structure:  XYZ/     .git/     XY1/     ABC/     XY2/   But I would like this instead:  XYZ/     .git/     XY1/     XY2/ ABC/     .git/     ABC/ What is the best way to remove and prevent leaf stains on concrete pavers? Is an Otyugh's grapple different from a standard grapple? In 5e to escape a Grapple check, I know you roll athletics (str) or acrobatics (dex) opposed by an athletics (str) check.  In the Otyugh entry in the Monster Manual it doesn't say there's any check, instead it assigns a set DC for an "escape" (DC 13... which is a str check with a 10 assigned as the "roll").    So the question is:    Does the Otyugh actually make a roll in this instance (like the general rule) or does it have a set DC (for the specific case) OR is it just a poorly communicated version of the GENERAL rule being played out?   A little clarification on this would be AWESOME. In the event of a "specific vs general" scenario, what actually is the "escape" roll?  What modifier is applied? None? Filling triangle doesn't seem to work with PGF/TikZ I was wondering if anyone had any clue on why my triangles are not filled in red within the following piece of code:  \documentclass{article} \usepackage{tikz} \usetikzlibrary{calc,3d}  \usepackage[active, pdftex, tightpage]{preview} \PreviewEnvironment[{[]}]{tikzpicture} \newcounter{number}  \begin{document} \begin{tikzpicture}[x  = {(-0.5cm,-0.5cm)},                     y  = {(0.9659cm,-0.25882cm)},                     z  = {(0cm,1cm)}] \begin{scope}[canvas is yx plane at z=0] \foreach \x / \y / \label in {1 / 0.4 / 1, 0.9 / 0.4 / 0, 1 / 0.5 / 1, 0.7 / 0.3 / 0, 0.8 / 0.4 / 0, 0.9 / 0.5 / 0, 1 / 0.6 / 1, 0.7 / 0.4 / 0, 0.4 / 0.4 / 0, 0.6 / 0.4 / 0, 0.5 / 0.4 / 0, 0.8 / 0.5 / 0, 0.9 / 0.6 / 0, 1 / 0.7 / 1, 0.7 / 0.5 / 0, 0.5 / 0.5 / 0, 0.4 / 0.5 / 0, 0.6 / 0.5 / 0, 0.8 / 0.6 / 0, 0.9 / 0.7 / 0, 1 / 0.8 / 1, 0.7 / 0.6 / 0, 0.6 / 0.6 / 0, 0.5 / 0.6 / 0} { \addtocounter{number}{1} \pgfmathparse{\value{number}}\let\k\pgfmathresult \draw (8 * \x, 8 * \y) node[inner sep=0, outer sep = -0.1pt,color=black,fill,circle] (node-\k) {}; };  \foreach \x / \y / \z / \trash in { 4/5/8/0, 9/11/16/0, 9/16/17/0, 11/10/18/0, 11/18/16/0, 10/8/15/0, 10/15/18/0, 8/5/12/0, 8/12/15/0, 5/2/6/0, 5/6/12/0, 2/1/3/0, 2/3/6/0, 17/16/24/0, 16/18/23/0, 16/23/24/0, 18/15/22/0, 18/22/23/0, 15/12/19/0, 15/19/22/0, 12/6/13/0, 12/13/19/0, 6/3/7/0} { \draw[fill=red]  (node-\x) --  (node-\y) --  (node-\z) -- cycle; } \end{scope} \end{tikzpicture} \end{document}) How can I get apt to use a mirror close to me, or choose a faster mirror? Get the index of the nth occurrence of a string? Getting all the combinations in an array What are the basis for co-countable and co-finite topologies How to design partitions for SSHD? Better extensible arrows Drawing text and line-dasharray Is it possible with TileMills and CartoCSS? Derivative of the product of operators and Derivative of exponential I'm asked to show that  $$\frac{d(\hat{A}\hat{B})}{d\lambda} ~=~ \frac{d\hat{A}}{d\lambda}\hat{B} + \hat{A}\frac{d\hat{b}}{d\lambda}$$ With $\lambda$ a continuous parameter. Should I use the definition $$\frac{d\hat{A}}{d\lambda} ~=~ \lim_{\epsilon \to 0} \frac{\hat{A}(\lambda + \epsilon) - \hat{A}(\lambda)}{\epsilon}$$ applied to $\hat{A}\hat{B}$ like $$\frac{d(\hat{A}\hat{B})}{d\lambda} ~=~ \lim_{\epsilon \to 0} \frac{\hat{A}(\lambda + \epsilon)\hat{B}(\lambda + \epsilon) - \hat{A}(\lambda)\hat{B}(\lambda)}{\epsilon}$$ and do some algebra to get the RHS of the first equation, or I'm missing something? Another interesting derivative to pay attention to is: $$\frac{d}{d\lambda}\exp(\hat{A}(\lambda) )~?$$ How to fix infinite bash loop (bashrc + bash_profile) when ssh-ing into an ec2 server? I mistakenly have source .bash_profile in the bashrc file and vice versa. Now when I tried to ssh into the machine (ec2), it will stuck at loading bash and get connection closed in a second. Is there a way I could fix it? Could I mount the disk to another ec2 instance to fix the bash files?  Update 1: I tried the following: %  ssh -i &quot;my-pem.pem&quot; -t ubuntu@&lt;server_address&gt;.amazonaws.com &quot;/bin/bash --noprofile --norc&quot; Connection to &lt;server_address&gt;.amazonaws.com closed.  Nothing else showed up. Do you have any idea on what was going wrong? For sanity check, if I do ssh -i &quot;my-pem.pem&quot;ubuntu@&lt;server_address&gt;.amazonaws.com, the message will be ...  28 packages can be updated. 0 of these updates are security updates. To see these additional updates run: apt list --upgradable  New release '20.04.2 LTS' available. Run 'do-release-upgrade' to upgrade to it.   Last login: Mon Feb 22 23:17:41 2021 from ip Connection to &lt;server_address&gt;.amazonaws.com closed.  Solution Just ssh onto the machine and immediately do Ctrlc and fix the bash files. Which types of particles are affected by the wave-particle duality? If we take the double slit experiment as a way of demonstrating the wave-particle duality, which types of particles would show an interference pattern?  For example, I know that electrons show such a pattern. But do protons, too? What about atoms? Where is the boundary between "wavey particles" and "classical particles"? I can easily find files with given string in name:  me@comp:/usr/local/hydra/hydra-7.4.2$ find -D stat -name "*hack*" ./hack881663129.txt ./hack881663129_7.txt ./hack881663129_5.txt ./hack881663129_4.txt ./hack881663129_4_7.txt ./hack881663129_6.txt ./hack881663129_1_6.txt ./hack881663129_8.txt   How can I also print file details, like size, date of creation etc.? How do I disable usb storage? I'm using Ubuntu 12.04 and I want to block usb storage on it. What is the correct way to do this?