I create a class Node which contains an int variable val When I try to get the val after I pop it from stack, the compiler throw a NullPointerException.  Does anyone knows how to fix this?  public class Node{   int val;   Node left;   Node right;   Node(int x) {val = x;} }   Node root = new Node(6); Stack s = new Stack(); s.push(root); Node node = (Node)s.pop(); System.out.println(node.val);
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?
Microdisplacements with adaptive subdivision turning black AMD GPU Material in "material preview mode" and "rendered mode", is entirely gray as background. Only outline is displayed I don't know why but now, when i go in material preview or rendered (in solid mode it works fine), my object is entirely gray, or invisible idk, because it has the same color of the background. I've been working for days on this project without having this issue, it suddenly showed up. I tried opening other old projects and suddenly they all do the same so i guess it is some blender program setting that applies for all the projects, but idk how it happened cuz i didn't change anything :/, anyone got any tips? Fiction novel or movie about sentient dinosaurs JSON URL from StackExchange API returning jibberish? Tracking Object Position - Firing on a Trajectory SElinux: allow httpd to connect to a specific port My system is running CentOS 6.4 with apache2.2.15. SElinux is enforcing and I'm trying to connect to a local instance of redis through my python/wsgi app. I get Error 13, Permission denied. I could fix this via the command:  setsebool -P httpd_can_network_connect  However, I don't exactly want httpd to be able to connect to all tcp ports. How can I specify which ports/networks httpd is allowed to connect to? If I could make a module to allow httpd to connect to port 6379 ( redis ) or any tcp on 127.0.0.1, that would be preferable. Not sure why my paranoia is so strong on this, but hey...  Anyone know? How to vertically align underbraces I am trying to balance underbraces in the equation below, how can I ensure that both underbraces at the same depth?  \begin{equation} y = \underbrace{ \left [ x^2 + r^2 (a^2+\frac{a^{0.3}}{b}) \right]}_\text{growth rate} \times  \underbrace{ C_0}_\text{initial} \end{equation} Short story about interstellar travel gone wrong Trying to find a story I read a couple of years ago in an anthology, not sure when the story dates from but likely to be recent - the last decade or so.   The story has passengers travelling between stars via a series of jumps. The calibration of the drive is affected by a special metal, any of which has to be stored in a shielded compartment (I think the jumps were measured in 'degrees'). One of the passengers unwittingly takes some of this metal on board, which throws off the calculations for the jumps, and they jump into an alternative universe where there are no signs of any humans ever existing. They start to try to reverse their steps to get back to the known universe, but the reverse calculation seems impossible.  That's as much as I read of the story before I lost the book. After many dreams (feverish nightmares!) re-living this story, I need to know how it ends. I know it's not much to go on. The story name, author, or the anthology it appears in would be fantastic. I can't really remember how long ago it was, but I think the book was marketed towards children/teens.  All I remember is the basic premise of the story.  Some story about these unusual little blue electric critters that only certain people could see, and these kids took it upon themselves to go about destroying them since they fed off energy or something like that.  Not sure whether they fed off people or off electricity... or both maybe?  I think the dominant method of destroying them was an electric baton that was taken from law enforcement.  I wanna call them buzz batons, but that's probably thanks to Artemis Fowl. Open Windows Terminal as admin with WIN+R I use shortcut  +R to run Windows Terminal [wt], but I cannot run it as Admin, which causes problems when I install something via choco.   To open it as Admin, I have to:  Press  > Right-click Terminal > More > Run as Admin  > ◀ > ENTER Some solutions say to use the following, but it can only open powershell, not wt:  +X > A > ◀ > ENTER While  say to use the following, but it also doesn't work:  +R > SHIFT+CTRL+ENTER   How do I open Windows Terminal as Admin via a command? Sending and Parsing JSON Objects in Android How can i show that two $R$-modules of finite rank are isomorphic if and only if they have the same rank, i.e., $R^n \cong R^m$ iff $n=m$. How do I turn on workspaces? (Why do I only have one workspace?) How do I write a correct micro-benchmark in Java? I am a Physics undergrad, and just started studying Topology. How do you define neighborhood and open set in Topology.Wikipedia gives a circular definition.   An  is defined as follows.      In topology, a set  is called an open set if it is a neighborhood of every point    While a  is defined as follows:     If $X$ is a topological space and $p$ is a point in $X$, a neighbourhood of $p$ is a subset $V$ of $X$, which includes an open set $U$ containing $p$   which itself contains the term open set.  How do you define it exactly? The Bash command :(){ :|:& };: will spawn processes to kernel death. Can you explain the syntax? I looked at  and can't understand how this works.  This command "exponentially spawns subprocesses until your box locks up".  But why? What I understand less are the colons.  user@host$ :(){ :|:&amp; };: Why must $a$ and $b$ both be coprime when proving that the square root of two is irrational? Suppose we wish to prove that the square root of two is irrational. We begin by assuming that it is rational.  Namely, where both $a$ and $b$ are integers $$\frac{a}{b} = \sqrt 2  % MathType!MTEF!2!1!+- % faaagCart1ev2aaaKnaaaaWenf2ys9wBH5garuavP1wzZbqedmvETj % 2BSbqefm0B1jxALjharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0x % bbL8FesqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yqaq % pepae9pg0FirpepeKkFr0xfr-xfr-xb9Gqpi0dc9adbaqaaeGaciGa % aiaabeqaamaabaabaaGcbaWaaSaaaeaacaWGHbaabaGaamOyaaaacq % GH9aqpdaGcaaqaaiaaikdaaSqabaaaaa!31D3! $$  Why is it so important that both $a$ and $b$ be coprime and the fraction be irreducible?  That is, if $a$ and $b$ are not coprime and the fraction is indeed reducible, then$$\frac{a}{b} \ne \sqrt 2  % MathType!MTEF!2!1!+- % faaagCart1ev2aaaKnaaaaWenf2ys9wBH5garuavP1wzZbqedmvETj % 2BSbqefm0B1jxALjharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0x % bbL8FesqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yqaq % pepae9pg0FirpepeKkFr0xfr-xfr-xb9Gqpi0dc9adbaqaaeGaciGa % aiaabeqaamaabaabaaGcbaWaaSaaaeaacaWGHbaabaGaamOyaaaacq % GHGjsUdaGcaaqaaiaaikdaaSqabaaaaa!3294! $$ From Rudin's Principles of Mathematical Analysis (Chapter 2, Exercise 6)  Let $E'$ be the set of all limit points of a set $E$. Prove that $E'$ is closed.  I think I got it but my argument is a bit hand wavy:  If $x$ is a limit point of $E'$, then every neighborhood of $x$ contains some $y\in E'$, and every neighborhood of $y$ contains some $z\in E$. Therefore every neighborhood of $x$ contains some $z\in E$, and so $x$ is a limit point of $E$. Then $x\in E'$, so $E'$ is closed.  The thing that's bugging me is the leap from one neighborhood to another. Is this formally correct? Why doesn't `sudo cd /var/named` work? Prove there are no other solutions of functional equation $f(x+y) = \frac{f(x)+f(y)}{1-f(x)f(y)}$