Making the following call:  SwingUtilities.updateComponentTreeUI(comp);   which sporadically results in the following error :  Exception in thread "AWT-EventQueue-0"  java.lang.NullPointerException     at javax.swing.plaf.synth.SynthArrowButton$SynthArrowButtonUI.getPreferredSize(SynthArrowButton.java:106)     at javax.swing.JComponent.getPreferredSize(JComponent.java:1662)     at javax.swing.plaf.basic.BasicScrollBarUI.layoutVScrollbar(BasicScrollBarUI.java:666)     at javax.swing.plaf.basic.BasicScrollBarUI.layoutContainer(BasicScrollBarUI.java:866)     at java.awt.Container.layout(Container.java:1511)     at java.awt.Container.doLayout(Container.java:1500)     at java.awt.Container.validateTree(Container.java:1696)     at java.awt.Container.validateTree(Container.java:1705)     at java.awt.Container.validateTree(Container.java:1705)     at java.awt.Container.validateTree(Container.java:1705)     at java.awt.Container.validate(Container.java:1631)     at javax.swing.RepaintManager$3.run(RepaintManager.java:711)     at javax.swing.RepaintManager$3.run(RepaintManager.java:709)     at java.security.AccessController.doPrivileged(Native Method)     at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)     at javax.swing.RepaintManager.validateInvalidComponents(RepaintManager.java:708)     at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1731)     at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)     at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)     at java.awt.EventQueue.access$500(EventQueue.java:97)     at java.awt.EventQueue$3.run(EventQueue.java:709)     at java.awt.EventQueue$3.run(EventQueue.java:703)     at java.security.AccessController.doPrivileged(Native Method)     at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)     at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)     at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)     at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)     at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)     at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)     at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)     at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)   Apparently some component is becoming null. Added the following IF condition but it doesn't help :  if (comp != null)       SwingUtilities.updateComponentTreeUI(comp);   Going through the stack and it is purely a java stack and no reference to custom code. There are actually other NULL exception similar but referencing other Java objects.   Is there a way to iterate through the components in this tree to find which component is returning a null?
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?
What is the point of throwing candy on the ground using the "Throw 10 candies on the ground"? All that seems to happen is that the text under it becomes more and more disappointed in me. ಠ_ಠ How to delete pre-installed apps in Catalina? I want to delete the pre-installed apps, like Music, Books, Siri etc. from my iMac running Catalina. How do I do this?  (NOTE:  asks the same thing but was closed as a duplicate of . Annoyingly, the supposed duplicate doesn’t give an obvious answer to the question, and the original was locked can’t be answered, hence a new question. I can’t add an answer to  either) How do you address unknown peers in email? This is one bit of academic etiquette I'm not sure about. When I write to another researcher, a peer (we're both professors). I've never exchanged with before and never met him but I know of him (and he probably knows of me).  Erring on the side of formality, I write to him “Dear professor Mortimer”. I sign with my first name, because that's how I've always signed my emails (there's a formal signature block below, with full name and contact details) and I think it indicates a willingness to be called by my first name (which I prefer, but I don't want to force people one way or the other).  Is that going to be well received? Will people take the hint? Or should I just start using first names from the start? (it does sound weird to write “dear Philipp” to someone you don't know) There is a bar of chocolate of size $N$ x $M$. Little square $(x, y)$ is poisoned. There are 2 players taking turns. In each move they can cut the bar along any of the horizontal or vertical lines, then the part which does not contain $(x, y)$ is being eaten. The player who is left only with square $(x, y)$ loses (beacause it is poisoned). Which player has the winning strategy? I guess that this game is equivalent to game of Nim due to Sprauge-Grundy theorem, but I don't know how. Callback animation to begin only after ALL children have finished animating I want to update my sources.list file with the fastest server from the command line in a fresh Ubuntu Server install. I know this is trivially easy with the GUI, but there doesn't seem to be a simple way to do it from from the command line? In Star Trek canon, what's the range of operation of Transporters? By "range of operation" of the transporter, I mean "distance up to which it could transport" or "distance from which it could transport something to its own location". The range of operation would have increased with time without a doubt but I want to know the average range at any period. It'll be better if you provide full stats of all times. Fuzzy string search library in Java I'd like to find the files in the current directory that contain the text "chrome".  $ find . -exec grep chrome find: missing argument to `-exec'   What am I doing wrong? What is the difference between new/delete and malloc/free? How does one prove that if $n \mid (a^{n}-b^{n}) \ \Longrightarrow$ $ \displaystyle n \mid \frac{a^{n}-b^{n}}{a-b}$ where $a,b, n \in \mathbb{N}$.  What i thought of is to consider $$(a-b)^{n} \equiv a^{n} + (-1)^{n}b^{n} \ (\text{mod} \ n)$$ and if we suppose that $n$ is odd then we have, $$(a-b)^{n} \equiv a^{n} -b^{n} \ (\text{mod} \ n)$$ and since $n \mid (a^{n} - b^{n})$ we have $$(a-b)^{n} \equiv 0 \ (\text{mod} \ n) $$  I think i am far away from the conclusion of the problem, but this is what i could work on regarding the problem. When to use LinkedList over ArrayList in Java? Contour graphing algorithm I am not sure if this belongs on Mathematics Stack Exchange, but it is somewhat relevant here.       The Problem If you've installed any graphing/plotting apps on your smartphone, you will notice that the app will only take the equation in form of a single variable for 2D (e.g  $y=f(x)$) or in 2 variables for a 3D plot (e.g $z=f(x,y)$). This is problematic as many equations are tough to reduce to the form of $y=f(x)$ or $z=f(x,y)$. The plotters like this because this is the only way they can plot it in form of lines connecting multiple points. These plotters compute the $y$ value at fixed intervals of $x$ and join the point to the previous one. The brute force approach to counter this is to check if the function satisfies it at each point on the image. The problem is that this not only takes more time, but gives discontinuous graph for the function as the image cannot have infinite resolution.         The Question The only way to fix this is to identify which point comes after the next in the plot and join them together by a line. That is where I am stuck. How do you identify which point is sequentially the next one for a given function? Keep in mind that for a human, this might be very easy to do. But, for a computer, this requires a general algorithm. Any ideas? Removing duplicate strings from an array? Using dpkg to install upgrade and dist-upgrade packages How to make cmd correctly display arabic script? Here is the script to create my tables:   CREATE TABLE clients (    client_i INT(11),    PRIMARY KEY (client_id) ); CREATE TABLE projects (    project_id INT(11) UNSIGNED,    client_id INT(11) UNSIGNED,    PRIMARY KEY (project_id) ); CREATE TABLE posts (    post_id INT(11) UNSIGNED,    project_id INT(11) UNSIGNED,    PRIMARY KEY (post_id) );   In my PHP code, when deleting a client, I want to delete all projects posts:  DELETE  FROM posts INNER JOIN projects ON projects.project_id = posts.project_id WHERE projects.client_id = :client_id;   The posts table does not have a foreign key client_id, only project_id. I want to delete the posts in projects that have the passed client_id.  This is not working right now because no posts are deleted. Group of order $p^2$ is abelian. Yes, I know that there are tons of solutions of this up here, but I, essentially, wanted to try it a different way and ah, well.  Let $|G| = p^2$ for some prime $p$.  Consider $x \in G$. So, $|x| = p, $ or $ p^2$. If it is the latter, then, we are done.  So, if $|x|= p$, then, let, $H = [h|  h=x^i, i \in Z] \Rightarrow |H|= p$.  As $p$ is the smallest prime dividing $|G|$, $H$ is a normal subgroup.  Now, consider some $y \in G$, but not in $H \Rightarrow y^p= e$. Then, its easy to show that $G = \langle x,y \rangle$.  Now, consider a homomorphism $f:G \rightarrow \operatorname{Aut}(H), g \to c_g $, wherein $c_g$ represents conjugacy by $g$. So, for example, $c_g(x) = gx g^{-1}$.  Then, it is clear that, $H \subset \operatorname{Ker}(f) $, as it is cyclic and therefore abelian. So, we need only look at $G/H$.  Basically, I am asking for a hint as to why $c_y(x) = yxy^{-1} = x = c_e(x)$, where $x$ and $y$ are the generators of $H$ and $G/H$, respectively?  Because, if I establish it for the generators, then it follows that it applies for for all elements, i.e, that $\operatorname{Im}(f)$ is trivial and therefore that $G$ is abelian.  Thank You! Will higher temperatures damage a computer over time, even when not reaching the maximum temperature? For example, if the maximum is 105 degrees celcius, will the computer break faster when continually having it on 80 degrees celsius instead of, for example, 60 degrees celcius. Why Dumbledore kept mum until the very end of the "Deathly Hallows"?