I feel like this is probably already answered somewhere, but I couldn't find it or maybe I'm not using the right keywords.  What I want to do is an initialize a value in a Servlet Filter with as little blocking as  possible. I am by no means an expert in threads, so here is what I came up with:  private static Object getObject() {     if(OBJECT == null) {         synchronized (MyClass.class) {             InputStream is = getInputStream("path_to_file");             OBJECT = loadObject(is);         }     }     return OBJECT; }   This method is used within the Servlet Filter, which will be called by multiple threads at once. I want to make sure that I load this when it is needed, and not on the init method.  My question is, is this thread safe? Once it is initialized I think it'll be good, but I want to make sure that it works when it's being initialized.  PS: Please redirect me to any question that already answers this if they exist. Thanks!
What is an efficient way to implement a singleton pattern in Java?
given value of matrix, getting it's coordinate Entropy increases when two substances mix with each other.  For example, the entropy of mixing of two different gases are given by $$\Delta S= 2Nk\ln\frac{V_f}{V_i}\;.$$  But, the entropy doesn't increase when the two gases mixing are same.  This is pointed by Daniel V Schroeder:     It's important to note that this result applies only if the two gases are different like helium and argon. If you start with the same gas on both sides, the entropy doesn't increase at all when you remove the partition.   Now, why is this so? Both the gases, though the same, increase their individual entropies when they expand, don't they?  So, why did Schroeder say there is no entropy of mixing? What actually happens such that there is no change in entropy when the gases are the same? Does every holomorphic function $f$ defined on an open set $U$ have a primitive? Can I SSH into a specific directory? I SSH into a multitude of different servers every day and some of them have different filesystem structures for their websites. Each time I change server, I have to work out where I'm going to get to the files I really want (because it's very rarely that I want to be in ~). I use Fabric files to automate all my actions (pushing to git, automated SSH to the server to pull from git, restarting the web application, changing dependencies, etc), so I was wondering if I couldn't also tell it that I want to SSH into the right directory so I can run fab ssh and be in a normal SSH session, just in the right place and start working immediately. This is not about Fabric though; please ignore it. Fabric is just running a batch of commands. My argument is with the ssh command. That's what I'll be telling Fabric to run. So how I can get ssh to cd into a specific directory on login? Note: this cannot be a server or user-wide thing. I have multiple websites under one user on some servers so sticking something in ~/.bashrc would not work for me. Showing that $|f(z)| \leq \prod \limits_{k=1}^n \left|\frac{z-z_k}{1-\overline{z_k}z} \right|$ Is there a privacy setting on Facebook that will stop the posting that is made on your friend's wall when you make a new friend?  I do not want this to appear on my friend's wall when I make a new friend Why not use a WAMP stack? This is a  about  the use of a *AMPP's stack.    I recently had a talk with some experienced people and they suggested to me not to use a WAMP stack, and instead install apache, mysql and php separately.  I don't understand why they have suggested this, though, so can anyone tell me?  Is there a particular disadvantage of WAMP, or a particular advantage to installing all of them separately?  Since a WAMP stack itself is composed of apache, mysql and php, then what's the difference between using the WAMP stack and installing them all separately? Prove $\int_0^\infty \frac{\sin^4x}{x^4}dx = \frac{\pi}{3}$ Make flash drive bootable with MULTIPLE Windows Installers I let my younger cousins play on my S4 from time to time, but now I have found that all the words they have used are the first to come up when I use my phone to text. It has gotten quite irritating. Samsung seems to have very hard to find settings compared to my old Sony Ericsson so I cannot find where to get rid of the word history typed onto my phone's keyboard. Why is it sensical for a proposition with a false antecedent to validate to true? In propositional logic, the statement "If pigs can fly, then elephants can lay eggs." validates to true because the antecedent validates to false.  In other words, given $a \rightarrow b$, if a is false, the entire statement is true. Why?  Just because the antecendent is false doesn't mean that another fact depends on it, right? How can I return the current action in an ASP.NET MVC view? I wanted to set a CSS class in my master page, which depends on the current controller and action. I can get to the current controller via ViewContext.Controller.GetType().Name, but how do I get the current action (e.g. Index, Show etc.)? I'm a newbie in WordPress. I'm trying to understand how it generates the XHTML pages. I guess it's the function of do_action( 'hook-name' ) , but I'm stuck with the understanding of the instruction isset( $wp_filter['all] ) when reading the function do_action because I don't understand what is the meaning of the word all.  Is it a PHP feature or a WordPress one? How old should you be when you begin Jedi training? Anakin Skywalker was about nine years old (see script link, below) during "The Phantom Menace". Yet according to various people, he was too old (or even "far too old") to begin training as a Jedi:   QUI-GON: "I'm afraid not. Had he been born in the Republic, we would have identified him early, and he would have become Jedi, no doubt...he has the way. But it's too late for him now, he's too old." OBI-WAN: "The boy will not pass the Council's tests, Master, and you know it. He is far too old." MACE WINDU: "He is too old. There is already too much anger in him." and even ANAKIN himself: "I'm with Qui-Gon...but...they're not going to let me be a Jedi. I'm too old."   All quotes above from .  However, during "The Empire Strikes Back" Yoda gives Luke's age (18 in chapter IV according to the ) as the reason to deny him training only after first saying that Luke is:   too impatient too angry not ready is unfocused ("Never his mind on where he was") and reckless   All from .  I would have thought that, if at 9 Anakin was too old, Yoda would have seen Luke at 18+ and started off by saying, "No chance, he has".  So, what is the age to begin Jedi training? I am using extensively trigonometric functions when an angle is given in degrees. Some of these functions like sine or cosine have rational values, for example, the well   known example is that $\cos(\theta) =0.6 $ and $\sin(\theta) =0.8 $.       However besides the case of multiplicity of $90^\circ$ it seems there are no rational numbers $\theta$  with simultaneously rational  values of  sine and cosine.      Is it possible somehow to prove that for rational values of an angle given in degrees there are  no values simultaneously rational of sine and cosine functions, beside obvious case of multiplicities of $90^\circ$? Sadly, yesterday I've spilled some water on my MacBook Pro (El Capitan). Before you yell at me - it was closed, turned off and it wasn't a lot of water, the laptop was wet only on the top and under - not inside.  Today I've turned him on. At first battery wasn't recharging for some reason, but SMC reset fixed the problem.  At the first booting speaker was like talking to me, hard to explain but it was weird. Also only right speaker was "talking", despite the fact that only left side of the laptop was watered. Then there was no sound at all, not from the speakers, nor from the headphones. Also microphone's not working. USB ports are okay. The water was spilled from the left side of the laptop. I know this is hardware issue, and something's broken, but I want to be 100% sure. No it's not muted. I've also tried NVRAM reset a few times, but it doesn't fix the problem :(  There might be one factor, before spilling, my laptop "died" for the first time (battery energy ran off), but I doubt it's the case. (but hey, every info matters, right?)  It wouldn't be weird for me, if only left speaker wouldn't work, but built-in microphone not working is kinda weird in this case, isn't it?  I just want to ask you guys, what else can I check, before I'll be 100% sure it's dead inside?  I've plugged in external microphone - it's not working too. Convert LaTeX to HTML Seeing as C# can't switch on a Type (which I gather wasn't added as a special case because is relationships mean that more than one distinct case might apply), is there a better way to simulate switching on type other than this?  void Foo(object o) {     if (o is A)     {         ((A)o).Hop();     }     else if (o is B)     {         ((B)o).Skip();     }     else     {         throw new ArgumentException("Unexpected type: " + o.GetType());     } } In System Info in 11.10, the graphics driver wasn't detected. I use ACER Aspire 4750 with Intel Core i3-2340M and Intel HD Graphics 3000 graphic card  How to install the driver for my Ubuntu? Assume the dice are numbered from $1$ to $k$. My hunch is that this will form a normal distribution with a median at $n\cdot\frac{k}{2}$. However, I have no idea as to    turn this fact into an answer (I have a minimal knowledge of stats, but I know that I am missing the standard distribution) and this is probably the wrong approach   How can I approach and solve this problem? (Aside, this is not for a class, stats or other, so any and all approaches welcome).  *Edit: * I want to find the number of ways that the sum of the numbers that are rolled has a particular value, if $n$ dice are rolled, and each has $k$ sides, numbered $1$ to $k$.