Why am i getting " java.lang.NullPointerException" in my android app?
What is a NullPointerException, and how do I fix it?
When looking for the path to an executable or checking what would happen if you enter a command name in a Unix shell, there's a plethora of different utilities (which, type, command, whence, where, whereis, whatis, hash, etc).  We often hear that which should be avoided. Why? What should we use instead? Where are my installed applications? Derivative of function with 2 variables I've leart in Calculus 1 that the derivetive of $f(x)$ is:  $$\lim_{h\to0} \frac{f(x+h) - f(x)}{h}$$.  suppose $f(x,y)$ is a function with 2 variables,  does $$f'(x,y) = \lim_{h\to0} \frac{f(x+h, y+h) - f(x,y)}{h}$$ ?  If not , what is the derivetive of function with 2 variables? Apostrophe difference (Ones' and Two's Complement) What is Stack Overflow's business model? I once , but that was . Now, , and they have , but where's the business model? The  its own Q&amp;A about its site (since it wasn't included in the ), and just about the only dig in there that halfway has merit is the business model dig:  We switched to a premium model to keep out of the Venture Capital business (been there, done that, got the t-shirt). Companies like StackExchange couldn't do it without the VC cookie jar. Where's the model? Huh, Spolsky? (I’m sorry. Did that slip out?)  It hurt, but I can't help but wonder if they're on to something.  How would Stack Overflow Internet Services pay back the VCs? Ad revenue alone?  There is only one 'paid' product, Stack Overflow careers, but it doesn't seem like that could sustain a business as large as Stack Overflow (Joel once mentioned on his blog that his jobs board brought in an extra million a year in revenue). So what does Stack Overflow do when the VC cash runs out? Is this a get-big-and-take-the-company-public maneuver? Is that the only viable business model for Stack Exchange? Edit: With the size of Stack Overflow's team, it isn't a stretch to say that payroll is at least $2 million  a year, probably as much as 4 when you think about benefits (average 120K for a developer in New York City; x2 for benefits/taxes/all-that-other-good-stuff). Office space can't be cheap, so how long does $6 million last before you need another VC infusion? Why doesn't the Enterprise use stealth/invisibility technology like the Romulans and Klingons? The stealth mode of starships plays a huge role in many Star Trek films and episodes and is a strong advantage in combat. I remember an episode revealing that the Federation developed a secret stealth technology (afair on Riker's former ship with another captain appearing in this episode) that even made it possible to fly with the ship through matter.   Nonetheless, while stealth technology seems standard for Romulans, Klingons (Ferengi, Vulcans?) and does not break any intergalactic laws (do these exist at all?), the Federation doesn't use it? On mu Ubuntu 12.04 setups my tmux clipboard copy and paste commands are setup as follows:  set -g prefix M-a unbind C-b bind C-c run "tmux save-buffer - | xclip -i -sel clipboard" bind C-v run "tmux set-buffer \"$(xclip -o -sel clipboard)\"; tmux paste-buffer"   This worked great for the longest time up until a month or so ago, when I suspect some configuration change or package change was made that broke the above. In GNOME terminal, pasting still works fine with both prefix+ctrl-v and ctrl-shift-v.   However the xclip copy command no longer works no matter what I do, and I have tried removing the custom prefix binding above, using -select instead of -sel, not using clipboard etc. This is pretty much a show stopper for a GVim user like me, since I don't even have the GNOME terminal workaround of ctrl-shift-c with tmux taking over the shell. I go into copy mode, select text with space+movement, and when I execute prefix+ctrl-c absolutely nothing happens. Before this broke, tmux would display a confirmation message in the notification section at the bottom.  Does anybody have suggestions as far as how one might debug this? This is a pretty big productivity hit. I can probably use the , but it'd be great to know just what happened to xclip. Session_ID_Remove Issue in API callouts in Salesforce How to prove that derivatives have the Intermediate Value Property How to find historical stock price for a de-listed or defunct company? With services like Google Finance or Yahoo Finance it is very convenient to find the historical stock price metric of a listed company, but if the company is not listed, I can't find the data.  For example, I want to see the price metrics for Macromedia, which had been listed in 1993 on NASDAQ (MACR), then acquired by Adobe in 2005. How could I find its historical price metrics? Just out of curiosity, I have developed a small app analysing and generating sitemaps for a given domain.   Can I tell search engines to look for the sitemap of a given domain on my app using a dynamic url i.e. can you use cross-domain sitemaps.xml without validating you are owner of the destination domain?  For example :   http://example.com/mydomain.com/sitemap.xml Is your mount an enemy of your target in terms of triggering sneak attack? I was working on building a Zorro character and this question came up: if a rogue is riding on a mount, is that mount considered an ally, or, specifically, an "enemy" of your target?  One of the ways Sneak attack can be trigged is...     "... if another enemy of the target is within 5 feet of it, that enemy   isn’t incapacitated, and you don’t have disadvantage on the attack   roll."   My assumption is yes. Every mount I'm aware of has some kind of attack at its disposal, and thus they represent a threat to your target in addition to the increased threat potential they grant you as the rider. Moreover, if a gnome can proc a sneak attack by acting as a distraction, I don't know why a warhorse wouldn't be able to. That said, is there anything in the books or elsewhere that defines what "enemy of the target" is allowed to include? Ending a two handed game It’s my turn. It is a two handed game of Canasta. I have three cards left in my hand. There is only one card left in the Stock pile. Can I pick up the one card and play my hand? Differentiability of this picewise function $$f(x,y) = \left\{\begin{array}{cc} \frac{xy}{x^2+y^2} &amp; (x,y)\neq(0,0) \\ f(x,y) = 0 &amp; (x,y)=(0,0) \end{array}\right.$$  In order to verify if this function is differentiable, I tried to prove it by the theorem that says that if $\frac{∂f}{∂x}$ and $\frac{∂f}{∂y}$ exist and are continuous at the point $(x_0,y_0)$, then the function is differentiable at this point. So I did:  $$\frac{\partial f}{\partial x}(0,0) = \lim_{h\to 0}\frac{f(0+h,0)-f(0,0)}{h} = 0$$ $$\frac{\partial f}{\partial y}(0,0) = \lim_{h\to 0}\frac{f(0,0+h)-f(0,0)}{h} = 0$$  so we have that the partial derivatives at point $(0,0)$ is $0$. Now, if we take the derivative at $(x,y)\neq (0,0)$ and then take the limit of it as $(x,y)\to(0,0)$, we can see if the derivatives are continuous or not. So here it is:  $$\frac{\partial f}{\partial x}(x,y) = \frac{y(y^2-x^2)}{(x^2+y^2)}$$  but   $$\lim_{(x,y)\to(0,0)} \frac{y(y^2-x^2)}{(x^2+y^2)} $$ does not exist (by wolfram alpha... but can anybody tell me an easy way to prove this limit does not exist? easier than taking the limit in different directions?), therefore the derivative is not continuous at $(0,0)$, so we can't say $f$ is differentiable at $(0,0)$, but for $(x,y)\neq (0,0)$ the function is continuous, as it is a quotient of continuous functions. So $f$ is at least differentiable at $(x,y)\neq (0,0)$.   Now, to verify differentiability at $(0,0)$ I think we must use the limit definition of differentiablity: A function is differentiable at $(0,0)$ iff: $$\lim_{(h,k)\to (0,0)} \frac{f(0+h,0+k)-f(0,0)-\frac{\partial f}{\partial x}(0,0)-\frac{\partial f}{\partial y}(0,0)}{\|(h,k)\|} = 0$$  Let's calculate this limit:  $$\lim_{(h,k)\to (0,0)} \frac{f(0+h,0+k)-f(0,0)-\frac{\partial f}{\partial x}(0,0)-\frac{\partial f}{\partial y}(0,0)}{\|(h,k)\|} = \\ \lim_{(h,k)\to (0,0)} \frac{\frac{hk}{h^2+k^2}}{\sqrt{h^2+k^2}} = \\ \lim_{(h,k)\to (0,0)} \frac{hk}{(h^2+k^2)\sqrt{h^2+k^2}}$$  which I think, it's a limit that does not exist, therefore the function isn't differentiable at $(0,0)$ Consider equations defined in $D_x = (0,\infty) \times (0,\infty)$  1) Find the maximal solution to  $$ \dot y = \frac{3t^2}{5y^4} $$ satisfying the initial value problem $y(1) = 2^{\frac{1}{5}}$  I have found the maximum solution to be $((0,\infty), (t^3+1)^\frac{1}{5})$  2) Find the maximal solution to   $$ \dot y = -\frac{2y}{5t}+\frac{3}{5y^4}$$  satisfying the IVP $y(1) = 2^{\frac{1}{5}}$.  (HINT: Use a transformation of the form $t^{\alpha}y(t)$.   I hope someone can explain how to use the transformation to find the maximal solution. I know in php you are able to make a call like:  $function_name = 'hello'; $function_name();  function hello() { echo 'hello'; }   Is this possible in .Net? Where is the itoa function in Linux? itoa() is a really handy function to convert a number to a string. Linux does not seem to have itoa(), is there an equivalent function or do I have to use sprintf(str, "%d", num)? In sketchup you can move an object based on two points. This makes it very easy to align a vertex in one object to vertex in another object. Here is an example:    I want to know how to do this in Blender. It seems that Translate can't align vertexs Problem with camera orientation in motion tracking sorry for my English, I write from Italy. I'll try to explain my problem. I'm tracking a footage with a solver error 0.2973. When I try to set the floor, the orientation of the camera is wrong (see screenshot).  So I tried to manually orient the camera, but even if well oriented respect to the xyz axes, geometry is incorrect as you can see from the screenshots below (the selected marker appears to be on the same plane but it is lower).  The video was recorded by a roof of a house so much higher than the road. It seems that Blender is not able to recognize this height and set the floor in a wrong point of view. Can someone help me solve this problem? Thank you I can’t for the life of me figure out where to use a and where to use the — and where there is no article at all. Is there a simple rule of thumb to memorize?   The standard rule you always hear:     “If a person knows which item you are talking about then use "the"   . . . doesn’t clear things up for me, as I have no idea whether or not they know.