ERROR: Exception in thread &quot;main&quot; java.lang.NullPointerException: Cannot invoke &quot;org.apache.poi.hssf.usermodel.HSSFSheet.getRow(int)&quot; because &quot;sheet&quot; is null     at newproject.MyClass.main(MyClass.java:76)
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?
I tried the Chrome development version for a few days, didn't like it because my tabs became broken and wouldn't load properly. I changed back to the live version, but I forgot to uninstall the development version, which gave me 2 chrome icons on my task bar, but they would both load a broken version of Chrome. I then uninstalled Chrome through the control panel and reinstalled the live version, but I now have one of those broken links on my task bar (a blank page with a folded corner) that points to a version of Chrome that's no longer there. Is there a way to remove this icon? Why is disk usage greater than the size of all files on it? Let $a,b,m,n$ be positive integers such that g.c.d.$(a,b)=1$ and $a^m+b^m|a^n+b^n$ , then is it true that $m$ divides $n$ ? Novel about interstellar war: Both sides win, both sides think they lost How to do joins in LINQ on multiple fields in single join Which is correct: I'll be moving next month or I'll be shifting next month? For changing one's home from one place to another, I've heard people in western part of the world using the sentence:      I'll be moving next month.   In India, even in the English news channels, I've heard many people using the phrase:     I'll be shifting next month.   Which one is the correct usage? Or are both of them correct and the difference in usage is regional? Is there an idiom for someone who gets what he deserved? Like someone receiving punishment for his evil deeds or someone getting awarded for his good deeds? What do repeating '^@' symbols in a logfile mean? A few times I have come across this in a log file:  Sep 17 09:35:57 00187d378926 ovpn-00187d378926[1701]: /sbin/ifconfig tun0 0.0.0.0 Sep 17 09:35:57 00187d378926 kernel: Kernel logging (proc) stopped. Sep 17 09:35:57 00187d378926 rsyslogd: [origin software="rsyslogd" swVersion="4.6.4" x-pid="1707" x-info="http://www.rsyslog.com"] exiting on signal 15. ^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@ ^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@ ^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@   What does it mean?  Sometimes it is a single line, other times it is lots of lines.  I usually see it happen on Debian 6 machines. If I took a picture of a windmill on the horizon — given that I know the sensor size and the focal length of the lens and other factors to do with the shot — could I calculate how far away an object is from the photographer? How can I delete a (null) app in usage? In my usage list(settings>general>storage &amp; iCloud usage>manage storage), there is an app that registers as (null) with no icon, a version number that doesn't match anything I've previously downloaded, and it's taking up storage space. I first noticed it about a week or two ago. And no, I can't download recent apps to find the one that it is, because I downloaded multiple apps recently, and didn't notice the (null) app until my storage was critically low and I was looking to delete something. I am running iOS 9.2.1 on an iPhone 5C. Is it possible to skip releases during an upgrade? How do I see the extensions loaded by PHP? Why is it true that "I before E, except after C"? How to replace spaces with newlines/enter in a text-file? I have simple text file named "example".  Reading with terminal command: cat example  Output:  abc cdef ghi jk lmnopq rst uv wxyz   I want to convert (transform) into following form: (expected output from cat example)  abc cdef ghi jk lmnopq rst uv wxyz   How can I do this via the command-line?   (This is only an example file, I want to convert word's position in vertical-column) Regression - How do I know if my residuals are normally distributed? Performing a regression and need to find out if my residuals are normally distributed. In An Introduction to Statistical Learning (James et al.), in section 3.7 exercise 5, it states that the formula for $\hat{\beta}_1$ assuming linear regression without an intercept is  $$\hat{\beta}_1 = \dfrac{\displaystyle\sum\limits_{i=1}^{n}x_iy_i}{\displaystyle \sum\limits_{i=1}^{n}x_i^2}\text{,}$$ where $\hat{\beta}_0 = \bar{y}-\hat{\beta}_1\bar{x}$ and $\hat{\beta}_1 = \dfrac{\displaystyle S_{xy}}{S_{xx}}$ are the usual estimates under OLS for simple linear regression ($S_{xy} = \displaystyle\sum\limits_{i=1}^{n}(x_i - \bar{x})(y_i - \bar{y})$).  This is not the actual exercise; I am merely wondering how to derive the equation. Without using matrix algebra, how do I derive it?   My attempt: with $\hat{\beta}_0 = 0$, we have $\hat{\beta}_1 = \dfrac{\bar{y}}{\bar{x}} = \dfrac{S_{xy}}{S_{xx}}$.  After some algebra, it can be shown that $\displaystyle S_{xy} = \sum\limits_{i=1}^{n}x_i^2 - n\bar{x}\bar{y}$ and $\displaystyle S_{xx} = \sum\limits_{i=1}^{n}x_i^2 - n\bar{x}^2$. From here, I'm stuck. How to test if a string is basically an integer in quotes using Ruby Let $b_n$ be a sequence which decreases monotonically to zero, prove $\sum b_nz^n$ converges for $|z|\leq 1$ and $z\neq 1$.  So I was able to prove it converges on the open unit disk in the complex plane, but except for a few special points on the boundary, such as $-1$ and $i$, where I was able to use the alternating series test, I can't seem to prove it converges in general on the boundary minus $z=1$.  I feel I have to somehow use the fact that all the numbers on the unit circle except $z=1$ rotate around and around as you raise them to higher and higher powers, thus you get the real and imaginary parts of your series to alternate between sets of positive and negative terms, but in a very irregular way for most initial angles.  I tried to simplify the problem by breaking the series into its real and imaginary parts, and then trying to prove they each converge separately, but I still can't prove either converges:  $$\sum_{n=0}^{\infty}b_ne^{n\theta i} = \sum_{n=0}^{\infty}b_n\cos(n\theta) +i\sum_{n=0}^{\infty}b_n\sin(n\theta).$$ Creating point to center of polyline? I'm trying to create a points in the middle of polylines. The data is now in the form of shapefile. Is there an easy way to do this? Some of my Apps Icons are changing to a generic app icon, and I have no idea why. Tried to restart the system and nothing. 3 days ago i updated my Skype and then the icon is gone.