Error: Attempt to invoke virtual method on a null object reference
What is a NullPointerException, and how do I fix it?
My awk program to change spaces into tabs doesn't work How to write a shell script which uses awk to read in the data file students.txt and output the data in the tabbed format as shown:  Surname Forename MSc Stream Date of Birth Smith John IT 15.01.1986 Taylor Susan IT 04.05.1987 Thomas Steve MIT 19.04.1986    Do not worry if tabbed columns don’t line up. The distance between each of (Surname, Forename, MSc Stream and Date of Birth) column is one tab.   Question:  Why this bellow code doesn't work for me?  awk 'BEGIN {IFS=" "} {OFS="\t"} {print $1,$2,$3,$4}' students.txt What does each part of the `ls -la` output mean? What do the following numbers represent? How do I save files edited with vim? Should the weight of question upvotes be reduced? After thinking about    which I declined for the reasons , and    I have a proposal.  How about if question upvotes were only worth +5 rep instead of +10? And this was applied retroactively? (answer upvotes would be unchanged from the current behavior, so this would be question specific.)  This seems to be a better solution to the particular  problem  without all the rather serious negative repercussions of increasing the punitive value and/or cost of downvotes across the board. What is the title of a 80's (i believe?) movie where humans in the far future are going extinct, and are aquiring new humans by going back in time to todays world, kidnapping air travelers that are known to have died that day, from the flying passenger aircraft just before its crashing.  A key visual:    Inside a hangar, a time portal opened, and a tail of a (flying) passenger aircraft extruded into the hangar where a stairs provided access to the back door of the aircraft.  The kidnappers made sure the surviving passengers remain seated and would forget about the whole ordeal. Kidnapped humans got replaced by lookalike dummies, including things they had on them. They tried hard not to alter the timeline. How to check iOS version? How to get the center line between two linestrings Where is the track of a ship when it is shipping always in the center of the   between the coasts of Great Britain and France?   I started with 2 linestrings and I created a lot of points on the linestring with a . After that I created a distance matrix with Raster/Analysis/Proximity in QGIS. Finally I created an aspect raster from the output raster.     Is the red line really the "center line" between these two linestrings?  Is there a more efficient way to do this? Eccentricity of an ellipse How is $\frac{PF}{PD} = e = \frac{C}{A}$ ? where e is eccentricity, P stands for any point on the ellipse. $F$ stands for one of the foci. $e$ stands for eccentricity. $D$ is a point on the directrix of the ellipse. 'C' is the distance from the center to the focus of the ellipse 'A' is the distance from the center to a vertex.   This is referring to an ellipse/hyperbola/parabola and their conic sections. The problem is not the proof for how $PF/PD = e$, or how $C/A = e$, but how the two equate to each other. (The letters stem from the points/foci of an ellipse of a cone and its directrix).  What is the answer that does NOT use analytic geometry? (only trigonometry) If an opponent casts  to gain control of  then sacrifices it who will draw the card? I have dual US and UK citizenship, but the US citizenship has only just been realized (wasn't aware I had claim to US citizenship earlier).  Unfortunately I haven't received my US passport yet. Can I still travel to the US only using my UK passport?  I understand that the US does require citizens to travel on a US passport, but would they even know that I am a dual citizen if I've only ever traveled on my UK passport? (Especially considering that my US passport has never been used before in any form - as I haven't received it).  If I was only entering the US for a small trip,  would it show on the screen that I also have US citizenship when they scan my UK passport? When is a cellular automaton "bidirectional"? Should I flag obviously wrong answers as NAA? What is the best way to filter a Java Collection? I want to filter a java.util.Collection based on a predicate. I'm looking to write a script that will find and replace any characters other than [^A-Za-z0-9._-] in a specific file type (in this case .avi) with an underscore _. I want this to exec recursively from the current path. I'm using Ubuntu.  I do not want or need to change any folder names as I have control over those at the time of creation. Here's what I have now after referencing the following link:    find . -depth -exec rename 's!([^/]*\Z)/[^A-Za-z0-9._-]/_/g' *.avi {} +   I'm clearly missing something. Please help. :) What really allows airplanes to fly? What aerodynamic effects actually contribute to producing the lift on an airplane?  I know there's a common belief that lift comes from the Bernoulli effect, where air moving over the wings is at reduced pressure because it's forced to travel further than air flowing under the wings. But I also know that this is wrong, or at best a minor contribution to the actual lift. The thing is, none of the many sources I've seen that discredit the Bernoulli effect explain what's actually going on, so I'm left wondering. Why do airplanes actually fly? Is this something that can be explained or summarized at a level appropriate for someone who isn't trained in fluid dynamics?  (Links to further reading for more detail would also be much appreciated) I use a Windows 10 machine remotely at work. Both the local and remote machine are in the same ISP network (not same local network, but same ISP). I've been using it like that for almost a year now and I had never experienced absolutely any issues.  Until recently. I don't know if anything specific triggered this behavior, but I feel that it started happening after I upgraded the remote to W10 1903. I later upgraded the local one as well and I feel that it only made the problem worse (but I don't really have any data).  From time to time, the session will freeze - the image stops updating. However, what I type and mouse clicks are still being sent. No error message is shown whatsoever, the image simply freezes. To make it work again, I have to close and re-open the session. Everything that I had typed and/or clicked while it was frozen is there.  Is there any way I can debug this? I have been attempting to solve this integral for several days in preparation for an exam and keep reaching dead ends.  I(a,b) = $\int_0^{\pi/2} \ln(a^{2}\cos^2(x)+b^{2}\sin^2(x)) dx$, where $a,b&gt;0$  My best approach so far is defining:  $F(x) = I(x, b)$  Then, I know:  $F(b) = I(b, b) = \pi/2 * \ln(b^2)$  Then I am now attempting to find $F'(x)$ using differentiation under the integral sign, and integrating from b to a.  This is where I am stuck - no matter what I do, I cannot solve the integral. Am I in the right direction or should this be solved in a different manner? I also considered something elliptical due to the acos, bsin in the original integral, yet it has also reached a dead end. Automatically convert quotations in the form of "abc" to become ``abc" Constructing the reciprocal of a segment How can one construct the reciprocal length of a line segment? For example, given any line segment a, how can $\frac{1}{a}$ be constructed?  I was told that it can be solved by creating similar triangles, but I do not get it. How to clip an Openlayers OSM Background to a Vector Boundary?