RecyclerView attempt to invoke virtual method getItemCount() on a null object reference
What is a NullPointerException, and how do I fix it?
Why does a helium filled balloon move forward in a car when the car is accelerating? I noticed that when I had a helium filled, latex balloon inside of my car, it moved forward in the cabin as I accelerated forward. The faster I accelerated forward, the faster the balloon went from the back of the car to the front of the car. The balloon didn't have a string.  This became a game with my 4 year old as we drove home. We figured out where the balloon would go based on how fast I accelerated, turned corners etc.  I expected that it would act a lot like the water in a cup does, but it was the total opposite it seemed.  What forces caused this behavior?  I assumed it has something to do with the fluid dynamics in the closed cabin, but I can't figure it out. Going through a text file line by line in C Looking at the properties for a Windows file I get two attributes, “Size” and “Size on disk,”  and "Size on disk" is always larger.  What do these two metrics mean? Why can't I connect to Pokemon Go via mobile data? Automating "Stay Awake" I use my personal Android smartphone for development. Every morning, I go to Settings \ Developer Options \ Stay Awake and check the box.  At the end of the day, I uncheck the box — since otherwise any notification (including the "fully charged" one) will trigger the screen on, and it will stay on from that point on.  How can these two operations be automated? Event button as a module or as user program? I'm working on a small game and i'd like to know how can i write/read a user settings file in ubuntu touch? I just installed a new version of Ubuntu and I want to roll it back to the previous version.  How can I do that? Is that even possible? What is the intuitive meaning of line integral with respect to x? I know how the line integral is calculated, but I want to know what the meaning of line integral with respect to x is, I mean, intuitively. The integral is $\int_C f(x,y) dx$ or it can be $\int_C f(x,y) dy$. ($C$ is a curve here) Thanks. I am currently developing a website which is going to take some time to finish. I need to move some 'stuff' to production frequently, while maintaining a under construction status. My front page will clearly mention that my website is under construction.  I know crawlers will access my URL before I am ready to go live. Should I tell them my site is under construction (no need to index)? If yes how?  P.S.: Eventually, I am ok to let them see my under construction front page. How to insert an item into an array at a specific index (JavaScript)? Compress a directory using tar/gz over SSH to local computer? I'd like to essentially tar/gz a directory on a remote machine and save the file to my local computer without having to connect back into my local machine from the remote one. Is there a way to do this over SSH? The tar file doesn't need to be stored on the remote machine, only on the local machine. Is this possible? Floating point arithmetic not producing exact results Getting the array length of a 2D array in Java I think I accidentally pressed something to make it like this:    See how the XYZ arrows are at the 3D cursor instead of those vertexes I've selected?  How do I fix that? Whenever I select on a vertex or something, those blue, red, green arrows would always be where I selected, now it just keeps going to the 3D cursor. Prove that the derivative of $x^w$ is $w x^{w-1}$ for real $w$ Can anyone give a proof of the derivative of this type of function? Specifically showing that  $\dfrac{d(x^w)}{dx} = wx^{w-1}$ for a real $w$?  I tried to use the Taylor series expansion for $(x+dx)^w$ and got the correct result. However, the proof of the Taylor series requires knowledge of the derivative of these functions. So this is essentially circular reasoning. I know that the same series is also given by the binomial expansion, but that's not entirely satisfactory either, because where's the proof that the binomial expansion works for all reals (isn't it only apparent for integers)? So far all of the arguments I've come across involve circular reasoning. I was thinking of showing that the binomial expansion is true for all reals using some form of proof by induction e.g. something like this.  I'm really not sure. Missing vertices in Edit Mode I have append a mesh (rex) from this .blend .  In edit Mode I see different vertices from objcet mode (see pictures). The vertices of belt are missing!  OBJECT MODE:   EDIT MODE:   How can I edit vertices of belt? Is It a bug?  This is my .blend  maybe can help. Solving an Op-amp circuit The image of the circuit is shown below and it required to find \$V_0\$,  My first attempt at solving this problem is by changing the current source into a voltage source with 1-V and 2k\$\Omega\$ resistance. The fact that the inverting and non-inverting terminals aren't grounded make this problem look difficult.To the point which, I don't know how to proceed with this question or where to start. I would appreciate any help. Following some thought and another schematic from a hint suggested by Alfred, I produced a schematic representing our work.  And  my solution for the problem is below, Using node equation at nodes A and B we have, $$\frac{V_A-V_B}{1k}=-0.5 \text{mA}$$ $$\frac{V_B-(2+V_A)}{1k}=-0.5-x$$ where x is the current that is sent in the output of the op-amp. Using KCL, at the bottom node near the current source we see that the same current that goes through the op-amp also goes through the \$2\text{k}\Omega\$ resistor. Hence, we have, $$\frac{-V_A}{2k}=x$$ Replacing this in the second equation, $$\frac{V_B-(2+V_A)}{1k}=-0.5+\frac{V_A}{2k}$$ And and solving the equations yields \$V_A=-2 \text{-V}\$ and \$V_B=-1.5 \text{-V}\$ Etymology for “Mc‑” and “O’‑” prefix in surnames There is clearly a prefix in names like McDonald, McChrystal, O’Brian, O’Neal.  What does this Mc- and O- prefix signify? It looks like Donald, Chrystal, Brian, Neal are perfectly fine names on their own, so why is there a prefix before it? Hi I have a question regarding finding the values of limit for the following question.  Let $a, b \in \mathbb R$. Find the limit $$\lim_{x\to+\infty}\left(\sqrt{(x+a)(x+b)}-x\right)$$