In the navigation drawer Im trying to open a new fragment on pop up menu item click in adapter. But getting the error as: Attempt to invoke virtual method 'androidx.fragment.app.FragmentManager androidx.appcompat.app.AppCompatActivity.getSupportFragmentManager()' on a null object reference. Unable to resolve. Below is the code PopupMenu popup = new PopupMenu(mCtx, holder.buttonViewOption);             //inflating menu from xml resource             popup.inflate(R.menu.options_menu);             //adding click listener             popup.setOnMenuItemClickListener(new PopupMenu.OnMenuItemClickListener() {                 @Override                 public boolean onMenuItemClick(MenuItem item) {                     switch (item.getItemId()) {                         case R.id.menu_map:                             //handle menu1 click                           //  Intent i = new Intent(mCtx, IntroActivity.class);                             //mCtx.startActivity(i);                              Fragment mFragment = null;                             mFragment = new PasswordFragment();                             FragmentManager fragmentManager = ((AppCompatActivity)context).getSupportFragmentManager();                             fragmentManager.beginTransaction()                                     .replace(R.id.container_view1, mFragment).commit();                             break;                         case R.id.menu_document:                             //handle menu2 click                             break;                     }
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?
Did Harry Potter lose Voldemort's powers when he lost his connection to Voldemort? Harry noted to Dumbledore that he had a number of things in common with Voldemort. And it was later revealed that some of Voldemort's power was transferred to Harry when Voldemort tried to kill him. So, what powers did Harry get from Voldemort (i.e. Parseltongue and others) and did they all leave him when he stopped being a horcrux?    Update In the Chamber of Secrets Dumbledore states that Harry has many qualities that Salazar Slytherin values in his hand picked students. Parseltongue - resourcefulness - determination - a certain disregard for rules.    So, does anyone know what specific powers Harry got from Voldemort and is there anyway to tell if he lost anything else besides Parseltongue? How to prove $\text{Rank}(AB)\leq \min(\text{Rank}(A), \text{Rank}(B))$? The bijection between homogeneous prime ideals of $S_f$ and prime ideals of $(S_f)_0$ Title is the question itself.  How can I show below without L’Hospital’s Rule  $$ \lim_{x\to \infty} ( 1 + x )^\frac{1}{x} = 1 $$ If $\gcd(a,b)=1$, then there exists integers $x$ and $y$ such that $xa + yb = 1$ Did not find this from this website...  If $$ \gcd(a,b)=1,$$ then there exists integers $x$ and $y$ such that $$xa+yb=1.$$  Now, the tip is to use particular corollary, that states:  The class $[m]_{n}$ generates $\mathbb{Z}/n\mathbb{Z}\Leftrightarrow \gcd(m,n)=1.$  I am totally lost with the corollary.  Let's assume that $\gcd(m,n)=1$. Then, $[m]_{n}$ generates $\mathbb{Z}/n\mathbb{Z}$.  OK! Then what?  There is also follow up, where I have to prove the converse. I am familiar with Bezout's lemma. Geometric meaning of the determinant of a matrix What is the geometric meaning of the determinant of a matrix? I know that "The determinant of a matrix represents the area of ​​a rectangle." Perhaps this phrase is imprecise, but I would like to know something more, please.  Thank you very much. I'm learning C#, so I made a little C# program that says Hello, World!, then compiled it with mono-csc and ran it with mono:  $ mono-csc Hello.cs $ mono Hello.exe Hello, World!   I noticed that when I hit TAB in bash, Hello.exe was marked executable. Indeed, it runs by just a shell loading the filename!   Hello.exe is not an ELF file with a funny file extension:  $ readelf -a Hello.exe readelf: Error: Not an ELF file - it has the wrong magic bytes at the start $ xxd Hello.exe | head -n1 00000000: 4d5a 9000 0300 0000 0400 0000 ffff 0000  MZ..............   MZ means it's a Microsoft Windows statically linked executable. Drop it onto a Windows box, and it will (should) run.  I have wine installed, but wine, being a compatibility layer for Windows apps, takes about 5x as long to run Hello.exe as mono and executing it directly do, so it's not wine that runs it.  I'm assuming there's some mono kernel module installed with mono that intercepts the exec syscall/s, or catches binaries that begin with 4D 5A, but lsmod | grep mono and friends return an error.   What's going on here, and how does the kernel know that this executable is special?    Just for proof it's not my shell working magic, I used  (aka sh) to run it and it still runs natively.    Here's the program in full, since a commenter was curious:  using System;  class Hello {   /// &lt;summary&gt;   ///   The main entry point for the application   /// &lt;/summary&gt;   [STAThread]   public static void Main(string[] args) {     System.Console.Write("Hello, World!\n");   } } How to show hidden folders in File Manager (Nautilus) on Ubuntu? Disk full, du tells different. How to further investigate? Short story with spaceship and humanity not realising they should be on it The plot is about:   A spaceship travelling through the universe Earth is a rogue planet whose inhabitants have been cut off and don't realise that their true purpose is to be part of the spaceship   The reason earth was in turmoil (wars etc.) was that they didn't realise that their true role was to fit into the ship  The ship contained 3 or 4 beings that made the thing work Planets throughout the universe are populated with the various beings which are components of the space craft   I seem to remember like an eye shaped being Each of them fulfil their role Humans are part of that as well.  There are loads of planets with humans on but earth has gotten overlooked so it never realised that's what they were designed for The story concludes with the ships picking up someone from Earth   and with the "human" realising that this is what they should really be doing  Humans have a different name that explains their role in the ship.   I probably read it around 30 years ago in English in a sci-fi collection book. What follows if we fail to reject the null hypothesis? Sort lines based on a variable-width number at a fixed position Clocks for multiple timezones in GNOME Shell I looked everywhere but I couldn't find a way to display multiple clocks in GNOME 3. In classic GNOME and Unity, it is possible. Also, for some strange reason, additional timezones cannot be added to the clock using the time-date applet in classic GNOME. Any solutions? Encode/Decode URLs in C++ Does anyone know of any good C++ code that does this? Back-transformation of regression coefficients I'm doing a linear regression with a transformed dependent variable. The following transformation was done so that the assumption of normality of residuals would hold. The untransformed dependant variable was negatively skewed, and the following transform made it close to normal:   $$Y=\sqrt{50-Y_{orig}}$$  where $Y_{orig}$ is the dependent variable on the original scale.   I think it makes sense to use some transformation on the $\beta$ coefficients to work our way back to the original scale. Using the following regression equation,   $$Y=\sqrt{50-Y_{orig}}=\alpha+\beta \cdot X$$  and by fixing $X=0$, we have   $$\alpha=\sqrt{50-Y_{orig}}=\sqrt{50-\alpha_{orig}}$$  And finally,  $$\alpha_{orig}=50-\alpha^2$$  Using the same logic, I found  $$\beta_{orig}=\alpha\space(\alpha-2\beta)+\beta^2+\alpha_{orig}-50$$  Now things work very well for a model with 1 or 2 predictors; the back-transformed coefficients resemble the original ones, only now I can trust the standard errors. The problem comes when including an interaction term, such as  $$Y=\alpha+X_1\beta_{X_1}+X_2\beta_{X_2}+X_1X_2\beta_{X_1X_2}$$  Then the back-transformation for the $\beta$s are not so close to the ones from the original scale, and I'm not sure why that happens. I'm also unsure if the formula found for back-transforming a beta coefficient is usable as is for the 3rd $\beta$ (for the interaction term). Before going into crazy algebra, I thought I'd ask for advice... If there is one noble left in line and you play "fled to England" to discard it (thus, ending the day) do you collect a noble from the new day? Persist variables between page loads Assume: $p$ is a prime that satisfies $p \equiv 3 \pmod 4$  Show: $x^{2} \equiv -1 \pmod p$ has no solutions $\forall x \in \mathbb{Z}$.  I know this problem has something to do with Fermat's Little Theorem, that $a^{p-1} \equiv 1\pmod p$. I tried to do a proof by contradiction, assuming the conclusion and showing some contradiction but just ran into a wall. Any help would be greatly appreciated. Confidence bands for QQ line This question doesn't specifically pertain to R, but I   chose to use R to illustrate it.  Consider the code for producing confidence bands round a (normal) qq-line:  library(car) library(MASS) b0&lt;-lm(deaths~.,data=road) qqPlot(b0$resid,pch=16,line="robust")   I'm looking for an explanation of (or alternative a link to a paper/online document explaining) how these confidence bands are constructed (I have  seen a reference to Fox 2002 in the help files of R  but sadly I don't have this book handy).   My question will be made more precise with an example.  Here's how R computes these particular CI's (I've  shortened/simplified the code used in car::qqPlot)  x&lt;-b0$resid good&lt;-!is.na(x) ord&lt;-order(x[good]) ord.x&lt;-x[good][ord] n&lt;-length(ord.x) P&lt;-ppoints(n) z&lt;-qnorm(P) plot(z,ord.x,type="n") coef&lt;-coef(rlm(ord.x~z)) a&lt;-coef[1] b&lt;-coef[2] abline(a,b,col="red",lwd=2) conf&lt;-0.95 zz&lt;-qnorm(1-(1-conf)/2) SE&lt;-(b/dnorm(z))*sqrt(P*(1-P)/n)     #[WHY?] fit.value&lt;-a+b*z upper&lt;-fit.value+zz*SE lower&lt;-fit.value-zz*SE lines(z,upper,lty=2,lwd=2,col="red") lines(z,lower,lty=2,lwd=2,col="red")   The question is: what is the justification for  the formula used to compute these SE (e.g. the line SE&lt;-(b/dnorm(z))*sqrt(P*(1-P)/n)).   FWIW this formula is very different from the formula of the usual "To mentor someone during a project" vs. "to mentor someone on a project" ..., whom I mentored during his final semester's project.   ..., whom I mentored on his final semester's project.      Which of these two is grammatically correct? Since I am not talking about the final semester but the final semester's project, I am not very sure about during. Can someone help me with this?