public class DisplayInformation extends MainActivity {     @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.second_screen);          confirmCode = (Button)findViewById(R.id.SmsCode); // Why is this null?         confirmCode.setOnClickListener(this); // NullPointerException here     }  }   The logcat:  Process: com.example.user.callarocket, PID: 2096     java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.user.callarocket/com.example.user.callarocket.DisplayInformation}: java.lang.NullPointerException             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)             at android.app.ActivityThread.access$800(ActivityThread.java:135)             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)             at android.os.Handler.dispatchMessage(Handler.java:102)             at android.os.Looper.loop(Looper.java:136)             at android.app.ActivityThread.main(ActivityThread.java:5017)             at java.lang.reflect.Method.invokeNative(Native Method)             at java.lang.reflect.Method.invoke(Method.java:515)             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)             at dalvik.system.NativeStart.main(Native Method)      Caused by: java.lang.NullPointerException             at com.example.user.callarocket.DisplayInformation.onCreate(DisplayInformation.java:55)             at android.app.Activity.performCreate(Activity.java:5231)             at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)             at android.app.ActivityThread.access$800(ActivityThread.java:135)             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)             at android.os.Handler.dispatchMessage(Handler.java:102)             at android.os.Looper.loop(Looper.java:136)             at android.app.ActivityThread.main(ActivityThread.java:5017)             at java.lang.reflect.Method.invokeNative(Native Method)             at java.lang.reflect.Method.invoke(Method.java:515)             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)             at dalvik.system.NativeStart.main(Native Method)   I was trying to run this piece of code which connects to a PHP script and i got this error:     Caused by: java.lang.NullPointerException.   I couldn't find any mistakes here as I have tried the same piece of code in another project and it works.
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?
If I wanted to say something occurred occurred between 1627 and 1629, I would use &quot;the late 1620s&quot;. However, if I wanted to say something occurred between 1607 and 1609, I couldn't use &quot;the late 1600s&quot; because that would imply it occurred between 1670 and 1699. I noticed there is a square knob sticking out of my basement wall. Does anyone know what this is? How do you get the index of the current iteration of a foreach loop? Is there some rare language construct I haven't encountered (like the few I've learned recently, some on Stack Overflow) in C# to get a value representing the current iteration of a foreach loop?  For instance, I currently do something like this depending on the circumstances:  int i = 0; foreach (Object o in collection) {     // ...     i++; } What is the name of this castle part? Alt-Tab shows two application switchers on Unity I started to have problems with my Ubuntu application switcher (Alt +  Tab) this morning. When I press Alt +  Tab, not only one, but two application switchers show up. Last night I customized some stuff using the Unity Tweak Tool. I thought I didn't touch the application switcher. So now it looks like this: PGFPlots: Understanding how to draw plots on different layers and arrange them I have gone through previous questions [,,], but I still couldn't figure out how to draw plots on different layers and change their order of drawing.  For this MWE, I would be grateful if I could figure out how to change the order of appearance of the three plots in such a way that the legend entries have the same order while keeping the plots commands/definitions in the same order as much as possible.  \RequirePackage{luatex85} \documentclass{standalone} \usepackage{pgfplots} \pgfplotsset{compat=newest}  \begin{document}  \begin{tikzpicture}     \begin{axis}[             legend style={                 at={([yshift=30pt]0.5,1)},                 anchor=north},             legend columns=-1,             xmin=0, xmax=10,             ymin=0, ymax=10,         ]         %         \addplot [red,      line width = 5pt,   domain=0:10]    {x}; \addlegendentry{$x$}            \addplot [black,    line width = 5pt,   domain=0:10]    {(x-5)^2}; \addlegendentry{$(x-5)^2$}         \addplot [yellow,   line width = 5pt,   domain=0:10]    {10-(x-5)^2}; \addlegendentry{$10-(x-5)^2$}     \end{axis} \end{tikzpicture}  \end{document} C# Change A Button's Background Color Find all integral solutions of $ x^4 + y^4 + z^4 -w^4 = 1995 $ What does ` (backquote/backtick) mean in commands? I came across the following command:  sudo chown `id -u` /somedir   and I wonder: what is the meaning of the ` symbol. I noticed for instance that while the command above works well, the one below does not:  sudo chown 'id -u' /somedir Windows service stops automatically What is a fragment in a fragment shader? Wikipedia says that:     In general, a fragment can be thought of as the data needed to shade the pixel, plus the data needed to test whether the fragment survives to become a pixel (depth, alpha, stencil, scissor, window ID, etc.)   So is it textures, vertices or something else? How to remove lines included in one file from another file? External hard disk detection issue what exactly happened:  I have a Seagate 1 TB external hard disk.It was working fine till now.The problem started when I plugged it in my android phone and then back in my pc then simply it does not showup anywhere and :  It does not appear in file explorer   It does not appear in disk management   It is not listed in Diskpart command   It only appears in Device Manager under disk drives   It does not appear in BIOS  only makes sound when plugged and unplugged.  disk spins and light is also showing.   . What I already tried:   updated usb drivers.  used new cable also.  plugged in another pc  usb mass storage is also enabled in BIOS  no issue in any usb port  Anything else i can do? Is it possible to disable X at boot time? I'm setting up a server so it would be nice if it wouldn't load the graphical interface every time I boot. How critical are macOS Mojave compatibility requirements? As per Apple support document, :     1. Check compatibility      You can upgrade to macOS Mojave from OS X Mountain Lion or later on   any of the following Mac models. Your Mac also needs at least 2GB of   memory and 12.5GB of available storage space, or up to 18.5GB of   storage space when upgrading from OS X Yosemite or earlier.         MacBook introduced in early 2015 or later   MacBook Air introduced in mid 2012 or later   MacBook Pro introduced in mid 2012 or later   Mac mini introduced in late 2012 or later   iMac introduced in late 2012 or later   iMac Pro (all models)   Mac Pro introduced in late 2013, plus mid-2010 or mid-2012 models with a recommended Metal-capable graphics card.      I am currently running macOS High Sierra on my Early 2011 MacBook Pro.  What does Apple's requirements mean for my ability to install Mojave? Is it possible or worthwhile to do so? Irreducibility check for polynomials not satisfying Eisenstein Criterion. How to put Json inside asynctask - Android How to remove lines included in one file from another file? I have two lists, a complete list and a partial list. I want a bash script that will search the complete list for any lines that are in the partial, if there are, they will be removed from the complete list. Can anyone help? What is Ubuntu's status on the Meltdown and Spectre vulnerabilities? Any questions relating to status updates, or asking if anything is going to be patched for these vulnerabilities should be closed as duplicates of this question.    are in the news right now and sound pretty severe. I don't see any security updates from Ubuntu that cover these vulnerabilities.  What is Ubuntu doing about these vulnerabilities, and what should Ubuntu users be doing?  These are CVE-2017-5753, CVE-2017-5715 and CVE-2017-5754. Let $f:X \to Y$ be a homeomorphism between topological spaces $X$ and $Y$. Let $A \subseteq X$ and $B = f(A) \subseteq Y$ (given the subspace topology) and let $g: A \to B$ be the restriction of $f$ given by $g(a) = f(a)$ for $a \in A$.  Showing that $g$ is a homeomorphism.   $g$ injective   Let $g(a_1) = g(a_2)$, $a_1, a_2 \in A$.  $\implies f(a_1) = f(a_2)$  $\implies a_1 = a_2$ as $f$ homeomorphic.   g surjective   Let $y \in B$  $\implies y = f(a)$ for some $a \in A$.  $\implies y = g(a)$ for some $a \in A$.   $g$ continuous   Let $U \subseteq B$ be an open set in $B$.  $f(a) = g(a)$ for all $a \in A =&gt; f^{-1}(b) = g^{-1}(b)$ for all $b \in B$.  So $g^{-1}(U) = f^{-1}(U)$ which is open in $A$ as $f$ homeomorphic.   $g^{-1}$ continuous   Let $U \subseteq A$ be an open set in $A$.  $g(U) = f(U)$ which is open in $B$ as $f$ homeomorphic.  So $g$ is homeomorphic.  Is my understanding correct, this seems almost too straightforward so I was wondering have I overlooked something?