Attempt to invoke virtual method 'boolean android.graphics.Bitmap.isMutable()' on a null object reference
What is a NullPointerException, and how do I fix it?
Let $F:\mathbb{R^2}\rightarrow \mathbb{R}$ be a function, such as : $$F(x,y)=\begin{cases}        0 &amp; (x,y)=(0,0) \\       \frac{xy}{x^2+y^2} &amp; (x,y)\neq (0,0)    \end{cases}$$      Is $F$ differentiable at $(0,0)$?   My attempt :  Consider the limit, $\lim_{(x,y)\rightarrow (0,0)} F(x,y)$ along the line $y=x$. The limit equals to $\frac{1}{2}$. However, $F(0,0)=0$. Hence $F$ is not continuous at $(0,0)$ which implies that $F$ isn't differentiable as well.  Is this approach okay ? Books for Condensed Matter after Ashcroft/Mermin What are some good condensed matter physics books that can fill the gap between Ashcroft &amp; Mermin and research papers? Suggestions for any specialized topics (such as superconductivity, CFT, topological insulators) are welcomed. I just learned that using CBC encryption with an IV which is predictable is not secure.  From what I understand, using certain plain texts, and then guessing the IV that it uses, the attacker can verify if the IV he guessed was right. How does this qualify as an attack? How can I install Ubuntu encrypted with LUKS with dual-boot? The Ubuntu installation disk has an option to install Ubuntu encrypted using LUKS. However, there is no option to perform an encrypted installation along-side existing partitions for a dual-boot scenario.  How can I install Ubuntu encrypted alongside another partition from the live disk? Show $\text{Hom}_{Ab}(A,\mathbb{Q}) = \{0\}$ for finite $A$ How to create a progress bar? Checking incoming call in iphone Does construction of a Lightsaber require use of the Force? It's true that Lightsaber construction is a mandatory skill for a trained Force user, but I am not sure about it in reverse.  @DVK has commented on  that:     Grievous wasn't a Jedi (or a Force Sensitive) at all. He couldn't construct one.   I always thought that a Lightsaber was a normal piece of technology which even non-Force users could construct, and you only need Force sensitivity to master  Lightsaber skills. But, I could be wrong.  Does construction of a Lightsaber require use of the Force? If yes, what role does the Force play in the construction of a Lightsaber? How to check if a shell is login/interactive/batch I think I understand the differences between an interactive, a login and a batch shell. See the following links for more help:    (from the sister site: )  (from A User's Guide to the Z-Shell)   My question is, how can I test with a command/condition if I am on an interactive, a login or a batch shell?   I am looking for a command or condition (that returns true or false) and that I could also place on an if statement. For example:  if [[ condition ]]    echo "This is a login shell" fi WPF TextBox and Scroll behavior I have a problem. I need to host grid with controls in ScrollViewer to prevent textbox from being either truncated or collapsed to zero-with at the UI. Also I want the with of textbox to be expanded when user change windows width. I'm setting Window's content to following code  &lt;DockPanel&gt;     &lt;TreeView DockPanel.Dock="Left" Width="150"/&gt;     &lt;ScrollViewer VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto"&gt;         &lt;Grid&gt;             &lt;Grid.ColumnDefinitions&gt;                 &lt;ColumnDefinition Width="Auto"/&gt;                 &lt;ColumnDefinition /&gt;             &lt;/Grid.ColumnDefinitions&gt;             &lt;Grid.RowDefinitions&gt;                 &lt;RowDefinition Height="Auto"/&gt;             &lt;/Grid.RowDefinitions&gt;             &lt;TextBlock Text="Name"                         Margin="5"                         VerticalAlignment="Center"/&gt;             &lt;TextBox Grid.Column="1"                      Text="Some Name"                      Margin="5"                      VerticalAlignment="Center"                      MinWidth="200"/&gt;         &lt;/Grid&gt;     &lt;/ScrollViewer&gt; &lt;/DockPanel&gt;   All work fine, but when user types very long text in TextBox it is being expanded and horizontal scroll appears. Is there any easy way to limit TextBox maximum width and allow it to be expanded only when user changes window size. I have a long line of code that I want to break up among multiple lines. What do I use and what is the syntax?  For example, adding a bunch of strings,  e = 'a' + 'b' + 'c' + 'd'   and have it in two lines like this:  e = 'a' + 'b' +     'c' + 'd' Showing order of $Z (G) =1$ or $pq$ Why aren't Java Collections remove methods generic? Why isn't  generic?    Seems like Collection&lt;E&gt; could have boolean remove(E o);   Then, when you accidentally try to remove (for example) Set&lt;String&gt; instead of each individual String from a Collection&lt;String&gt;, it would be a compile time error instead of a debugging problem later. How was this answer posted after this question was closed? Question:   Answer:   The question was closed at 11:57:57 (according to the hover-over-"x-mins-ago" bar) and the answer was posted at 12:16:35. It's a fairly lengthy answer, so it's possible that it was started to be written before it was closed but I swear I've started to answer a question then I got a banner along the top saying "this question has been closed" and the answer was no longer post-able. Undefined Symbol ___gxx_personality_v0 on link I've been getting this undefined symbol building with this command line:  $ gcc test.cpp Undefined symbols:   "___gxx_personality_v0", referenced from:   etc...   test.cpp is simple and should build fine.  What is the deal? How to parse a date? every rank 2 oriented matroid is realizable Is this a correct way for passing an environment variable in sudo ?  sudo -u www-data -b env FOOBAR="foobar" /home/user/folder/daemon How can I increase the JVM memory? In an enumerate environment, I want a few (not all) of the items to have asterisks which appear before the number (I don't feel strongly about whether the number or the asterisk is horizontally aligned with the other numbers).  Is there a less crufty way to do this than, say, using multiple lists (or &mdash; horrors &mdash; numbering manually)?  I'm already using enumitem for an unrelated purpose, if that helps, although I didn't find a solution after a quick skim of its manual.