I get the exception whenever I run the program without passing in  the usDollarAmount or cancel the program... I'm using swing components to accept user input. The program works fine otherwise.  Please show me how to handle this... Thanks  THE Currency CALCULATOR Exception in thread "main" java.lang.NullPointerException at sun.misc.FloatingDecimal.readJavaFormatString(Unknown Source) at sun.misc.FloatingDecimal.parseDouble(Unknown Source) at java.lang.Double.parseDouble(Unknown Source) at Currency.main(Currency.java:28)  import java.text.DecimalFormat;  import javax.swing.JOptionPane;   public class Currency{  public static void main(String[] args) {     // declare and construct variables       double usDollarAmount, poundsAmount,eurosAmount,rublesAmount;      DecimalFormat twoDigits = new DecimalFormat("####.00");      //print prompts and get input     System.out.println("\tTHE Currency CALCULATOR");      //print prompts and get input     usDollarAmount = Double.parseDouble(JOptionPane.showInputDialog(null, "Enter your dollar amount:"));        // calculations      poundsAmount = 0.64 * usDollarAmount;     eurosAmount = 0.91 * usDollarAmount ;     rublesAmount = 61.73 * usDollarAmount ;       // output   JOptionPane.showMessageDialog(null, "YOUR DOLLAR AMOUNT OF  " + twoDigits.format(usDollarAmount) + " is equal to  " + twoDigits.format(eurosAmount)  + " euros"  + " ," + twoDigits.format(poundsAmount)  + " pounds"  + " " + " and " + twoDigits.format(rublesAmount) + " rubles"  );   System.exit(0);  }   }
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?
Poisson Distribution of sum of two random independent variables $X$, $Y$ $X \sim \mathcal{P}( \lambda) $ and $Y \sim \mathcal{P}( \mu)$ meaning that $X$ and $Y$ are Poisson distributions. What is the probability distribution law of $X + Y$. I know it is $X+Y \sim \mathcal{P}( \lambda + \mu)$ but I don't understand how to derive it. Calculating extent of custom tiles? Problem  The picture is just an example of my case, but I am left with blank space on the left when I load all the tiles and I get error message in the console:   Failed to load resource: the server responded with a status of 404 (Not Found) http://path/to/the/tile//base/8/6/53.png  etc.     I could guess and limit the extent of the layer, but I would really like to know is there any way to calculate it and also be able to get rid of the error message.  What I know about the tiles?   Originally they are served as TMS They represent png images 256*256 They are in projection: EPSG:3857 In OpenLayers2 they were called like TMS layer In OpenLayers3 (v.3.8.2) they are called like XYZ layer (that includes -y):     var baseLayer = new ol.layer.Tile({   source: new ol.source.XYZ({     url: 'http://path/to/the/tiles/base/{z}/{x}/{-y}.png'   }) });   Thoughts and questions   How can I calculate extent of the tiles? Or in another words how can I find out zmin zmax xmin ymin xmax ymax or whole range of x/y?  What does x/y really represent? Are they coordinates of the left bottom corner or center of the tile? In which direction does x/y increase? How can I figure out x/y of at least one of the tile, for example left bottom one?    (I have modified the question for better understanding of the problem) Why can Naruto still use Rasenshuriken? When Naruto first used Rasenshuriken against Kakuzu the effect was massive. He destroyed   two of Kakuzu's hearts, but there were a huge after effect to him. If I'm not mistaken, Tsunade advised Kakashi to ask Naruto never to use the technique again due to the huge risk. So why is Naruto still able to use the technique and have no side effects after that battle? I may have missed out on something. Android: Using default styles Find all primes $p$ such that  $ p^3-4p+9 $ is a perfect square.   I tried a few different values for $p$, namely $2,3,5,7,$ and $11$. The prime $p =2,7,11$ all worked but $p =13$ didn't so it makes me wonder. How can I find all primes such that it is a perfect square?  EDIT: it turns out this is problem P25 in post number #63 at I need to calculate the time complexity of the following code:  for (i = 1; i &lt;= n; i++) {   for(j = 1; j &lt;= i; j++)   {    // Some code   } }   Is it O(n^2)? Generating random numbers without repetitions Links that are italics and bold not showing as links in Mobile Web Feed randomly gone on SE Android app SE Android App Version: 1.0.84  There is a display issue in the Feed section. The Earlier today, the below sections and the post title are not visible.  There is no exact steps to reproduce. It doesn't happen when the app is initializing. But if you open the Stack Exchange app, navigate to some other apps, then come back after few minutes, I can see this issue (but not all the time).  When the issue is there, if I manually click the Feed option in the navigation sidebar, the display issue is fixed.  I have recorded the issue: How to make a surface with a transparent background in pygame I read this great answer about how ReLu could approximate non-linear functions. However, if ReLu can approximate such functions, why cant linear activation function do the same? If they can, why linear activation functions are strictly prohibited in NN? Trying to install Atom. Unable to start Clangd language server sed fails to remove newline character Stealing and reattaching a running process It's sunday and I left an instance of R running in the office. The script finished running, and I would like to save my workspace without going to the office. I can ssh to the computer, but I have not run the program into screen.  Is there a way, over ssh, to disown the process (I don't have access to jobsid of the terminal the program is running on!) and reattach it with say, reptyr? Reptyr and reptyr -T don't work without previously disowning the process  28845 3diag       20   0  139.3m  10.4m   0.0  0.1   0:12.06 S  `- urxvtd                                                                                        5327 3diag       20   0   16.4m   4.6m   0.0  0.1   0:00.02 S      `- bash                                                                                      5335 3diag       20   0 3499.3m 3.244g   0.0 43.3 171:57.89 S          `- R    The unique part of my question is that I explicitly require to detach a program without having access to the terminal in which it was created. Is it possible to extend a tourist Schengen visa (12 more days)? My daughter applied at the Italian Consulate in Philadelphia, and they gave her two entries for 20 days (we thought it was for 20 days each entry). She was in Italy 12 days, then went to England for 1 week and flew to Venice to take a 10-day cruise. She couldn't make it because they said she has only 8 remaining days, but she still has a land tour already paid this week.  What can she do? Can she extend the Schengen visa for 12 more days? Right now, she is in Madrid, but next week she has to fly from Barcelona to Paris to start with the tour. What would happen if she decides to take the tour with the expired visa? She is a US permanent resident. Notify close voters when a closed question gets edited As mentioned in , a question closed because it was not well-phrased is currently very unlikely to be reopened after an edit clarifying the issues. Those who voted to close should at least be asked to reconsider and reopen the question after such an edit. I am working on the following problem from group theory:     If $G$ is a group of order $2n$, show that the number of elements of $G$ of order $2$ is odd.   That is, for some integer $k$, there are $2k+1$ elements $a$ such that $a \in G,\;\; a*a = e$, where $e$ is the identity element of $G$. Center Gravity in Unity a 2d planet in the center, where the players will be able to walk on it and also will be affected by center gravity. planet will be static and there won't be any other planets. so;  Is there a way that I can change the gravity position? yes you can adjust gravity via project settings but I want it to be in the middle x,y,z(0,0,0)  I have tried creating my own gravity but it wasn't very useful because objects wouldnt fall, stand on foot and also in contact with another object they would vibrate instead of standing still. (if they stand on the planet they would vibrate)          private void FixedUpdate() {     foreach(GameObject item in grav)     {         Attact(item);     } }  private void Attact(GameObject gmf) {     Rigidbody2D rgg = gmf.GetComponent&lt;Rigidbody2D&gt;();      Vector3 direction = rg2d.position - rgg.position;     float distance = direction.sqrMagnitude;      float forceMag = (10 * rgg.mass) / distance;     Vector3 force = direction.normalized * forceMag * 10;      rgg.AddForce(force); }   I can create a script that will change the gravity by the opposite of the object's position. this does work but I cannot see any way to do it for more than one object. 16.04 VirtualBox VM (from vhd file) hangs at "non blocking pool is initialized" I'm trying to boot Canonical's Azure 16.04 .vhd file in a VirtualBox VM and it hangs at "non blocking pool is initialized".  It works in Hyper-V, so it may only be a bug in virtual box (5.0.20). A "non-believer" of C# was asking me what the purpose to extension methods was. I explained that you could then add new methods to objects that were already defined, especially when you don't own/control the source to the original object.  He brought up "Why not just add a method to your own class?" We've been going round and round (in a good way). My general response is that it is another tool in the toolbelt, and his response is it is a useless waste of a tool... but I thought I'd get a more "enlightened" answer.  What are some scenarios that you've used extension methods that you couldn't have (or shouldn't have) used a method added on to your own class?