java.lang.NullPointerException on version upgrade
What is a NullPointerException, and how do I fix it?
What are the left and right ideals of matrix ring? How about the two sided ideals? For any prime $p > 3$, why is $p^2-1$ always divisible by 24? How to get Total Pupulation From Tables In Sqlserver? On which Stack Exchange site should I post subjective questions? I lost my .keystore file? Why does the Lasso provide Variable Selection? Should I use 'was' or 'were' in this example?     I was always delighted when my brother or one of my sisters was/were asked to do them. If we pick randomly two points inside a circle centred at $O$ with radius $R$, and draw two circles centred at the two points with radius equal to the distance between them, what is the expected area of the intersection of the two cirlces that contain the origin $O$. 18.04.01 Not available for server Energy conservation and interference I have a problem with energy conservation in case of interfering waves.  Imagine two harmonic waves with amplitudes $A$. They both carry energy that is proportional to $A^2$, so the total energy is proportional to $2A^2$. When they interfere, the amplitude raises to $2A$, so energy is now proportional to $4A^2$ and bigger than before.  The equivalent question is what happens to the energy with the superposition of two waves that interfere destructively.  Also, if someone could comment on the statement about this problem in my physics book (Bykow, Butikow, Kondratiew):      The sources of the waves work with increased power during the interference because they feel the wave from the other source. What is the difference between 'su -' , 'sudo bash' and 'sudo sh'? I would like to know what is the difference between    su - sudo bash sudo sh   I cannot login as root while doing su - because I do not have the root password - I mean I have my user password with which I can sudo "command"  but this password does not work for su - I can log in as root with sudo bash with my user password and I can login while doing sudo sh but then I get a command prompt in this form # The three files are read in this order?  .bash_profile .profile .bashrc   This is not happening when I first open a terminal.   I have trace statements in the files appending to the file init.log. Please take a look at the following. It begins after opening a terminal. I have placed a comment to show where the log picks up after the su command.  stephen@debian:~$ cat init.log reading .bashrc done reading .bashrc stephen@debian:~$ su - stephen Password:  stephen@debian:~$ cat init.log reading .bashrc done reading .bashrc # # after su # reading .bash_profile reading .profile reading .bashrc done reading .bashrc done reading .profile done reading .bash_profile stephen@debian:~$    So the su - login triggers the expected sequence however the initial login reads only the bashrc. This cannot be correct. Can someone explain under what conditions this would occur. I could modify the bashrc and profile files so that the initial read includes all expected files but I would rather get to the root of the problem and fix it there. Let's say that there is a question asked in a bad manner, in a way that the question is missing. For example the user shows the code and doesn't explain very well which is the problem.    Usually someone starts digging in the OP mind posting comments and asking him for details. It may happen that the question gets edited and the OP edits it to a valid question.    Do you usually give the OP time to edit the question or you immediately vote to close it? Do we have always $f(A \cap B) = f(A) \cap f(B)$? Suppose $A$ and $B$ are subsets of a topological space and $f$ is any function from $X$ to another topological space $Y$. Do we have always $f(A \cap B) = f(A) \cap f(B)$?  Thanks in advance Using round function in QGIS expression? Empty intersection and empty union I'm using C++ to understand how exactly pointers work. I have this piece of code using arrays, which I'm using just to understand how the equivalent works with pointers.  int main() {         int arr[10] = {1,2,3};         char arr2[10] = {'c','i','a','o','\0'};     cout &lt;&lt; arr &lt;&lt; endl;     cout &lt;&lt; arr2 &lt;&lt; endl; }   However when I run this, arr outputs the address of the first element of the array of ints (as expected) but arr2 doesn't output the address of the first element of the array of chars; it actually prints "ciao".  What is it that I'm missing or that I haven't learned yet about this? How does Cooper get out of the black hole? In the movie Interstellar,  Cooper (played by ) enters a black hole and sends a message to Murph (his daughter). Later, he is found by rangers, but I don't understand how he gets out of the black hole?  As far as I know, nothing can get out of a black hole, as it has extremely high gravity. Where can a tourist apply for a UK Visitor (standard) Visa? Punctuation of a Direct Quote Is any particular punctuation necessary when a quotation is the subject of a sentence? For example, in the sentence below, should there be a comma after export? Or anything else?     "Laughter is America's most important export" is a well-known quote of Walt Disney's.