I’ve read about hash and HashMap coming from here:   In particular:  //Depends only on account number @Override public int hashCode() {     final int prime = 31;     int result = 1;     result = prime * result + accountNumber;       return result; }   Why the hash code is calculated with prime * result + accountNumber and not only from accountNumber  What meaning has the fixed value prime * result?  Why not just prime (result value is 1 so prime time 1 is prime)?
What issues / pitfalls must be considered when overriding equals and hashCode?
How to automatically archive a directory? I want to do one way synchronization.  I am having Folder A on my computer which is constantly updated with content. Another Folder B is used for backup purpose which is on external HDD.  Now what I expect is that whatever extra which is present in folder A should go to folder B. However something which is present in B and NOT in A ""shall NOT be copied to A"".  In a nutshell, the backup folder may copy everything from the source folder, however nothing should be copied form backup folder to the source. Someone edited my message on StackOverflow, but it really bugs me out.  I'm not sure what's wrong with it:   As you see, the bigger the circle becomes, the more vertices I need for hiding the straight lines. As you see, the bigger the circle becomes, the more vertices I need to hide the straight lines.   The latter (corrected) sentence just bugs me out for some reason. It would sound better if there was a comma after the word "need", perhaps, but I guess that would be even worse in means of grammatical correctness.  Can someone explain why is the first sentence bad and the second sentence correct? And could there even be better solution to fix the first sentence? Query Activity for ORDER BY xxx DESC LIMIT The indefinite article "an" is used when the following word starts with a vowel sound. For example one write "an adjective" but "a noun".   However, when an abbreviation is used, should this be based on the expanded meaning, or on the letters of the abbreviation.  Specifically, one writes "a noun phrase". However, here on ell.se, the abbreviation "NP" for 'noun phrase" is commonly used. If this were pronounced by sounding out its letters, as "En-Pee" it would call for "an". If it were read as "noun phrase"  it would call for "a". Should one write "a NP" or "an NP" here on ELL?  I incline to "a NP" and i used that in  just a few minutes ago, but I am not at all sure. I uninstalled my wine, and installed a higher version of wine by compiling source.  However when I run an exe executable (whose path is in $PATH), it doesn't work as it used to:  $  PDFXCview.exe     run-detectors: unable to find an interpreter for /home/tim/program_files/document/pdfxchange_portable/PDFXCview.exe   I am not sure why I was able to run this executable directly before I uninstalled my old wine and installed the new one. How can I make $  PDFXCview.exe work now?  Note:  $ wine /home/tim/program_files/document/pdfxchange_portable/PDFXCview.exe   will work.   $ wine PDFXCview.exe   wine: cannot find L"C:\\windows\\system32\\PDFXCview.exe"   doesn't work. Note that the path of PDFXCview.exe is still in $PATH  Thanks. Exclude TeX.SE question titles from MathJax parsing in Hot Network Questions Recently, the TeX.SE question  got onto the Hot Network Questions list.  Like many questions on TeX.SE, it contains some TeX markup in its title.  Unfortunately, on , this markup gets parsed and rendered as math:    I propose that either:   the entire Hot Network Questions sidebar should be excluded from MathJax parsing by styling it with the tex2jax_ignore class, or at least, any hot questions from TeX.SE should be so excluded from MathJax parsing.   Or I suppose we could always implement , which asks for all questions with TeX in the title to be excluded from Hot Network Questions entirely.  That might be throwing out the baby with the bathwater, though.    Ps. This issue is partially masked by , which causes MathJax not to be rendered in many elements that are hidden by default and revealed using jQuery .show(), such as all but the first five questions in the Hot Network Questions sidebar.  The  user script contains a generic fix for that bug, so users who have it installed will see this issue more frequently, but it occurs even without SOUP whenever a question with TeX markup in the title happens to show up among the top five hot questions that are always visible by default. Is it OK to borrow a ground wire from a different circuit? I've got a circuit in the basement carrying a handful of outlets.  This circuit has no ground wire.  I want to add a couple of outlets and want them to be grounded.  I may even want gfci outlets on them in the future.  I have no reasonable way of getting a ground wire to the panel but I do have easy access to another circuit that does have a ground wire.  Is it acceptable to borrow or share a ground wire between circuits?  If not, why not?  Both circuits are 20A with 12 AWG wire. variable-variables in PHP "Give a hand" vs "Help" Currently, I'm working in an international company with many non-native speakers (so am I) but with pretty good level of English, I suppose.  I have noticed that my colleagues usually use phrase "give a hand" when asking for  help or even talking about it and seems to be avoiding the strong word "help".  It looks a bit confusing to me especially after I made people laugh when I said  "I will help you" in one of our conversations, don't remember exact context though...  So could you please describe the difference between them? And is there something wrong or hidden meaning with help-based phrase? What happens if my order exceeds the bid or ask sizes? What will happen if my order exceeds the bid or ask size? For example, if the bid or ask is 100 shares, but I want to buy 100,000 shares, is it possible to do so? Easier way to debug a Windows service Unknown error: '' ("The cache has no package named wine-staging-i386") Find the sum of angles without trigonometry? When I open the game, the screen stays at the Mojang sign and then crashes. It has something to do with my texture pack, but I can't start the game to change it and it won't let me delete the texture pack folder alone.  I've already deleted the bin and resources folders in the minecraft folder and downloaded the game again. I opened it but the same problem was still there.  How do I properly perform a clean uninstall of the game? Is the power set of the natural numbers countable? Find the condition on $a$ and $b$ so that the two tangents drawn to the parabola $y^2=4ax$ from a point are normals to the parabola $x^2=4by$ Find the condition on $a$ and $b$ so that the two tangents drawn to the parabola $y^2=4ax$ from a point are normals to the parabola $x^2=4by.$  The required condition is $a^2&gt;8b^2$.I dont know how to prove it.I tried.  Let $(h,k)$ be the point from where tangents are drawn to the parabola $y^2=4ax$.Let $m_1,m_2$ be the slopes of the tangents.Equations of tangents are $y=m_1x+\frac{a}{m_1}$ and $y=m_2x+\frac{a}{m_2}$ and equations of normal to the parabola $x^2=4by$ is $y=m_1x-2bm_1-bm^3_1$ and $y=m_2x-2bm_2-bm^3_2$.  As $y=m_1x+\frac{a}{m_1}$ and $y=m_1x-2bm_1-bm^3_1$  are the same lines.Therefore,$\frac{a}{m_1}=-2bm_1-bm^3_1$  $bm^4+2bm^2+a=0$  And I am stuck.I cannot get the desired condition.Please help me. How to bond two different internet connections I have two internet connection, each with 4mbps bandwidth. I want to join/merge both of them to get 8mbps speed, both routers have WLAN and LAN capability.    How to Double my speed using two routers? Can I bond two connections in Windows and double my internet connection speed? Any download manager that accepts two internet connection to download? How to integrate a paper where I am third author, but did a lot of work, into my master's thesis? I am writing my thesis currently and I want to include papers that I am included on as an author.  I will have a couple papers where I am 1st author, but there is one where I am third but did a lot of the work.  How do I go about integrating it into my thesis?  Can I copy/paste most of the text still or do I need to rewrite it?  I don't want to plagiarize but don't want to rewrite it if I don't have to. When I try to save a new file in TextEdit, I always get a popup saying I can't save with ".txt" at the end of the file name.  I want to save a plaintext file, but TextEdit insists on saving as .rtf instead. It also lists .doc, .odt, .html, and some others.  How can I save a TextEdit document as plain text? Adding 3 electron spins I've learned how to add two 1/2-spins, which you can do with C-G-coefficients. There are 4 states (one singlet, three triplet states). States are symmetric or antisymmetric and the quantum numbers needed are total spin and total z-component.  But how do you add three 1/2-spins? It should yield 8 different eigenstates. Which quantum numbers do you need to characterise the 8 states?  It is not as easy as using C-G-coefficients and the usual quantum numbers as for the total momentum the doubly degenerate 1/2 state and quadruple degenerate 3/2 state can describe only 6 or the 8 states. You will need an additional quantum number for the degeneracy.  So how do you get the result?  (I actually tried out myself with a large 8x8 matrix. The total spin 1/2 is each doubly degenerate. For the additional quantum number I chose the cyclic permutation. Spin 1/2 states are neither symmetric nor antisymmetric. But what is the usual way to derive this?)  EDIT: For reference I'm adding my results for up to 4 spins from some time ago:   If you recall the basics of quantum mechanics with matrices it is actually a straightforward matrix diagonalization and requires no specialized knowledge. However, you still need to find an additional operator which breaks degeneracy. I chose the cyclic permutation, which seems to do the job. Please refer to the below answer, since I haven't checked all details.