Is it more Java-thonic to throw an exception or return null?
Should a retrieval method return 'null' or throw an exception when it can't produce the return value?
Illustrating op amp feedback without control theory We are teaching op amps in a high school class, before calculus.  So we can't use control theory to teach how op amps respond.  All the same, I would like to have an intuitive explanation for how a feedback circuit works.  Take the negative feedback, for instance.  Is there a clear way to show how an initial difference of delta between V+ and V- leads to a very large (G(V+ - V-)) difference in the output, which then leads to ...  I'd like to be able to take that argument and show how the voltage of the output converges in addition to the standard virtual short argument.  Can anyone explain it clearly? Spambots causing server crashes. How do I block them? Question about set notation: what does $]a,b[$ mean? In the following question  the notation $c\in ]a,b[$ is used. What does this mean? I have never seen it before. If $f$ is measurable and $fg$ is in $L^1$ for all $g \in L^q$, must $f \in L^p$? My question is about one particular usage of static keyword. It is possible to use static keyword to cover a code block within a class which does not belong to any function. For example following code compiles:  public class Test {     private static final int a;         static {         a = 5;         doSomething(a);     }     private static int doSomething(int x) {         return (x+5);     } }   If you remove the static keyword it complains because the variable a is final. However it is possible to remove both final and static keywords and make it compile.   It is confusing for me in both ways. How am I supposed to have a code section that does not belong to any method? How is it possible to invoke it? In general, what is the purpose of this usage? Or better, where can I find documentation about this? What are those characteristics by which every sound can be identified uniquely? Label rows of a matrix by characters Can somebody help me understanding the fouries series coefficients?  I know that if we have:  $$f(t) = \sum_{n=1}^N A_n \sin(2\pi nt + Ph_n) \tag{where $Ph_n$ = phase}$$  And because of the $\sin(a+b)$ formula: $$ f(t) = \sin(2\pi nt + Ph) = \sin(2\pi nt )\cos(Ph_n) + \cos(2\pi nt )\sin(Ph_n)$$ Then:  $$\sum_{n=1}^N A_n \sin(2\pi nt + Ph) = \sum_{n=1}^N (A_n\sin(2\pi nt )\cos(Ph_n) + A_n\cos(2\pi nt )\sin(Ph_n))$$ And by definition: $$a_n = A_n\cos(Ph_n)$$ $$b_n = A_n\sin(Ph_n)$$ Then:  $$f(t) = \sum_{n=1}^N A_n \sin(2\pi nt + Ph) = \sum_{n=1}^N (a_n\sin(2\pi nt ) + b_n\cos(2\pi nt ))$$ Where does the $A_n$ in the first formula come from? Could somebody explain me where does the $\frac{a_0}{2}$ in the formula below come from? $$f(x) = \frac{a_0}{2}+\sum_{n=1}^\infty (a_n \cos(nx) + b_n \sin(nx))$$  And the most important:  How can I proof the formulas for the $a_n$ and $b_n$ coefficients?  PLEASE, I'm searchinf for these answers for DAYS. Yesterday we started relativity on our physics class, and my professor taught us a few concepts. We did some examples on how things changed by looking them from different reference systems, and a paradox came to my mind from an example he did on the blackboard:  Suppose that we have a bug inside a hole (and suppose that the height of the bug is $0$). This hole is $L$ meters deep. There's a man outside the hole that wants to kill the bug with a nail of length $l$, with $l&lt;L$. Obviously, it's not possible for him to kill the bug by trying to push the nail in the hole. However, if the nail moves really fast, from it's reference system the hole will have a height $L_n&lt;L$, and if it moves fast enough then it will happen that $L_n&lt;l$, so he finally could kill the bug. On the other hand, the bug knows that from his reference system, the nail will have a lenght $l_b&lt;l$, so it would never reach him.  Is it possible to quantify the problem? How is it possible that the nail reach the bug from it's reference system and from the bug's reference system it's even smaller than before? How do I grep for lines containing either of two words, but not both? I'm trying to use grep to show only lines containing either of the two words, if only one of them appears in the line, but not if they are in the same line.  So far I've tried  grep pattern1 | grep pattern2 | ... but didn't get the result I expected. In Visual Studio, How do I show all classes inherited from a base class?    For example, in ASP.NET MVC there are several '' types -- and they all inherit from / implement the base class ActionResult.    It looks like unless you just 'know' that View and Json are valid ActionResult types, there is no way you can easily find this information out.    Please prove me wrong.  Is there something in the object browser that makes this easy to find out?  I'm even up for suggestions of tools outside of Visual Studio to discover this information about various classes.  For example: is there something in Resharper that will help me out? R's lmer cheat sheet $ not defined using jQuery in WordPress Is it required for panel data to use dummy variables? I am doing research considering seven countries and I have panel data. My question is: do I need to include dummy variables every time I use panel data in regression, or is enough to do it as a fixed effect in Stata, without dummy variables? Is there an Attribute I can use in my class to tell DataGridView not to create a column for it when bound to a List Every time i publish a new post i want to display an icon like a badge (NEW) and after a specific time it will disappear.  Lets say i publish a new post and i want to display that it is a new post. So i want to insert an icon that for example a star, in each new post in the blog list view into the metadata of the posts. Also i want to be removed after a specific time will pass. Lets say in one day or 24 hours the icon should be removed automatically.  How can i do this? "Pending" vs "Impending". Are they synonyms? They appear to me to mean almost if not exactly the same thing, but I am not sure. Are there differences in meaning between them? Anthology series with suspended animation and a galaxy-wide empire? What are suitable TV sizes and viewing distances for couch surfing on a 1080p LCD HDTV? I've seen the phrase "sh compatible" used usually in reference to shells. I'm not sure if it also applies to the programs that might be run from within shells.  What does it mean for a shell or other program to be "sh compatible"? What would it mean to be "sh incompatible"?  Edit: This question asking the difference between bash and sh is very relevant:   I'd still like a direct answer to what it means to be "sh compatible". A reasonable expectation might be that "sh compatible" means "implements the Shell Command Language" but then why are there so many "sh compatible" shells and why are they different?