Error when indexing tweets In order to index my collection of tweets I have written this code but an error appears I do not know is what I should initialize a few things, thank you.  public static void main (String args []) throws IOException{         String line;         Gson gson = new GsonBuilder().create();         File f = new File(Constante.DATA);         BufferedReader reader;         Indexer indexer = new Indexer(Constante.INDEX);         int nbrtweet =0, i=0;                 for(File file : f.listFiles())         {                 System.out.println(file.getName());         reader = new BufferedReader( new FileReader(file));         while((line = reader.readLine())!=null){         Tweet tweet;                 try{            tweet = gson.fromJson(line, Tweet.class);            }catch( JsonSyntaxException e){             i++;             continue;                 }         nbrtweet++;         indexer.indexTweet(tweet);                 }   The error shows up on this line:   for(File file : f.listFiles())   and prints  Exception in thread "main" java.lang.NullPointerException     at ApprocheTwitter.src.Indexer.IndexationThematiqueTwits.main(IndexationThematiqueTwits.java:26)
What is a NullPointerException, and how do I fix it? 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?
Can't see a glass object through another glass object in Eevee I was just testing Eevee and found that you cannot view a glass object through another glass. Is there any workaround? It works fine only If I rotate the glass window or put my camera to the back side of the scene How can I cancel an edit I made on another user's question or answer? Edits made by users who do not have edit privileges are placed in a queue until it is peer reviewed.  E.g., I edited someone else's answer. but changed my mind. If I'd edited my own question/answer, there would be a "Rollback" button, but there's no such button in the scenario I'm describing (at least none that I can see).  This is more relevant today because users can only have at most five edits waiting in the queue, and allowing one to cancel an edit would permit them to free up one of their five pending edits for something else.  Is there a way to remove one's edit from the peer review queue? Why we use $L_2$ Space In QM? I asked this question for many people/professors without getting a sufficient answer, why in QM Lebesgue spaces of second degree are assumed to be the one that corresponds to the Hilbert vector space of state functions, from where this arises? and why 2-order space that assumes the following inner product:  $\langle\phi|\psi\rangle =\int\phi^{*}\psi\,dx$  While there is many ways to define the inner product.  In Physics books, this always assumed as given, never explains it, also I tried to read some abstract math books on this things, and found some concepts like "Metric weight" that will be minimized in such spaces, even so I don't really understand what is behind that, so why $L_2$? what special about them? Who and how physicists understood that those are the one we need to use? How can I get a history of running processes Proving $\frac{\sin x}{x} =\left(1-\frac{x^2}{\pi^2}\right)\left(1-\frac{x^2}{2^2\pi^2}\right) \left(1-\frac{x^2}{3^2\pi^2}\right)\cdots$ Triangle Inequality for norm integral $\|f\|_1=(\int_a^b [|f|^2+|f'|^2]\mathsf dx)^{1/2}$. Define $C_1^1[a,b]$ to be the space of continuously differentiable functions on $[a,b]$, with norm $$\|f\|_1=\left(\int_a^b[|f|^2+|f'|^2]\mathsf dx\right)^{1/2}.$$ Show that this is a proper definition of a norm. Is this normed space complete?  I'm stuck on the triangle inequality part. I'm having a hard time showing that $$\|f+g\|_1\leq \|f\|_1+\|g\|_1.$$ Any hints or solutions are greatly appreciated. Misleading chat usercard on SE 2.0 chat Why multiple PTR records in DNS is not recommended? I   that using multiple PTR records in a DNS configuration is not recommended.   However, the reasons are often vague, or not so obvious, naming:    "it can cause problems",  "can trigger bugs in programs expecting a single answer" : it's the software's problem then, isn't it?! "can make DNS answer packet too large" : isn't this fixed with ?   Are these good reasons? Do you know of any other (good) reasons? All this kinda looks like a "legacy fear"... Why are mirrorless cameras much slower than DSLRs? This is a follow-up to a .  I've been comparing cycle times (or shot to shot times in single shot mode) of mirrorless cameras with interchangeable lenses with traditional DSLRs and all of them, even the highest end ones (like the Olympus OM-D EM-5 or Panasonic GH3) are slower than even entry level DSLRs.  How's that possible? The electronics should be the same and they don't have the burden of a mirror going up and down between each shot. I think that there must be a technical reason for that (maybe some cpu time is going to feed the live-view before the image is encoded?), because from a commercial point of view it does not make sense.  I've seen the cycle times under the performance tab in the reviews pages of .  The average CSC has cycle times around 0.7~1.2s, the flagship ones around 0.5s. The average DSLR has cycle times around 0.4s and the flagships around 0.25s. Windows Metro apps have wrong icon in search UI I want to show without using Sylow theorem that     Any group of order $2p$ has a subgroup of order $p~$($p$ being a prime)   My attempt: Since $|G|=2p,$ even $\exists~a\neq e\in G$ such that $a^{-1}=a.$ Then $H=\{e,a\}\leq G.$ Let $S$ be the set of all left cosets of $H$ in $G.$ Now $S$ forms a group w.r.t. the composition $g_1Hg_2H=(g_1g_2)H.$ The mapping $\sigma:S\to G:gH\mapsto g$ is an $1-1$ homomorphism. Then $S\simeq\sigma(S)\leq G.$ Thus $G$ has a subgroup of order $p.$  Please tell me whether I'm right!     I'm skecptical as I didn't use that $p$ is prime. How do you start Windows 10 in safe mode? Viruses: Adaptation to a new host through repeated host jumps A friend told me, during a 3 minute discussion, that viruses that are endemic in host $A$ and make repeated jumps to host $B$ but can't be transmitted between individuals of species $B$, may slowly adapt (through these repeated jumps) to be able to be transmitted between individuals of host $B$ and become epidemic.  I don't know much about epidemiology. I don't understand how a virus population that is endemic to host $A$ may adapt to host $B$ with repeated jumps while the viruses that jump to host $B$ are dead end because they cannot be transmitted further more. Or Are these viruses able to jump back to host $A$ to bring back their newly acquired adaptations to host $B$?  Also, I might misunderstand the meaning of "repeated host jumps transmission". I first thought it meant repeated jumps from a reservoir population in host $A$ to host $B$, but it is also possible that it describes the dynamic of a virus population that is adapted to jump from species to species and they actually gain this ability by keeping jumping and jumping. But then, how could a virus species get adapted to jump from species to species? I'm a bit confused…. Can you give me some hints about this process of cross-species transmission through repeated jumps? Lookup list dependant on the contents of another cell hope someone can help to crack this one?  I've got a table on my data entry tab (sheet 1) where each row includes two data validation drop down lists (source is on sheet2). I want the options in the second drop down list (k13) to be dependant on what is selected in the first drop down list (g13)...  ie. if you select 'Diary' in the first drop down box then the second drop down only has 'Cheese', 'Milk' and 'Yogurt' - likewise, if you select 'Fruit' in the first box - I want the only options to be available specific to that categories column in the lookup table on sheet 2 (which are 'apple', 'pear' and 'banana' in this case)?  Please help - i think this is a vlookup indirect formula?? Letter O with vertical line: operator null Okay setting the scene. I have three tables, (Table1, Table2 and DataTable) and I want to insert into Table1 and Table2 using DataTable as source. So for every row in DataTable I want a row in Table1 and Table2, and Table2 needs to have the inserted id (PK) from Table1...  If I were to do this...  INSERT INTO Table1 SELECT A, B, C FROM MyTable INSERT INTO Table2 SELECT IDENTITY_INSERT(), D, E, F FROM MyTable   I'd get the ID of the last inserted record into Table1.  Is a CURSOR or WHILE loop the only ways to do this? The coefficient of kinetic friction can be calculated using $\mu_k = F_k/F_n$. A change in angle does not affect the normal force, but doesn't a change in angle affect the friction? As in if the angle is steeper, wouldn't the object have a greater propensity to move down the slope, thus there will be less friction? According to preliminary estimates, the project should be completed by 2020 at a cost of about RUB 40 bn.    or      According to preliminary estimates, the project should be completed by 2020 at the cost of about RUB 40 bn.    Are both options okay, or is the first better?   We are introducing the sum to the reader, so logically it should be the first option. But there's a lot of google-hits for "at the cost of" too. What are the possible values of the Hibernate hbm2ddl.auto configuration and what do they do I really want to know more about the update, export and the values that could be given to hibernate.hbm2ddl.auto I need to know when to use the update and when not? And what is the alternative?  These are changes that could happen over DB:   new tables new columns in old tables columns deleted data type of a column changed a type of a column changed its attributes tables dropped values of a column changed    In each case what is the best solution? Can anyone explain precisely what IOWait is?