I am trying to validate the format of a date before assigning it to an instance variable in a constructor. Each time, it throws a null pointer exception even though I provide a value for the field each time I try to instantiate an object. I think the problem must be with the parse statement, but I cannot understand why there would be a null value there.   Moreover, I was wondering if it is proper practice to validate a date in the constructor in the first place. Should I have a method doing this instead?   Thank you so much for your help. This is my first exposure to java.   import java.text.*;  public class Photograph {     private String caption;     private final String filename;     private String dateTaken;     private int rating; //declares fields       public Photograph(String caption, String filename) {         this.caption = caption;         this.filename = filename;         this.dateTaken = "1901-01-01";         this.rating = 0;    //constructor      }      public Photograph(String caption, String filename, String dateTaken,   int rating) {         this.caption = caption;         this.filename = filename;          SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");         sdf.setLenient(false);          try {             sdf.parse(this.dateTaken);             this.dateTaken = dateTaken;         } catch (ParseException e) {             this.dateTaken = "1901-01-01";         }     }  }  public static void main(String[] args) {     // Testing       //valid date     Photograph test_photo1 = new Photograph("cap1", "pic1", "2017-09-30", 3);     System.out.println(test_photo1.toString());     Photograph test_photo2 = new Photograph("cap2", "pic2", "2017-12-25", 0);     System.out.println(test_photo2.toString());     Photograph test_photo3 = new Photograph("cap3", "pic3", "2018-14-25", 5);     System.out.println(test_photo3.toString());     Photograph test_photo4 = new Photograph("cap4", "pic4", "2018-03-27", 4);     System.out.println(test_photo4.toString());     Photograph test_photo5 = new Photograph("cap5", "pic5", "2018-03-29", 2);     System.out.println(test_photo5.toString());     Photograph test_photo6 = new Photograph("cap6", "pic6", "2018-10-21", 9);     System.out.println(test_photo6.toString()); }
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?
Is there any app for Stack Exchange? Is there any app available in the Android Market for Stack Exchange sites? The literature defines multinomial coefficients (or extended bnomial coefficients) as   $$ \binom{n}{r_1,r_2,\cdots,r_l} = \frac{n!}{r_1!r_2!\cdots r_l!}$$ where $$ r_1+r_2+\cdots+r_l = n$$  Which is the coefficient of $x_1^{r_1}x_2^{r_2}\cdots x_l^{r_l}$ in expansion of $(x_1+x_2+\cdots+x_l)^n$.  Question: How do we call the coefficients of $x^r$ in expansion of $(x+x^2+\cdots+x^l)^n$. Is it bad to name them multinomial coefficients?  In case of binomial coefficients the two are same in expansion of $(x_1+x_2)^n$ and the expansion of $(x+x^2)^n$. Obviously the two are different for multinomial expansion.   Example: As $(x_1+x_2+x_3)^2= x_1^2+2 x_2 x_1+2 x_3 x_1+x_2^2+x_3^2+2 x_2 x_3$. The coefficients are $\{ 1,2,2,1,1,2 \}$. Are these MULTI-NOMIAL?  $ As (x+x^2+x^3)^2=x^2 + 2 x^3 + 3 x^4 + 2 x^5 + x^6$. The coefficients are $\{1,2,3,2,1\}$. Are these too the MULTI-NOMIAL?  I prefer different terminologies to avoid confusion. Therefore want to know how these are distinguished in existing literature. Automatically use specific language in Notepad++ for files with no extension How can I avoid SSH's host verification for known hosts? Why does my water pressure fluctuate? No, not like this:    Hear me out!  As Stack Overflow grows, it becomes increasingly ridden with extremely newbie and mediocre questions. It's just natural. The team are fighting hard to keep the crap out, but there are many very basic questions that are okay on their own. They don't deserve to be kicked out* - they are just very, very, very basic and/or localized.   Too much basicness tires users who are interested in discussing more advanced topics.  As has been discussed many times over, the voting system is not suited to identify advanced questions due to what is known as the . A group of people will vote on what they understand, which puts truly expert questions at a natural disadvantage that a democratic voting system can't overcome.  Now, there is a small number of users on Stack Overflow who I, as a user, know share my basic values (like, teaching a man to fish rather than fishing for them); I also know they are experts in their fields. It is safe to assume that what these users regard an interesting question worthy of attention, is likely to be interesting to me as well.      The idea is to introduce an opt-in "follow" mechanism that allows followed users to share what they find interesting, with very little fuss and on a purely opt-in basis.   Allow every SO user to follow any other SO user with a certain minimum rep, say 3000.   To every user who is being followed (or every user), display a new link underneath every question, "Recommend".    Introduce a new filtered view somewhere, named "Recommended". The view will show me all questions that my followed users are recommending.   Whenever I browse a list of questions, highlight those questions that have been recommended by my followed users. (I haven't put much thought into how to implement this graphically - you get the drift.)      I'm not especially fond of social networking, and I think most of it doesn't have a place on SO. But I'm starting to think this specific thing could be the right way to go in order to tell apart expert content from the 10000th dreary "how do I upload a file" question.  This is an entirely opt-in feature - unless I actively follow some users, I will never notice this.   Maybe for a bit of ego-stroking, the profile page can show how many users follow you. Or even who. Everything else would simply be kept under wraps - no publication of follower data in the data dump, no follower leagues or anything like that.      * At least not under current SO rules. Maybe those rules need to be changed - but that is a different discussion. Let $p,q,r$ be integers such that the symmetric sum of fractions $\dfrac{pq}{r}+\dfrac{qr}{p}+\dfrac{pr}{q}$ is an integer. Prove that each of the numbers: $\dfrac{pq}{r},\dfrac{qr}{p},\dfrac{pr}{q}$ is an integer. How to do this? Someone posted but deleted soon. How many different (circular) garlands can be made using $3$ white flowers and $6m$ red flowers? This is my first question here.   I'm given $3$ white flowers and $6m$ red flowers, for some $m \in \mathbb{N}$. I want to make a circular garland using all of the flowers. Two garlands are considered the same if they can be rotated to be the same. My question is: how many different garlands can I make?  I tried an approach in which I divided the question into three parts:   No. of ways in which the white flowers are all together No. of ways in which two white flowers are together No. of ways in which no two white flowers are together   Can you please help?  The answer is $3m^2+3m+1$ in the book that I saw  Thanks How to add appendix in thesis using LyX I want to add an appendix (questionnaire etc.) at the end of my thesis after the references. I use the article class and work with LyX: Document -&gt; Start appendix here. But I am not getting it in the TOC.  \documentclass[12pt,english]{article} \usepackage[T1]{fontenc} \usepackage[latin9]{inputenc} \setlength{\parskip}{\smallskipamount} \setlength{\parindent}{0pt} \usepackage{color} \definecolor{document_fontcolor}{rgb}{0, 0, 0} \color{document_fontcolor} \usepackage{babel} \usepackage{float} \usepackage{textcomp} \usepackage{pdfpages} \usepackage{tipa} \usepackage{amsmath} \usepackage{graphicx} \usepackage{setspace} \PassOptionsToPackage{normalem}{ulem} \usepackage{ulem} \usepackage[unicode=true,  bookmarks=true,bookmarksnumbered=false,bookmarksopen=false,  breaklinks=false,pdfborder={0 0 0},backref=false,colorlinks=false]  {hyperref}  \makeatletter  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% LyX specific LaTeX commands. %% Because html converters don't know tabularnewline \providecommand{\tabularnewline}{\\} %% A simple dot to overcome graphicx limitations \newcommand{\lyxdot}{.}   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands. \numberwithin{equation}{section}  \@ifundefined{date}{}{\date{}} \@ifundefined{showcaptionsetup}{}{%  \PassOptionsToPackage{caption=false}{subfig}} \usepackage{subfig} \makeatother  \appendix \noindent \begin{center} \textbf{Questionnaire (Case Study 1)} \par\end{center} Subgroup of $\mathbb{R}$ either dense or has a least positive element? Windows registry entries for default explorer view I'm making an automated installer for Windows XP, and I want the default view to be the "details" view on all folders - that is, the effect gotten by setting one folder to the details view, then going to Tools -> Folder Options -> View -> "Apply to All Folders". I also want the status bar to be showing, same as going View -> Show Status Bar.  What registry entries should I modify, and to what, to have this be the case? Permutation count of AABBC Given a string:  $AABBC=A^2B^2C^1$   I am trying to find the Total Permutations (this may be incorrect):  $\dfrac{5!}{2!\cdot2!}=30$  My question is how would I find the partial sums (perhaps the wrong choice in words) when added together would give an index such that:   $AABBC=1$ $ABABC=2$ $CBABA=29$ $CBBAA=30$   (30 might be the wrong number here but is suppose to be the last)   I have tried to draw out a tree structure to get an idea and thought with as the minimum:  $CAABB$  $4(\dfrac{4!}{2!\cdot2!})$ + 1  And:  $CABAB$  I would get something like:  $4(\dfrac{4!}{2!\cdot2!}) + 0 + 1(\dfrac{2!}{2!}) + 0 + 0$  But I am missing something here. How would I find the sums needed for $BACAB$ ?  More specifically I am trying to find the index of a word that is arbitrarily long in alphanumeric.  So the string may also be something like:  $AAAAABBBBDDEEFFFGGGGGHIIIIJJJJKLMOOOPZZZZZ$ Motion Tracking objects that go out of frame and re-enter later I am motion tracking a video in which the camera turns away from one scene and then shortly returns to it. However, the segments that went off-screen lose their track and do not continue tracking once the camera returns. I am wondering if there is a manual way to tell a segment where and when to continue looking for it's tracking target. Is this possible to do? Is every metric space a subspace of some connected metric space? Invoking JavaScript code in an iframe from the parent page How to find the last field using 'cut' After lethal first strike damage to an indestructible creature, will it still do damage? If I were to attack with a 4/4 indestructible creature and it's blocked by a 4/2 creature with first strike, would my indestructible creature still attack during the second combat damage step, or would it just tap, not assigning any damage? I'm about to travel to the US, where iPods / AirPods / iPhone are much cheaper than in my country.   I wonder if Apple's warranty is international, and/or what warranty coverage exists when I return to my country.  Where can I understand what Apple Warranty covers in the US when products leave the US? When using the Profile Tool plugin I get a graph that looks like the 2nd derivative of what I want. That is, it looks like the rate of change of the slope of the elevation profile. I just want the elevation profile! The data is present for good 1' contour lines. Why is the Profile Tool interpreting the data in such a bizarre way? What am I missing? And what are the Min and Max units? "physical block size is 2048 bytes, but Linux says it is 512" when formatting USB - How to create a bootable USB without this error I was using:  dd  if=/path/to/my/ubuntuiso/ubuntu.iso  of=/dev/sdb1  bs=4M  &amp;&amp;  sync   In order to create bootable Ubuntu USB drives on older Ubuntu versions. It was working perfectly. Sometimes I used the Startup Disk Creator program, which  worked well.  But when I use the same methods on Ubuntu 16.04 LTS, I get some warnings while formatting it afterwards.   The installation media works as expected, but when I try to format that USB stick after my work is done, I get the following warning :    The partitioning of that Pendrive looks strange :    And it also shows my 16GB pendrive as 64GB.  After struggling a lot with Gparted, I will somehow format it. But Why is it happening like this ?? Is there any better methods of creating bootable Ubuntu in 16.04 ?  EDIT : there is a related question . But my question is not about how to format it properly. My question is "How to create a bootable USB without that errors." &amp; "Wht are the causes of that error"