Null Pointer Exception Linked Lists I'm getting a NullPointerException for the code below, and I don't quite understand why (or how to fix it). I know that my first node (next) is null, and that I can't call any methods on it without causing a null pointer exception, but I can't make it 'not null' without calling any methods on it :(. The null pointer exception is thrown in the first for loop in the insert function.   Here's my node class:  public class Node {  public Node next; public String word;  public Node(String word){     this.word = word; }  public void display(){     System.out.println (word); }  public String toString(){     return word; }   And here's my linked list class  public class WordLinkedList {  private int size; //integer to store the size of the linked list (# of words) private Node firstnode; // reference to first node  public WordLinkedList(){     size = 0;     firstnode = new Node(null); // setting first node to null - dummy node ... I think ???!!! }     public WordLinkedList(String[] arrayOfWords){     for (int i = 0; i &lt; arrayOfWords.length; i++){         this.insert(arrayOfWords[i]);     } }   public void insert(String newword){     Node newNode = new Node(newword);     for (Node iterator = firstnode.next; iterator != null; iterator =        newNode.next){         if (iterator.word.compareTo(newword) &lt; 0){             if (iterator.next.word.compareTo(newword)&gt;0){                 newNode.next = iterator.next;                 iterator.next = newNode;             }         }         if (iterator.word.compareTo(newword) &gt; 0){             iterator.next = iterator;             firstnode.next = newNode;         }     } }  public void display(){     Node theNode = firstnode;      while (theNode != null){         theNode.display();         System.out.println("Next node: " + theNode.next);         theNode = theNode.next;         System.out.println();     } }   }
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?
OpenGL Line Width Near the beginning of the definition of the amsmath environments aligned and gathered (and also mathtools' multlined) there's an explicit thin space \,. This often  has to be cancelled  with  \!, as has been observed  various  times:         I'd like to know why this explicit \, is there &ndash; what would go wrong without it?    From amsmath.dtx:  \newcommand{\start@aligned}[2]{%     \RIfM@\else         \nonmatherr@{\begin{\@currenvir}}%     \fi     \savecolumn@ % Assumption: called inside a group %    \end{macrocode} %    The \cs{null} here is to keep the \cs{,} glue from causing the %    invocation of the clause in \tex/'s built-in tag placement %    algorithm that can cause an equation to be shifted all the way over %    to the margin. %    \begin{macrocode}     \null\,% Let $\alpha,\beta,\gamma$ be the roots of $x^3-3x^2+1 = 0$. Prove that $$\alpha^n+\beta^n+\gamma^n \equiv 4^n+5^n+(-6)^n \pmod{17}$$ where $n$ is an integer.   It sort of makes sense why they are congruent since we can say $x^3-3x^2+1 \equiv (x-4)(x-5)(x+6) \pmod{17}$, but since $\alpha,\beta,\gamma$ aren't integers, how do we prove this? Change the URL in the browser without loading the new page using JavaScript How would I have a  action that may have some effects on the current page but would also change the URL in the browser so if the user hits reload or bookmark, then the new URL is used?    It would also be nice if the back button would reload the original URL.  I am trying to record JavaScript state in the URL. This user is only displaying 56 rep.   But surely (s)he has much more than that.        The user has also never given out any bounties.  Also, badges are missing: Check for column name in a SqlDataReader object How to change SQL Server collation Changes to "close as duplicate" (part deux) Recently I have been working on a mini-game that gives a few players custom spawn eggs. I want the mob to simply have a name and be on a team. I'm attempting to use this command:   /give @p[x=-102,y=48,z=81,distance=..1] shulker_spawn_egg{Team:PurpleTeam,CustomName:"\"Purple Turret\""}   It does give the player a spawn egg, but the resulting shulker spawn does not have a team or a name. Any ideas why this isn't working? It seems Kylo has an obsession of Darth Vader and has an interest in the dark side because of it.   Does he not realize that Darth Vader turned good and helped destroy the Empire (Spoilers?)? I feel like Kylo would've seen how much better the Light Side was as it was able to turn Vader from the Dark Side if he had known this. Title is removed/cleared/erased when starting editing Writing string at the same position using Console.Write in C# 2.0 I have a console application project in C# 2.0 that needs to write something to the screen in a while loop. I do not want the screen to scroll because using Console.Write or Console.Writeline method will keep displaying text on console screen incremently and thus it starts scrolling.  I want to have the string written at the same position. How can i do this?  Thanks Get bytes from std::string in C++ I'm working in a C++ unmanaged project.  I need to know how can I take a string like this "some data to encrypt" and get a byte[] array which I'm gonna use as the source for Encrypt.  In C# I do    for (int i = 0; i &lt; text.Length; i++)     buffer[i] = (byte)text[i];   What I need to know is how to do the same but using unmanaged C++.  Thanks! How is $\dfrac1{(1-x)^5}=\sum_{n\geq0}{n+4\choose4}x^n$ Can someone please explain to me how is $$\dfrac1{(1-x)^5}=\sum_{n\geq0}{n+4\choose4}x^n$$  Thanks! What is the explanation of the non-existence of magnetic monopoles? Just for curiosity, sometimes while installing software throught apt-get install software_name the terminal prompt me for a confirmation "type Yes or No", but not allways.  I dont mean the sudo password prompt, but the message "Will be downloaded and configrated XXX Kb, continue?" (or something similar)  Why? Should flawed questions be closed immediately? Possible Duplicate:          Given a question that is not clear at first, which satisfies the criterion for being "not a real question":      "It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot   be reasonably answered in its current form."   should that question be closed immediately, or should the OP get a chance to improve the quality of his question.  My modus operandi is this: When I encounter a question which is "not a real question", I respond via a comment - I might ask one or more counter-questions (about the issue), I might comment that I don't understand the question. So, I let the OP know that his question should be improved. I then give the OP say 15 minutes to respond (and in most cases the OP does respond). However, if the OP doesn't respond, then I consider voting for "not a real question".  Is this how SO should work?   I have witnessed in the past several questions for which I saw potential, being closed immediately, and I wonder if that is appropriate.   Edit: Of course, questions that are duplicates should be closed immediately, and off-topic questions should also be moved / closed immediately. In this question I am specifically referring to those questions which "have potential" (which could be corrected / improved).. Spurious @ symbol at start of bash prompt in Debian Jessie What is the regular expression to match strings (in this case, file names) that start with 'Run' and have a filename extension of '.py'?  The regular expression should match any of the following:  RunFoo.py RunBar.py Run42.py   It should not match:  myRunFoo.py RunBar.py1 Run42.txt   The SQL equivalent of what I am looking for is ... LIKE 'Run%.py' .... How to change the voltage polarity in an inverting amplifier? I want to implement a simple circuit which can map -10V  +10V voltage range to 0V 5V voltage range. Thus I'm planning to use two op-Amps, one for the slope and the other for the offset. So basically I'm planning to use an inverting amplifier and a summing amplifier consecutively.  But Inverting amplifiers multiplies the input voltage with a minus sign. I need a positive slope. How can I overcome this?  Also Could you gibe me any ideas for a cheap op-Amp models for this purpose?