NullPointerException while writing to an excel file using Apache POI
What is a NullPointerException, and how do I fix it?
How does the momentum operator act on state kets? Prove that $f\in L^1(A)\Leftrightarrow \sum_{n}^{\infty}m(\{ x\in A : f(x)\geq n \}) < \infty$ This problem comes from another equation on another question (. I tried to split it in half but I found out that $$\sum_{k=0}^\infty \frac{k}{2^k}$$ can't be divided. Knowing that $$\sum_{k=0}^\infty x^k=\frac{1}{1-x}$$ I wrote that $$\sum_{k=0}^\infty \frac{k}{2^k}=\sum_{k=0}^\infty \left(\frac{\sqrt[k] k}{2}\right)^k=\frac{1}{1-\frac{\sqrt k}{2}}=\frac{2}{2-\sqrt[k] k}$$ But that's not what I wanted. Could anyone help me? Prove that formula is not tautology. Show Kripke's model proving that the following formula isn't tautology in intuitionistic logic. $ \neg ( p \wedge q ) \implies (\neg p \vee \neg q) $  Please help/hint me ;) If $A, B$ are two non empty sets of real numbers and for every $a$ from $A$ and $b$ from $B$     $a &lt; b$,   how can I prove that      $\sup A \leq \inf B$? I am trying to follow the best practice design patterns for triggers but get the error Constructor not defined: [CaseTriggerHandler].(). What am I missing?  Trigger  trigger CaseTrigger on Case (before insert, before update, before delete, after insert, after update, after delete, after undelete) {      CaseTriggerHandler handler = new CaseTriggerHandler();      //Before Insert     if(Trigger.isInsert &amp;&amp; Trigger.isBefore){         handler.OnBeforeInsert(Trigger.new);     }     //After Insert     else if(Trigger.isInsert &amp;&amp; Trigger.isAfter){         handler.OnAfterInsert(Trigger.new);     }     //Before Update     else if(Trigger.isUpdate &amp;&amp; Trigger.isBefore){         handler.OnBeforeUpdate(Trigger.old, Trigger.new, Trigger.newMap);     }     //After Update     else if(Trigger.isUpdate &amp;&amp; Trigger.isAfter){         handler.OnAfterUpdate(Trigger.old, Trigger.new, Trigger.newMap);     }     //Before Delete     else if(Trigger.isDelete &amp;&amp; Trigger.isBefore){         handler.OnBeforeDelete(Trigger.old, Trigger.oldMap);     }     //After Delete     else if(Trigger.isDelete &amp;&amp; Trigger.isAfter){         handler.OnAfterDelete(Trigger.old, Trigger.oldMap);     }     //After Undelete     else if(Trigger.isUnDelete){         handler.OnUndelete(Trigger.new);     } }   Handler      public with sharing class CaseTriggerHandler {      private boolean isExecuting = false;      public CaseTriggerHandler(boolean isExecuting){         this.isExecuting = isExecuting;     }      public void OnBeforeInsert(List&lt;Case&gt; newCases){         //EXECUTE BEFORE INSERT LOGIC     }      public void OnAfterInsert(List&lt;Case&gt; newCases){         //EXECUTE AFTER INSERT LOGIC     }      public void OnBeforeUpdate(List&lt;Case&gt; oldCases, List&lt;Case&gt; updatedCases, Map&lt;Id, Case&gt; CaseMap){         //BEFORE UPDATE LOGIC     }      public void OnAfterUpdate(List&lt;Case&gt; oldCases, List&lt;Case&gt; updatedCases, Map&lt;Id, Case&gt; CaseMap){         //AFTER UPDATE LOGIC     }      public void OnBeforeDelete(List&lt;Case&gt; CasesToDelete, Map&lt;Id, Case&gt; CaseMap){         //BEFORE DELETE LOGIC     }      public void OnAfterDelete(List&lt;Case&gt; deletedCases, Map&lt;Id, Case&gt; CaseMap){         //AFTER DELETE LOGIC     }      public void OnUndelete(List&lt;Case&gt; restoredCases){         //AFTER UNDELETE LOGIC     }      public boolean IsTriggerContext{         get{ return isExecuting;}     } } What is the difference between "INNER JOIN" and "OUTER JOIN"? Also how do LEFT JOIN, RIGHT JOIN and FULL JOIN fit in? In order to solve  $$y''-2y'+y=te^t$$ I did:  $$y = (at+b)e^t\implies\\ y' = ate^t+ae^t+be^t\implies \\y''= ate^t+2ae^t+be^t$$  but when I put these in the differential equation I'm getting zeros eveywhere. Does somebody know what I'm doing wrong? Why isn't a virus "alive"? When would you call java's thread.run() instead of thread.start()? Is an Otyugh's grapple different from a standard grapple? In 5e to escape a Grapple check, I know you roll athletics (str) or acrobatics (dex) opposed by an athletics (str) check.  In the Otyugh entry in the Monster Manual it doesn't say there's any check, instead it assigns a set DC for an "escape" (DC 13... which is a str check with a 10 assigned as the "roll").    So the question is:    Does the Otyugh actually make a roll in this instance (like the general rule) or does it have a set DC (for the specific case) OR is it just a poorly communicated version of the GENERAL rule being played out?   A little clarification on this would be AWESOME. In the event of a "specific vs general" scenario, what actually is the "escape" roll?  What modifier is applied? None? Limiting double to 3 decimal places How to enable read/write for windows partitions? With 11.04 I could read and write my partitions that was created in Windows however after upgrading to 11.10 somehow I can only read those partitions. How do I enable this ability in 11.10? How can I get back to Blender's default theme? Even though I am relatively new to Blender I am recording a tutorial for a Blender exporter. I have been advised that I should use the Default colour scheme (for the UI). I have no idea which one that is. Can anyone tell me? Check if an image is loaded (no errors) with jQuery Custom cursor in WPF? I want to use an image or icon as a custom cursor in WPF app. What's the best way to do it? As a result, I am able to be understanding and accommodating to the people that I work with, including clients, which I believe is an important asset for a professional to have.   I wanted to make sure it is absolutely known that I mean to acknowledge clients as a class of people I can work with. Is this the right way to include it with commas? Multiple "order by" in LINQ I have two tables, movies and categories, and I get an ordered list by categoryID first and then by Name.  The movie table has three columns ID, Name and CategoryID. The category table has two columns ID and Name.  I tried something like the following, but it didn't work.  var movies = _db.Movies.OrderBy( m =&gt; { m.CategoryID, m.Name }) I tried to prove this question but without a success: Let $K_1$ and $K_2$ be definable sets, prove that $K_1\cup K_2$ is definable.  What I tried to do is to assume: $K_1=Ass(X)=\{ v|v \vDash X \}$ and $K_2=Ass(Y)=\{ v|v \vDash Y \}$ and to say that $K_1\cup K_2=Ass(X \cap Y)$. but I couldn't prove that: $K_1\cup K_2 \supseteq Ass(X \cap Y)$ .   I would like to get help with this proof Prove the following: If $a \mid bc$, then $a \mid \gcd(a, b)c$. Prove the following: If $a \mid bc$, then $a  \mid \gcd(a, b)c$.   I tried to set $\gcd(a, b)$ to $b$ and used the fundamental theorem of arithmetic to prove that it is divisible by $a$, but I can't prove that $a \mid bc$, if and only if $a\mid b$ and $a\mid c$. Please help. Thanks.