So the method is to see if a book exists in the library:   public boolean exists(Book l) {                           for (int i = 0; i &lt; index; i++) {         if (books[i].equals(l)) {             return true;         }     }     return false; }   Output:   Exception in thread "main" java.lang.NullPointerException at Library.exists(Library.java:13)   What am i doing wrong here?   So i got the answer thanks to you guys :) So the problem was that i did not add books inside the library! The method to add books:   public void addBook(Book l) {     if (l == null) {         System.out.println("Book is unini..!");         return;     }      if (exists(l)) {         System.out.println("Book exists!");         return;     }      if (books.length == index) {         Book[] temp = new Book[books.length + 3];         for(int i=0;i&lt;books.length;i++){             temp[i]=books[i];         }         books=temp;     }     books[index++] = l;  }
129
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?
23917
Which is the right thing to do?  if (myObj['key'] == undefined)   or  if (myObj['key'] == null)   or  if (myObj['key'])
18864
Is there any way other than looping through the Files to check whether the folder or files exist in document library using clientcontext - client object model..  Can someone confirm me whether the below code will work or not. Or will it create any exception??  using(var clientContext = new ClientContext(site))  {  Web web = clientContext.Web;  Microsoft.SharePoint.Client.File file = web.GetFileByServerRelativeUrl("/site/doclib/folder/filename.ext");  bool bExists = false;  try { clientContext.Load(file);  clientContext.ExecuteQuery();   bExists = file.Exists;  } catch{ }  if (bExists ) { . .  }
5718
The following will cause infinite recursion on the == operator overload method      Foo foo1 = null;     Foo foo2 = new Foo();     Assert.IsFalse(foo1 == foo2);      public static bool operator ==(Foo foo1, Foo foo2) {         if (foo1 == null) return foo2 == null;         return foo1.Equals(foo2);     }   How do I check for nulls?
46114
First of all, I swear this is not homework, it's a question I was asked in an interview. I think I made a mess of it (though I did realise the solution requires recursion). Here is the question:  Implement the count() method which returns the number of nodes in a tree. If a node doesn't have either a left or right child, the relevant getXXChild() method will return null  class Tree {    Tree getRightChild() {     // Assume this is already implemented   }    Tree getLeftChild() {     // Assume this is already implemented   }    int count() {     // Implement me   } }   My reason for asking the question is simply curious to see the correct solution, and thereby measure how bad mine was.  Cheers, Tony
105546
When modelling classes, what is the preferred way of initializing:   Constructors, or Factory Methods   And what would be the considerations for using either of them?  In certain situations, I prefer having a factory method which returns null if the object cannot be constructed. This makes the code neat. I can simply check if the returned value is not null before taking alternative action, in contrast with throwing an exception from the constructor. (I personally don't like exceptions)  Say, I have a constructor on a class which expects an id value. The constructor uses this value to populate the class from the database. In the case where a record with the specified id does not exist, the constructor throws a RecordNotFoundException. In this case I will have to enclose the construction of all such classes within a try..catch block.  In contrast to this I can have a static factory method on those classes which will return null if the record is not found.  Which approach is better in this case, constructor or factory method?
42562
How could I go about detecting (returning true/false) whether an ArrayList contains more than one of the same element in Java?  Many thanks, Terry  Edit Forgot to mention that I am not looking to compare "Blocks" with each other but their integer values. Each "block" has an int and this is what makes them different. I find the int of a particular Block by calling a method named "getNum" (e.g. table1[0][2].getNum();
91805
Short prestory. Typical problem: you have a lot of small string and you want to concatenate them all in one big string. The code like for(Integer i = 0; i &lt; smallStringsSize; i++){     bigString += smallStrings[i]; }  will solve this problem, but it has to produce a lot of unnecessary string objects that consume your heap. That is why Java has classes like StringBuilder and StringBuffer that allow you to solve this problem wihtout consuming too many memory. Thus, I decided to try to do my own StringBuilder in apex. String class has two methods that looked suitable for it:   and  . Here is the code that i used to test these methods against &quot;+=&quot; concatenantion (ready to execute in anonymous): List&lt;String&gt; chars = new List&lt;String&gt;(); List&lt;Integer&gt; charsCode = new List&lt;Integer&gt;(); Integer size = 10000; String chartToAdd = 'Ω'; for(Integer i = 0; i &lt; size; i++){     chars.add(chartToAdd);     charsCode.add(String.valueOf(chartToAdd).charAt(0)); } String joinS = '', forS = '', fromS = ''; Integer initialHeapSize = Limits.getHeapSize(); Integer initialCPUtime = Limits.getCpuTime(); System.debug('initialHeapSize ' + initialHeapSize); System.debug('initialCPUtime ' + initialCPUtime); joinS = String.join(chars, ''); Integer joinHeapSize = Limits.getHeapSize() - initialHeapSize; Integer joinCPUtime = Limits.getCpuTime() - initialCPUtime; System.debug('-------- String.join ---------'); System.debug('joinHeapSize ' + joinHeapSize); System.debug('joinCPUtime ' + joinCPUtime); for(Integer i = 0; i &lt; size; i++){     forS += chars[i]; } Integer forHeapSize = Limits.getHeapSize() - joinHeapSize - initialHeapSize; Integer forCPUtime = Limits.getCpuTime() - joinCPUtime - initialCPUtime; System.debug('-------- for each += ---------'); System.debug('forHeapSize ' + forHeapSize); System.debug('forCPUtime ' + forCPUtime); fromS = String.fromCharArray(charsCode); Integer fromHeapSize = Limits.getHeapSize() - forHeapSize - joinHeapSize - initialHeapSize; Integer fromCPUtime = Limits.getCpuTime() - forCPUtime - joinCPUtime - initialCPUtime; System.debug('-------- String.fromCharArray ---------'); System.debug('fromHeapSize ' + fromHeapSize); System.debug('fromCPUtime ' + fromCPUtime); System.debug(''); System.debug('-------- strings ----------'); System.debug('joins ' + joins); System.debug('fors ' + fors); System.debug('fromS ' + fromS);  The result of this code execution:  initialHeapSize 161166 initialCPUtime 242 -------- String.join --------- joinHeapSize 10016 joinCPUtime 17 -------- for each += --------- forHeapSize 10024 forCPUtime 48 -------- String.fromCharArray --------- fromHeapSize 10016 fromCPUtime 5  As you can see, the heap size for each approach is so close to 10 kilobytes, what is a size of the string of 10 000 characters (size) if each character takes only one byte. In case of creating 10 000 additional string objects, the heap size would be a bit more than 50 megabytes (10 000 * (1 + 10 000)/2). Taking into consideration the above,  Does not the '+=' concatenation create any additional string objects that devour your heap? Did not I miss anything? I am so amazed.  As you see, I use Omega 'Ω' as a chartToAdd. It has  the high-surrogate code point equals to 937, that needs at least 2 bytes to be encoded. How can 10 000 Omegas consume only 10 000 bytes of the heap size?  What weightful reasons Salesforce has that they do not to add the character data type? Any ideas?
120685
It seems to me that a lot of my debugging time is spent chasing down null-reference exceptions in complex statements. For instance:  For Each game As IHomeGame in _GamesToOpen.GetIterator()   Why, when I get a NullReferenceException, can I get the line number in the stack trace, but not the name of the object that equals null. In other words, why:  Object reference not set to an instance of an object.   instead of  _GamesToOpen is not set to an instance of an object.   or  Anonymous object returned by _GamesToOpen.GetIterator() is null.   or  game was set to null.   Is this strictly a design choice, meant to protect the anonymity of the code or is there a compelling reason in compiler design not to include this information in the debug-time exception?
90464
We noticed that lots of bugs in our software developed in C# (or Java) cause a NullReferenceException.  Is there a reason why "null" has even been included in the language?  After all, if there were no "null", I would have no bug, right?  In other words, what feature in the language couldn't work without null?
95092
Why does list.index throw an exception, instead of using an arbitrary value (for example, -1)? What's the idea behind this?  To me it looks cleaner to deal with special values, rather than exceptions.  EDIT: I didn't realize -1 is a potentially valid value. Nevertheless, why not something else? How about a value of None?
16290
This is a canonical question and answer developed by the community to help address common questions. If you've been directed here, or your question has been closed as a duplicate, please look through the resources here and use them to shape more specific questions. To browse all canonical questions and answers, navigate to the  tag. This question and answer is owned by the community! If you can improve it and make it more useful, you're welcome to edit it. I am building a trigger, a unit test, or other Apex code. I'm working with sObject records, and trying to access related record information. For example, I might have a structure like this: Account a = new Account(Name = 'Test'); insert a;  Opportunity o = new Opportunity(Name = 'Donation', AccountId = a.Id); insert o;  System.assertEquals(o.Account.Name, 'Test', 'Correct Account associated'); // Assertion fails!  Or, in a trigger, I might be aiming for something like this: trigger ContactTrigger on Contact (after insert) {     for (Contact c : Trigger.new) {         if (c.Account.Name == 'Test') {             // Take some action here         }      } }  Why doesn't this work? Why am I getting null values, or NullPointerExceptions?
87341
An array is defined to be maxmin equal if it contains at least two different elements and the number of times the maximum value occur is the same as the number of times the minimum value occur. So {11, 4, 9, 11, 8, 5 , 4, 10} is maxmin equal, because the max value 11 and min value 4 both appear two times in the array.   if the input array is {} , isMaxMinEqual should return false (array must have at least two different elements) status = false; if the input array is {2}, isMaxMinEqual should return false (array must have at least two different elements). if the input array is {1,1,1,1,1,1}, isMaxMinEqual should return false (array must have at least two different elements).  if the input array is {2,4,6,8,11}, isMaxMinEqual should return true (Both max value(11) and min value 2 appear exactly one time). if the input array is {-2,-4,-6,-8,-11}, isMaxMinEqual should return true(Both max value(-2) and min value -11 appear exaclty one time).   I wrote the class MaxMin which satisfied the above condition.  public class MaxMin {  public static void main(String args[]) {     System.out.println("The result is: " + isMaxMinEqual(new int[]{2, 2})); }  public static boolean isMaxMinEqual(int[] a) {     boolean status = false;     int count = 0;     if (a.length &lt; 2) {         status = false;         return status;     }     for (int i = 1; i &lt; a.length; i++) {         if (a[i - 1] != a[i]) {             count++;         }     }      int max = a[0];     int min = a[0];     for (int i = 0; i &lt; a.length; i++) {         if (a[i] &gt; max) {             max = a[i];         } else if (a[i] &lt; min) {             min = a[i];         }     }      int maxCount = 0;     for (int j = 0; j &lt; a.length; j++) {         if (a[j] == max) {             maxCount++;         }     }      int minCount = 0;     for (int j = 0; j &lt; a.length; j++) {         if (a[j] == min) {             minCount++;         }     }     System.out.println(count);     System.out.println("max is: " + max);     System.out.println("min is: " + min);     System.out.println("Max Count is: " + maxCount);     System.out.println("Min Count is: " + minCount);      if (maxCount == minCount &amp;&amp; count &gt;= 1) {         status = true;     }      return status; } }
9792
Yesterday I had a two-hour technical phone interview (which I passed, woohoo!), but I completely muffed up the following question regarding dynamic binding in Java.  And it's doubly puzzling because I used to teach this concept to undergraduates when I was a TA a few years ago, so the prospect that I gave them misinformation is a little disturbing...  Here's the problem I was given:  /* What is the output of the following program? */  public class Test {    public boolean equals( Test other ) {     System.out.println( "Inside of Test.equals" );     return false;   }    public static void main( String [] args ) {     Object t1 = new Test();     Object t2 = new Test();     Test t3 = new Test();     Object o1 = new Object();      int count = 0;     System.out.println( count++ );// prints 0     t1.equals( t2 ) ;     System.out.println( count++ );// prints 1     t1.equals( t3 );     System.out.println( count++ );// prints 2     t3.equals( o1 );     System.out.println( count++ );// prints 3     t3.equals(t3);     System.out.println( count++ );// prints 4     t3.equals(t2);   } }   I asserted that the output should have been two separate print statements from within the overridden equals() method: at t1.equals(t3) and t3.equals(t3).  The latter case is obvious enough, and with the former case, even though t1 has a reference of type Object, it is instantiated as type Test, so dynamic binding should call the overridden form of the method.  Apparently not.  My interviewer encouraged me to run the program myself, and lo and behold, there was only a single output from the overridden method: at the line t3.equals(t3).  My question then is, why?  As I mentioned already, even though t1 is a reference of type Object (so static binding would invoke Object's equals() method), dynamic binding should take care of invoking the most specific version of the method based on the instantiated type of the reference.  What am I missing?
74579
I am writing this java program to find all the prime numbers up to num using the Sieve of Eratosthenes, but when I try to compile, it says I can't use a long var as an array index, and it expects an int var in its place. But I'll be working with large numbers, so I can't use int. What can I do?  import java.util.*; import java.lang.*;  public class t3{     public static void main(String[] args){         long num = 100;          //declaring list and filling it with numbers         ArrayList&lt;Long&gt; numlist = new ArrayList&lt;Long&gt;();         for(long x=2 ; x&lt;num ; x++){             numlist.add(new Long(x));         }          //sieve or eratosthenes         for(long x=0 ; x&lt;Math.sqrt(num) ; x++){             for(long y=x+1 ; y&lt;numlist.size() ; y++){                 if(numlist[y]%numlist[x] == 0){                     numlist.remove(y);                 }             }         }          //print list         for(Object item : numlist){             System.out.println((Long)item);         }     } }
119160
This Trigger is on lead (Before Insert, Before Update) and it errors (System.NullPointerException: Attempt to de-reference a null object). I'm getting this error when i'm posting "Web to Lead". However i do have the right ORG iD and Record type id included on the xml file:   &lt;input type=hidden name="oid" value="xxxxxxxxxxxxx"&gt; &lt;input type=hidden name="recordType" id="recordType" value="zzzzzzzzzzzzzzzzzz"&gt;    trigger IAAILeadTrigger on Lead (before insert, before update){ //BEGIN: S-444340 T-561808 - Process only IAA Remarketing Recordtype data     Map&lt;ID,Schema.RecordTypeInfo&gt; rt_Map = Lead.sObjectType.getDescribe().getRecordTypeInfosById();      //Looks up the branches based on the Lead PostalCode     if(trigger.isBefore){          for(Lead ld : trigger.new){                      if (rt_map.get(ld.recordTypeID).getName().containsIgnoreCase('IAA Remarketing') )                if( trigger.isInsert ||                    (trigger.isUpdate &amp;&amp; ld.Branch__c == null) ||                   (trigger.isUpdate &amp;&amp; ld.PostalCode != trigger.oldMap.get(ld.id).PostalCode)               ){                   leads.add(ld);               }           }           }                 //Only check the leads that are in the lead list and NOT the full trigger.new list         if(!leads.isEmpty()){             LeadController.GetLeadBranches(leads);           }     }  }
77222
I have a really simple Java class that effectively decorates a Map with input validation, with the obvious void set() and String get() methods.  I'd like to be able to effectively call those methods and handle return values and exceptions from outside the JVM, but still on the same machine Update: the caller I have in mind is not another JVM; thanks @Dave Ray  My implementation considerations are typical    performance ease of implementation and maintenance (simplicity?) reliability flexibility (i.e. can I call from a remote machine, etc.)   Is there a 'right way?' If not, what are my options, and what are the pro/cons for each?  (Stuff people have actually done and can provide real-life feedback on would be great!)