Reserved Keywords in Salesforce

Total Number of  Reserved Keywords-130

abstract	global	private
activate*	goto*	protected
and	group*	public
any*	having*	retrieve*
array	hint*	return
as	if	returning*
asc	implements	rollback
autonomous*	import*	savepoint
begin*	inner*	search*
bigdecimal*	insert	select
blob	instanceof	set
break	interface	short*
bulk	into*	sort
by	int	stat*
byte*	join*	super
case*	last_90_days	switch*
cast*	last_month	synchronized*
catch	last_n_days	system
char*	last_week	testmethod
class	like	then*
collect*	limit	this
commit	list	this_month*
const*	long	this_week
continue	loop*	throw
convertcurrency	map	today
decimal	merge	tolabel
default*	new	tomorrow
delete	next_90_days	transaction*
desc	next_month	trigger
do	next_n_days	true
else	next_week	try
end*	not	type*
enum	null	undelete
exception	nulls	update
exit*	number*	upsert
export*	object*	using
extends	of*	virtual
false	on	webservice
final	or	when*
finally	outer*	where
float*	override	while
for	package	yesterday
from	parallel*
future	pragma*

Note:

               Keywords marked with an asterisk (*) are reserved for future use.

The following are special types of keywords that aren't reserved words and can be used as identifiers.

• after
• before
• count
• excludes
• first
• includes
• last
• order
• sharing
• with

Salesforce : APEX : Class ( Simple/Regular Vs Abstract Vs Virtual )

Note : simple/regular means, class without abstract and virtual keyword

Before going into differences between these, understand regular, abstract, virtual methods :
Regular Method  : Has code body or code implementation  and cannot be overridden

Virtual Method    : Has code body or code implementation and can be overridden by extending class method using override keyword

Abstract Method : Doesn't have body or code implementation and can be overridden by extending class method using override keyword



Regular Class:
a) Can be constructed ( can create object ).
b) Can not be extended
c) Can have regular methods
d) Can have virtual methods
e) Can not have abstract methods

Virtual Class:
a) Can be constructed ( can create object ).
b) Can be extended
c) Can have regular methods
d) Can have virtual methods
e) Can not have abstract methods

Abstract Class:
a) Can not be constructed ( can not create object ).
b) Can be extended
c) Can have regular methods
d) Can have virtual methods
e) Can have abstract methods

Note:
i) Extending class can not reduce visibility of super method by overriding it.
ii)Extending class can not change return type of super method by overriding it.
iii)Extending can do method over loading e.g.

Super class has 
public void getName(){
return 'My Name'; 
}

and Extending class can have
public getName(String str){
return str; 
}

iv) class can't extend two classes but can extend one class (virtual/abstract) and implements multiple interfaces.  

Attempt to de-reference a null object salesforce

Thought of posting about a very common error we come across in apex. You must have seen this a number of times.
Visualforce Error: System.NullPointerException: Attempt to de-reference a null object

This error occurs whenever you try to access value/values from a variable/collection whose new new instance has not been created or is null in other words.

for example: if you have a list of accounts (List<account> accList) defined and you have not created a new instance of this list. And then, accidentally you try to access record from this list; then you will get the above error.

As can be seen from this example:

Visualforce Page

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<apex:page controller="DeferenceDemoController">  <apex:form >    <apex:commandButton value="Call Method" action="{!Demo_method}"/>  </apex:form> </apex:page>

Controller

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Public with sharing class DeferenceDemoController { List<account> accList;    Public DeferenceDemoController(){          }   Public void Demo_method(){    for(Account ac:accList )        system.debug('****'+ac.name);   }  }

A list "accList" of accounts has been defined but a new instance of this list has not been created. So when we press the button "Call Method" we get the error "Attempt to deference a null object" as in the called method we are trying to access the "accList" list.

This error can be avoided by creating a new instance of the accList before trying to access it as below:

Modified controller:

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Public with sharing class DeferenceDemoController { List<account> accList;    Public DeferenceDemoController(){       accList =New List<account>();     }   Public void Demo_method(){    for(Account ac:accList )        system.debug('****'+ac.name);   }  }