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);   }  }