Its not like always to use the static method .. but dependes on the contect .. 
 

public class P { 
   public static boolean firstRun = true; 
}

 

trigger T1 on Account (before delete, after delete, after undelete) { 
       if(Trigger.isBefore){
          if(Trigger.isDelete){
             if(p.firstRun){
                 Trigger.old[0].addError('Before Account Delete Error');
                  p.firstRun=false;
              } 
           }
        }
}

 

Hi Pradeep
,
If the class contains a static method, it can directly call by its Class Name and its related Member Name and 
If the Class contains Non - static member than it gets called by after instantiating the class, then while 
using instantiation name and Non-static member Name, you can make calls to that member.

For example -- 
 

public Class ABC 
{
public static void showName()
{
System.debug('Name present in static member');
}
public void showName2()
{
System.debug('Name present in Non - static member');
}
}

In Execute anonymous  --
ABC.showName();  // It will get Executed Static member
ABC.showName2(); // It will Not get executed like this because Non - static member cannot get executed directly by class Name.

ABC ob = new ABC();
ob.showName2(); // Now non - static member gets executed.

Hope it might help you.

 Thanks 
 Akshay

@Pradeep, We can use both static or nonstatic in the trigger.

if you use static than no need to instance object and called direct.

for static method : 
Trigger: 
trigger AccountTrigger  on Account (before insert){
    if ((Trigger.isBefore) && (Trigger.isInsert)){    
        AccountTriggerHandler.CreateAccounts(Trigger.New);     
    }
}

Apex class :
public class AccountTriggerHandler{
  public static void CreateAccounts(List<Account> acc){
      List<Account> accts = new List<Account>();
      for(Account a:acc){
    if(a.BillingState!=a.ShippingState){
     a.BillingState.addError('BillingState must be same as ShippingState');
      }
      }
  }
}

for Non static method : 
Trigger: 
trigger AccountTrigger  on Account (before insert){
    if ((Trigger.isBefore) && (Trigger.isInsert)){
  AccountTriggerHandler obj  = new AccountTriggerHandler(); // Here We need to first create Object than call apex claxx
        obj.CreateAccounts(Trigger.New);     
    }
}

Apex class :
public class AccountTriggerHandler{
  public void CreateAccounts(List<Account> acc){
      List<Account> accts = new List<Account>();
      for(Account a:acc){
    if(a.BillingState!=a.ShippingState){
     a.BillingState.addError('BillingState must be same as ShippingState');
      }
      }
  }
}
  
  if you found this answer helpful then please mark it as best answer so it can help others.   
  
  Thanks 
  Akshay