Iterator:
________
An iterator traverses through every item in a collection. For example, in a while loop in Apex, you define a condition for exiting the loop, and you must provide some means of traversing the collection, that is, an iterator. 

The start() method returns either a Database.QueryLocator object or an iterable that contains the records or objects passed to the job.
When you’re using a simple query (SELECT) to generate the scope of objects in the batch job, use theDatabase.QueryLocator object. If you use a QueryLocator object, the governor limit for the total number of records retrieved by SOQL queries is bypassed. For example, a batch Apex job for the Account object can return aQueryLocator for all account records (up to 50 million records) in an org. Another example is a sharing recalculation for the Contact object that returns a QueryLocator for all account records in an org
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Use the Iterable to create a complex scope for the batch job. You can also use the iterable to create your own custom process for iterating through the list

The iterator method must be declared as global or public. It creates a reference to the iterator that you can then use to traverse the data structure.

To use custom iterators, you must create an Apex class that implements the Iterator interface.

If you do not want to use a custom iterator with a list, but instead want to create your own data structure, you can use theIterable interface to generate the data structure.
The Iterable interface has the following method:

Name                         Returns             Description
iterator                       Iterator class    Returns a reference to the iterator for this interface.

The iterator method must be declared as global or public. It creates a reference to the iterator that you can then use to traverse the data structure.
In the following example a custom iterator iterates through a collection:
 

global class CustomIterable 
   implements Iterator<Account>{ 

   List<Account> accs {get; set;} 
   Integer i {get; set;} 

   public CustomIterable(){ 
       accs = 
       [SELECT Id, Name, 
       NumberOfEmployees 
       FROM Account 
       WHERE Name = 'false']; 
       i = 0; 
   }   

   global boolean hasNext(){ 
       if(i >= accs.size()) {
           return false; 
       } else {
           return true; 
       }
   }    

   global Account next(){ 
       // 8 is an arbitrary 
       // constant in this example
       // that represents the 
       // maximum size of the list.
       if(i == 8){return null;} 
       i++; 
       return accs[i-1]; 
   } 
}

The following calls the above code:

global class foo implements iterable<Account>{
   global Iterator<Account> Iterator(){
      return new CustomIterable();
   }
}

The following is a batch job that uses an iterator:

global class batchClass implements Database.batchable<Account>{ 
   global Iterable<Account> start(Database.batchableContext info){ 
       return new foo(); 
   }     
   global void execute(Database.batchableContext info, List<Account> scope){ 
       List<Account> accsToUpdate = new List<Account>(); 
       for(Account a : scope){ 
           a.Name = 'true'; 
           a.NumberOfEmployees = 69; 
           accsToUpdate.add(a); 
       } 
       update accsToUpdate; 
   }     
   global void finish(Database.batchableContext info){     
   } 
}

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