//Database is Class provided by Salesforce.
//Batchable is an global interface which is inside of the Database class.
//Batchable include 3 method prototypes: 1. start 2. execute 3. finish
//start and finish methods will execute only one time.
//execute method executes multiple times.
global class BatchUsage implements Database.Batchable <sobject>, Database.Stateful {
 //at a time we can inherit multiple interfaces but we cannot inherit multiple classes.
 //By default batch class is stateless (variable info. store in one method cannot be remembered in other method), 
 //to make it stateful we should inherit Database.Stateful interface.
 String name = '';
 global Database.queryLocator start(Database.BatchableContext bc) {
   
  //From asynchronous to asynchronous we cannot call 
  //(from batch class we cannot call future mehthod vice versa.).
  name += 'start';
  system.debug('@@@Start Method: '+name);
  //Collects the records to process. It will forward these records to execute method.
  //If we use Iterable<sobject> as return type for this start method it can hold 
  //max. of 50k records only.
  //If we use Database.queryLocator as return type for this start method it can hold 
  //max. of 50 million records.
  return Database.getQueryLocator('select id, name, Location__c from Department__c where Location__c = \'\'');
 }
 global void execute(Database.BatchableContext bc, LIST<sobject> sObjLst) {
  name += 'execute';
  system.debug('@@@Execute Method: '+name);
  //Split the records forwarded by start method into batches.
  //Default batch size is: 200.
  //Max. batch size is: 2000.
  List<department__c> depUPdLst = new List<department__c>();
  for(SObject sObj: sObjLst) {
   Department__c dept = (Department__c) sObj;//Type Casting.
   dept.Location__c = 'Bangalore';
   depUPdLst.add(dept);
  }
  if(depUPdLst.size() > 0)
   update depUPdLst;
 }
 global void finish(Database.BatchableContext bc) {
  name += 'finish';
  system.debug('@@@Finish Method: '+name);
  //Post Commit logic like sending emails for the success or failures of the batches.
  AsyncApexJob a = [select id, Status, NumberOfErrors, JobItemsProcessed,
  TotalJobItems, CreatedBy.Email from AsyncApexJob where id =: bc.getJobId()];
   
  Messaging.singleEmailMessage mail = new Messaging.singleEmailMessage();
  mail.setToAddresses(new String[]{a.CreatedBy.Email});
  mail.setSubject('Batch Class Result');
  mail.setHtmlBody('The batch Apex job processed ' + '<b>' + a.TotalJobItems + '</b>' +
  ' batches with '+ '<b>' + a.NumberOfErrors + '</b>' + ' failures.');
  //sendEmail methods
  Messaging.sendEmail(new Messaging.singleEmailMessage[]{mail});
   
  /*** Scheduling in minutes or hours ***/
  /*//Create object for schedulable class
  SchedulableUsage su = new SchedulableUsage();
  //Preparing chron_exp
  Datetime sysTime = System.now();
  sysTime = sysTime.addminutes(6);
  String chron_exp = '' + sysTime.second() + ' ' + sysTime.minute() + ' ' +
  sysTime.hour() + ' ' + sysTime.day() + ' ' + sysTime.month() + ' ? ' + sysTime.year();            
  System.schedule('Dep Update'+sysTime.getTime(),chron_exp, su);*/
 }
}
 </department__c></department__c></sobject></sobject></sobject>