How to write Test Class for a Batch Apex Class?

In continuation to my previous post on Batch Apex, here we will try to understand how to write test class for a batch apex class.

So let's have a sneak peek into it. 

Apex Class

global class AccountUpdate implements Database.Batchable {

     String query, field, value;

     global AccountUpdate(String f, String v){

          field = f;

          value = v;

          query = 'SELECT' + field + 'FROM Account';

     }

     global Database.QueryLocator start(Database.BatchableContext BC){

          return Database.getQueryLocator(query);

     }

     global void execute(Database.BatchableContext BC, List scope){

          for(sObject s: scope){

               s.put(field, value);

          }

     update scope;

     }

     global void finish(Database.BatchableContext BC){

     //

     }

}

Test class for above Batch Class

@isTest

public class AccountUpdateTest {

    static testMethod void test() {

        // Create an instance of query locator

        Database.QueryLocator QL;

        // Create an instance of batchable context

        Database.BatchableContext BC;

        List AcctList = new List();

        AccountUpdate oAccountUpdate = new AccountUpdate('Name', 'Test');

        // execute start method

        QL = oAccountUpdate.start(bc);       

        // Create an instance of Query Locator Iterator

        Database.QueryLocatorIterator QLI =  QL.iterator();

        while (QLI.hasNext())

        {

            Account Acc = (Account)QLI.next();        

            System.debug(Acc);

            AcctList.add(Acc);

        }               

        // initiate execute method

        oAccountUpdate.execute(BC, AcctList);

        // execute finish method

        oAccountUpdate.finish(BC);    

    }

}