How can I write a schedule for this batch inside the same class

Question

    global class NumberOfContactsBatch implements Database.Batchable<SObject>, Database.Stateful {

    global Database.QueryLocator start(Database.BatchableContext context) {
        return Database.getQueryLocator([select Id from Account order by Name]);
    }

    global void execute(Database.BatchableContext context, List<Account> scope) {
        List<Account> accountUpdate = new List<Account>();
        for (AggregateResult ar : [
                select AccountId a, count(Id) c
                from Contact
                where AccountId in :scope
                group by AccountId
                ]) {
            accountUpdate.add(new Account(
                    Id = (Id) ar.get('a'),
                    Number_of_Contacts__c = (Decimal) ar.get('c')
                    ));
        }
        update accountUpdate;
    }

    global void finish(Database.BatchableContext context) {
    }
    global void execute(SchedulableContext SC){
        database.executeBatch(new NumberOfContactsBatch());
        String cronExpression = '';
        System.schedule(cronExpression);        
    } 
}

I got stuck here where I want write a scheduler which runs every six hours

Answer 1

You are trying to Schedule a batch class without a scheduler to Implement SchedulableContext, for this Schedulable Interface should be implemented

For Example:

Class NumberOfContactsSchedulabler implements Schedulable {
     global void execute(SchedulableContext SC) { 
        database.executeBatch(new NumberOfContactsBatch());
    }
}

Run the following Code in anonymous window (One time)

NumberOfContactsSchedulabler m = new NumberOfContactsSchedulabler();
String sch = '0 0 0/6 1/1 * ? *'; //Schedules the job to run every 6 hours
String jobID = system.schedule('Contacts Schedulable', sch, m);

Cron maker help can be found here

Also the Method "global void execute(SchedulableContext SC)" can be removed