Apex class that illustrates how a completely self-contained Batchable actually works

BatchableExperiment.cls

/*
 * Database.Batchable that can be executed, assuming you have a custom object
 * named Experiment__c that already exists in your org, with a custom
 * Text field named Title__c
 */
global class BatchableExperiment
implements Database.Batchable<Task>, Iterable<Task>, Iterator<Task> {
    
    /*
     * The ID of the Experiment record created for this batch job
     */
    private Id experimentId;

    /*
     * A fun, memorable title that will be repeated to create
     * subsequent titles. Try to keep the title at 4 or fewer characters
     * in length.
     */
    private String experimentTitle;

    /*
     * A numeric counter used to keep track of when key Batchable,
     * Iterable and Iterator methods are called
     */
    private Integer stepNumber;

    /*
     * Default constructor which sticks in an experiment title, "BaFa"
     */
    global BatchableExperiment() {
        this('BaFa');
    }
    
    /*
     * @param experimentTitle
     *            The value to store in the new Experiment record's Title field
     */
    global BatchableExperiment(String experimentTitle) {
        this.experimentTitle = experimentTitle;
        stepNumber = 0;
        experimentId = createNewExperiment(experimentTitle);
        recordNewStepNumber('BatchableExperiment()');
    }

    /*
     * @param  title The Title value for the new Experiment record
     * @return the Experiment ID of the newly created record
     */
    private static Id createNewExperiment(String title) {
        Experiment__c newExperiment = new Experiment__c(
            Title__c = title);
        insert newExperiment;
        return newExperiment.Id;
    }
    
    /*
     * Create a new Note record associated with the experiment,
     * to record the execution of a new key step in the batch job
     *
     * @param description
     *            A brief description of the step number
     */
    private void recordNewStepNumber(String description) {
        stepNumber++;  // Move to the next step number
        
        String noteBody = 'BatchableExperiment: ' + this + '\n\n';
        noteBody +=  'Step ' + stepNumber + ': ' + description;

        Note experimentNote = new Note(
            Body = noteBody,
            ParentId = experimentId,
            Title = 'BatchableExperiment step ' + stepNumber);

        insert experimentNote;
    }
    
    /*
     * Implementation of Database.Batchable.execute()
     */
    global void execute(Database.BatchableContext context, List<Task> records) {
        recordNewStepNumber('execute(' + records + ')');
    }
    
    /*
     * Implementation of Database.Batchable.finish()
     */
    global void finish(Database.BatchableContext context) {
        recordNewStepNumber('finish()');

        if (experimentTitle.length() < 10) {
            Database.executeBatch(
                new BatchableExperiment(experimentTitle + experimentTitle));
        }
    }
    
    /*
     * Implementation of Iterator.hasNext()
     */
    global Boolean hasNext() {
        recordNewStepNumber('hasNext()');
        return Math.random() >= 0.25;
    }
    
    /*
     * Implementation of Iterable.iterator()
     */
    global Iterator<Task> iterator() {
        recordNewStepNumber('iterator()');
        return this;
    }
    
    /*
     * Implementation of Iterator.next()
     */
    global Task next() {
        recordNewStepNumber('next()');
        return new Task(Subject = 'Step ' + stepNumber);
    }
    
    /*
     * Implementation of Database.Batchable.start()
     */
    global Iterable<Task> start(Database.BatchableContext context) {
        recordNewStepNumber('start()');
        return this;
    }
}