Solving Schedule Jobs Using the Apex Scheduler

The Trailhead challenge says:

Create an Apex class that uses Scheduled Apex to update Lead records.

• Create an Apex class that implements the Schedulable interface to update Lead records with a specific LeadSource. Write unit tests that achieve 100% code coverage for the class. This is very similar to what you did for Batch Apex.
• Create an Apex class called 'DailyLeadProcessor' that uses the Schedulable interface.
• The execute method must find the first 200 Leads with a blank LeadSource field and update them with the LeadSource value of 'Dreamforce'.
• Create an Apex test class called 'DailyLeadProcessorTest'.
• In the test class, insert 200 Lead records, schedule the DailyLeadProcessor class to run and test that all Lead records were updated correctly.
• The unit tests must cover all lines of code included in the DailyLeadProcessor class, resulting in 100% code coverage.
• Run your test class at least once (via 'Run All' tests the Developer Console) before attempting to verify this challenge.

So first, you must to create the DailLeadProcessor class:

global class DailyLeadProcessor implements Schedulable {
    
    global void execute(SchedulableContext ctx) {
        
        //Retrieving the 200 first leads where lead source is in blank.
        List<Lead> leads = [SELECT ID, LeadSource FROM Lead where LeadSource = '' LIMIT 200];

        //Setting the LeadSource field the 'Dreamforce' value.
        for (Lead lead : leads) {
            lead.LeadSource = 'Dreamforce';
        }

        //Updating all elements in the list.
        update leads;
    }

}

And then you can create the test class.

@isTest
private class DailyLeadProcessorTest {
    
    @isTest
    public static void testDailyLeadProcessor(){

        //Creating new 200 Leads and inserting them.
        List<Lead> leads = new List<Lead>();
        for (Integer x = 0; x < 200; x++) {
            leads.add(new Lead(lastname='lead number ' + x, company='company number ' + x));
        }
        insert leads;

        //Starting test. Putting in the schedule and running the DailyLeadProcessor execute method.
        Test.startTest();
        String jobId = System.schedule('DailyLeadProcessor', '0 0 12 * * ?', new DailyLeadProcessor());
        Test.stopTest();

        //Once the job has finished, retrieve all modified leads.
        List<Lead> listResult = [SELECT ID, LeadSource FROM Lead where LeadSource = 'Dreamforce' LIMIT 200];

        //Checking if the modified leads are the same size number that we created in the start of this method.
        System.assertEquals(200, listResult.size());

    }
}