How to properly test invocable callout class

Question

I am a total beginner in Apex, however I created the following class that calls an external API endpoint and is invoked by a flow:

public class SubscribeToStatuspage {
    @InvocableMethod(label='Call Status Page' description='Subscribe the contact to status page' category='Contact' callout=true)
    public static void callStatusPageAPI(List<ID> contactIDs) {

        // Get the contact details
        List<Contact> contacts = [SELECT Name, Email FROM Contact WHERE Id in :contactIDs]; 
        
        String contactName;
        String contactEmail;
        for (Contact contact : contacts) { 
            contactName = contact.Name;
            contactEmail = contact.Email;
        
        
            Http http = new Http();
            HttpRequest tokenRequest = new HttpRequest();
            // First request an auth token with the refresh token
            tokenRequest.setEndpoint('https://accounts.zoho.com/oauth/v2/token?');
            tokenRequest.setMethod('POST');                 
            tokenRequest.setBody('client_id=XXX&refresh_token=XXX&grant_type=refresh_token'); 
            HttpResponse tokenResponse = http.send(tokenRequest);
            //Parse the response to get the auth token
            JSONParser parser = JSON.createParser(tokenResponse.getBody());
            String accessToken;
            while (parser.nextToken() != null) {
                if ((parser.getCurrentToken() == JSONToken.FIELD_NAME) &&
                    (parser.getText() == 'access_token')){
                        parser.nextToken();
                        accessToken = parser.getText();
                    }
            }
    
            // Use the access token to subscribe the user
            HttpRequest subscribeRequest = new HttpRequest();
            subscribeRequest.setEndpoint('https://www.site24x7.com/sp/api/subscribers/email');
            subscribeRequest.setMethod('POST');
            subscribeRequest.setHeader('Content-Type', 'application/json;charset=UTF-8');
            subscribeRequest.setHeader('Accept', 'application/json; version=2.0');
            subscribeRequest.setHeader('Authorization', 'Zoho-oauthtoken ' + accessToken);
            
            JSONGenerator jsonBody = JSON.createGenerator(true);
            jsonBody.writeStartObject();      
            jsonBody.writeStringField('subscriber_name', contactName);
            jsonBody.writeStringField('email_id', contactEmail);
            jsonBody.writeStringField('subscribe_statuspage_info','{"enc_statuspage_id":"XXX"}');
            jsonBody.writeEndObject();    
            
            String jsonS = jsonBody.getAsString();
            subscribeRequest.setBody(jsonS);
            
            //subscribeRequest.setBody('{"subscriber_name": + contactName, "email_id": + contactEmail, "subscribe_statuspage_info": {"enc_statuspage_id":"XXX"}}');
            HttpResponse subscribeResponse = http.send(subscribeRequest);
            
            System.debug(subscribeResponse.getBody());
        }
    }
}

This works fine, but I'm stuck writing the proper test class for it. I got this far:

@isTest
private class SubscribeToStatuspageTest{
  @testSetup
  static void setupTestData(){
    test.startTest();
    Contact contact_Obj = new Contact(LastName = 'LastName835', Email = 'Email41@test.com', HasOptedOutOfEmail = false, HasOptedOutOfFax = false, DoNotCall = false, Christmas_Card__c = false, Newsletter_Opt_In2__c = false, CSM_contact__c = false, Active__c = false, Main_contact__c = false, Email_opt_in_via_website_newsletter__c = false, Opt_in_after_bounce__c = false, Escalation_contact__c = false, Release_contact__c = false, Buyer_contact__c = false, Billing_contact__c = false, SSP_User__c = false, Security_contact__c = false, dupcheck__dc3DisableDuplicateCheck__c = false, Customer_panel_member__c = false, Personal_Data_Breach_Contact__c = false, TK_Double_Opt_In__c = false);
    Insert contact_Obj; 
    test.stopTest();
  }
  static testMethod void test_callStatusPageAPI_UseCase1(){
    List<Contact> contact_Obj  =  [SELECT Id,Name,Email,HasOptedOutOfEmail,HasOptedOutOfFax,DoNotCall,Christmas_Card__c,Newsletter_Opt_In2__c,CSM_contact__c,Active__c,Main_contact__c,Email_opt_in_via_website_newsletter__c,Opt_in_after_bounce__c,Escalation_contact__c,Release_contact__c,Buyer_contact__c,Billing_contact__c,SSP_User__c,Security_contact__c,dupcheck__dc3DisableDuplicateCheck__c,Customer_panel_member__c,Personal_Data_Breach_Contact__c,TK_Double_Opt_In__c from Contact];
    System.assert(contact_Obj.size()>0);
    SubscribeToStatuspage obj01 = new SubscribeToStatuspage();
    SubscribeToStatuspage.callStatusPageAPI(new List<ID>());
  }
}

This succeeds, however the code coverage is only 14% because the test never enters the for loop. What am I missing here?

Any advise would be very much appreciated. Thank you!

Answer 1

You forgot to send the contact id to function call, change:

SubscribeToStatuspage.callStatusPageAPI(new List<ID>());

to

Id contactId = contact_Obj[0].Id;
SubscribeToStatuspage.callStatusPageAPI(new List<Id>{ contactId });