To test Apex code that performs an HTTP callout, Salesforce requires the use of a mock callout. The standard mechanism for this is to create a test class that implements the HttpCalloutMock interface. This interface requires a respond method, which intercepts the callout and returns a specified HttpResponse without making an actual external request. This allows the developer to test how their code handles various responses (e.g., success, error codes) in a controlled, predictable test environment.

1. Salesforce Apex Developer Guide

"Testing HTTP Callouts": This official guide explicitly states

"To test callouts

you need to use a mock callout. You can do so by creating a class that implements the HttpCalloutMock interface." It provides a clear example: global class YourHttpCalloutMock implements HttpCalloutMock { ... }.

2. Salesforce Apex Developer Guide

"HttpCalloutMock Interface": The documentation for the interface itself details its purpose and required respond method signature

confirming it is the correct tool for mocking HttpRequest and HttpResponse objects used in REST callouts.

3. Salesforce Apex Developer Guide

"Testing Web Service Callouts": This section clarifies the use of the WebServiceMock interface for testing callouts made from WSDL-generated code (SOAP)

differentiating it from HttpCalloutMock. It states

"To test your web service callouts

you can use a mock object in your test class by implementing the WebServiceMock interface." This confirms WebServiceMock is for a different use case.