Hi Naomi, 

Since test classes do not support web service callouts, you have to take a more roundabout way of simulating the callout. 

The first thing you will want to do is to implement the HttpCalloutMock interface which will serve as the 'response' you will expect from the web service. In this class you will set some mock response values and then return them. 

Next you will want to call the mock class you just made from within the actual test class. Do this by calling: 
 

Test.setMock(HttpCalloutMock.class, new YourHttpCalloutMockImpl());

This part takes care of replacing the callout to the actual web service with a callout to the mock class you created in step 1.


Here is an example given by Salesforce documentation which outlines the various pieces in code form: 

YourSampleMockResponse:
 

@isTest
global class MockHttpResponseGenerator implements HttpCalloutMock {
    // Implement this interface method
    global HTTPResponse respond(HTTPRequest req) {
        // Optionally, only send a mock response for a specific endpoint
        // and method.
        System.assertEquals('http://example.com/example/test', req.getEndpoint());
        System.assertEquals('GET', req.getMethod());
        
        // Create a fake response
        HttpResponse res = new HttpResponse();
        res.setHeader('Content-Type', 'application/json');
        res.setBody('{"example":"test"}');
        res.setStatusCode(200);
        return res;
    }
}

YourTestClass: 
 

@isTest
private class CalloutClassTest {
     @isTest static void testCallout() {
        // Set mock callout class 
        Test.setMock(HttpCalloutMock.class, new MockHttpResponseGenerator());
        
        // Call method to test.
        // This causes a fake response to be sent
        // from the class that implements HttpCalloutMock. 
        HttpResponse res = CalloutClass.getInfoFromExternalService();
        
        // Verify response received contains fake values
        String contentType = res.getHeader('Content-Type');
        System.assert(contentType == 'application/json');
        String actualValue = res.getBody();
        String expectedValue = '{"example":"test"}';
        System.assertEquals(actualValue, expectedValue);
        System.assertEquals(200, res.getStatusCode());
    }
}

Here is the documentation for the full explanation: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_restful_http_testing_httpcalloutmock.htm

Hope this helps!