MockCalloutTestClass.cls

@isTest
private class AnimalsCalloutTest {

    @isTest static void testFetCallout() {
        // Create the mock response based on a static resource
        StaticResourceCalloutMock mock = new StaticResourceCalloutMock();
        mock.setStaticResource('GetAnimalResource');
        mock.setStatusCode(200);
        mock.setHeader('Content-Type', 'application/json;charset=UTF-8');
        // Associate the callout with a mock response
        Test.setMock(HttpCalloutMock.class, mock);
        // Call method to test
        HttpResponse result = AnimalsCallouts.makeGetCallout();
        // Verify mock response is not null
        System.assertNotEquals(null, result, 'The callout returned a null response.');
        // Verify status code
        System.assertEquals(200, result.getStatusCode(), 'The status code is not 200.');
        // Verify content type
        System.assertEquals('application/json;charset=UTF-8',result.getHeader('Content-Type'),'The content type value is not expected.');
        // Verify the array contains 3 items
        Map<String, Object> results = (Map<String, Object>) JSON.deserializeUntyped(result.getBody());
        List<Object> animals = (List<Object>) results.get('animals');
        System.assertEquals(3,animals.size(),'The array should only contain 3 items.');
    }
}