public class AnimalLocator
{
    
    public static String getAnimalNameById(Integer id)
    {
        Http http = new Http();
        HttpRequest request = new HttpRequest();
        request.setEndpoint('https://th-apex-http-callout.herokuapp.com/animals/'+id);
        request.setMethod('GET');
        HttpResponse response = http.send(request);
        String strResp = '';
        system.debug('******response '+response.getStatusCode());
        system.debug('******response '+response.getBody());
        // If the request is successful, parse the JSON response.
        if (response.getStatusCode() == 200) 
        {
            // Deserializes the JSON string into collections of primitive data types.
            Map<String, Object> results = (Map<String, Object>) JSON.deserializeUntyped(response.getBody());
            // Cast the values in the 'animals' key as a list
            Map<string,object> animals = (map<string,object>) results.get('animal');
            System.debug('Received the following animals:' + animals );
            strResp = string.valueof(animals.get('name'));
            System.debug('strResp >>>>>>' + strResp );
        }
        return strResp ;
    }
    
}

@isTest
public class AnimalLocatorTest {
    @isTest public static void AnimalLocatorMock() {
        Test.setMock(HttpCalloutMock.class, new AnimalLocatorMock());
        string result = AnimalLocator.getAnimalNameById(1);
        system.debug(result);
        String expectedResult = 'chicken';
        System.assertEquals(result,expectedResult );
    }
}

@isTest
global class AnimalLocatorMock implements HttpCalloutMock {
    // Implement this interface method
    global HTTPResponse respond(HTTPRequest request) {
        // Create a fake response
        HttpResponse response = new HttpResponse();
        response.setHeader('Content-Type', 'application/json');
        response.setBody('{"animal":{"id":1,"name":"chicken","eats":"chicken food","says":"cluck cluck"}}');
        response.setStatusCode(200);
        return response; 
    }
}