APEX: Web Service Callouts using WebServiceMock

1. Implement the WebServiceMock 

@isTest
global class MyWebServiceCalloutMock implements WebServiceMock
{
    // Need to override this method
    global void doInvoke(
                   Object stub,

           Object request,
           Map<String, Object> response,
           String endpoint,
           String soapAction,
           String requestName,
           String responseNS,
           String responseName,
           String responseType)

    {
        try
        {
             MyService.myElement test = MyService.MyElement();
             MyElement.Result = 'Test';

       response.put('response_x', test); 

        }
        catch(Exception ex)
        {
           system.debug(ex.getMessage() + ' ' + ex.getStackTraceString());
        }
        return res;
     }
}

2. Create the test class

@isTest
public class WebServiceTest 
{
     public static testMethod void testRestGetService() 
     {
        try
        {
           Test.startTest();
             Test.setMock(WebServiceMock.class, new MyWebserviceCalloutMock());
        
             String response = MyCalloutClass.processService('Test');
        
             // provide your assertions here

           Test.stopTest();
        }
        catch(Exception ex)
        {
           system.debug(ex.getMessage() + ' ' + ex.getStackTraceString());
        }
    }
}
This is the sample Callout Class
public class MyCalloutClass
{
    public static String processService(String input) 
    {

        try
        {

          MyService.MyElement elm = new MyService.MyElement();

          elm.endpoint_c = 'your endpoint url';
          return elm.myServiceMethod(input);
        }
        catch(Exception ex)
        {
           system.debug(ex.getMessage() + ' ' + ex.getStackTraceString()); 
        }
        return '';
    }
}

This is a sample generated class

public class MyService
{
    public class MyElement
    {
       public static String Result;
       public String myServiceMethod(String input)
       {
           // Autogenerated code here
       }
    }
}

You can find more information to the original article here: 

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_callouts_wsdl2apex_testing.htm

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_callouts_wsdl2apex_gen_code.htm