Magento 2 – Fix REST API Error ‘Class Does Not Exist’

I've created a test Magento 2.0.2 REST web service based on Alan's blog: http://alankent.me/2015/07/24/creating-a-new-rest-web-service-in-magento-2/

I'm using Postman to call the custom web service and get the following error:

"message": "Class  does not exist",
  "code": -1,
  "trace": "#0 P:\\wwwroot\\Magento202_com_loc\\Web\\vendor\\magento\\framework\\Webapi\\ServiceInputProcessor.php(128): ReflectionClass->__construct('')\n#1 P:\\wwwroot\\Magento202_com_loc\\Web\\vendor\\magento\\framework\\Webapi\\ServiceInputProcessor.php(262): Magento\\Framework\\Webapi\\ServiceInputProcessor->_createFromArray(NULL, '30')\n#2 P:\\wwwroot\\Magento202_com_loc\\Web\\vendor\\magento\\framework\\Webapi\\ServiceInputProcessor.php(99): Magento\\Framework\\Webapi\\ServiceInputProcessor->convertValue('30', NULL)\n#3 P:\\wwwroot\\Magento202_com_loc\\Web\\vendor\\magento\\module-webapi\\Controller\\Rest.php(262): Magento\\Framework\\Webapi\\ServiceInputProcessor->process('Test\\\\Calculator...', 'add', Array)\n#4 P:\\wwwroot\\Magento202_com_loc\\Web\\vendor\\magento\\module-webapi\\Controller\\Rest.php(160): Magento\\Webapi\\Controller\\Rest->processApiRequest()\n#5 P:\\wwwroot\\Magento202_com_loc\\Web\\var\\generation\\Magento\\Webapi\\Controller\\Rest\\Interceptor.php(24): Magento\\Webapi\\Controller\\Rest->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#6 P:\\wwwroot\\Magento202_com_loc\\Web\\vendor\\magento\\framework\\App\\Http.php(115): Magento\\Webapi\\Controller\\Rest\\Interceptor->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#7 P:\\wwwroot\\Magento202_com_loc\\Web\\vendor\\magento\\framework\\App\\Bootstrap.php(258): Magento\\Framework\\App\\Http->launch()\n#8 P:\\wwwroot\\Magento202_com_loc\\Web\\index.php(39): Magento\\Framework\\App\\Bootstrap->run(Object(Magento\\Framework\\App\\Http))\n#9 {main}"

I can successfully call Magento out-of-the-box REST web services.

app/code/Test/Calculator/registration.php

<?php

\Magento\Framework\Component\ComponentRegistrar::register(
\Magento\Framework\Component\ComponentRegistrar::MODULE,
'Test_Calculator',
__DIR__
);

app/code/Test/Calculator/etc/module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Test_Calculator" setup_version="1.0.0"/>
</config>

app/code/Test/Calculator/etc/webapi.xml

<?xml version="1.0"?>
<routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Webapi:etc/webapi.xsd">

    <route url="/V1/calculator/add/:num1/:num2" method="GET">
        <service class="Test\Calculator\Api\CalculatorInterface" method="add"/>
        <resources>
            <resource ref="anonymous"/>
        </resources>
    </route>
</routes>

app/code/Test/Calculator/etc/di.xml

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Test\Calculator\Api\CalculatorInterface" type="Test\Calculator\Model\Calculator" />
</config>

app/code/Test/Calculator/Api/CalculatorInterface.php

<?php

namespace Test\Calculator\Api;

interface CalculatorInterface
{
    public function add($num1, $num2);
}

app/code/Test/Calculator/Model/Calculator.php

<?php

namespace Test\Calculator\Model;

use Test\Calculator\Api\CalculatorInterface;

class Calculator implements CalculatorInterface
{
    public function add($num1, $num2) {
        return $num1 + $num2;
    }
}

REST url that returns the error:

http://local.magento202.com:81/index.php/rest/V1/calculator/add/30/70