Phpunit test simple getPath() method

Question

*This is a beginner's question in phpunit so i don't know if it belongs here or not.

I want to test that the var path is correct, but I'm getting Failed asserting that null matches expected '/magento2/var'.

I suppose because I am using a mock for DirectoryList, instead instantiating the class. How does one test it? Should I instantiate DirectoryList? I am having issues with it's constructor...

<?php

namespace Vendor\Module\Cron;

use Magento\Framework\App\Filesystem\DirectoryList;

class Products {

    /**
     * Retrieve var path
     *
     * @return string
     */
    protected $directory_list;

    public function __construct(
        DirectoryList $directory_list
    )
    {
        $this->directory_list = $directory_list;
    }

    /**
     * Retrieve var path
     *
     * @return string
     */   
    public function getVarFolderPath()
    {
        return $this->directory_list->getPath('var');
    }
}

and my test:

<?php

namespace Vendor\Module\Test\Unit\Model;

use Magento\Framework\App\Filesystem\DirectoryList;
use Vendor\Module\Cron\Products;

class ProductsTest extends \PHPUnit_Framework_TestCase
{
    /**
     * @var string
     *
     * dummy var path
     */
    const VAR_PATH = '/magento2/var';

    /**
     * @var Vendor\Module\Cron\Products;
     */
    protected $products;

    /**
     * setup tests
     */
    protected function setUp()
    {

        /** @var \PHPUnit_Framework_MockObject_MockObject|DirectoryList */
        $directoryListMock = $this->getMockBuilder(DirectoryList::class)
            ->disableOriginalConstructor()
            ->getMock();
        $this->products = new Products(
            $directoryListMock
        );
    }


    /**
     * @test Vendor\Module\Cron\Products::getVarFolderPath()
     */
    public function testGetVarFolderPath()
    {
        $this->assertEquals(self::VAR_PATH, $this->products->getVarFolderPath());
    }
}

Update: so Kandy's answer was ok. in unit test you test just that specific class.

Answer 1

All mocked methods returns null, so you need describe in your test what getPath method will return:

$directoryListMock->expects($this->any())
->method('getPath')
->with('var')
->willReturn(self::VAR_PATH);