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<?php declare(strict_types = 1); namespace App { class Product { private $id; private $name; private $volumes; /** * Product constructor. * @param int $id * @param string $name * @param array $volumes * @throws \InvalidArgumentException */ public function __construct(int $id, string $name, array $volumes) { $this->id = $id; $this->name = $name; if (count($volumes) === 0) { throw new \InvalidArgumentException('Your product must have 1 product at least.'); } $this->volumes = $this->prepareVolumes($volumes); } private function prepareVolumes(array $volumes) { foreach ($volumes as $volume) { $volume->setProduct($this); } return $volumes; } } class Volume { protected $id; protected $product; protected $description; public function __construct(string $description) { $this->description = $description; } public function setProduct(Product $product) { $this->product = $product; } } class ProductTest { public function __construct() { $volume1 = new Volume('part-1'); $volume2 = new Volume('part-2'); $volume3 = new Volume('part-3'); $product = new Product(1, 'BED', [$volume1, $volume2, $volume3]); $invalidProduct = new Product(2, 'Table', []); } } return new ProductTest(); }