How can we identify whether the class is injectable or not in Magento 2

Question

1. How can we identify whether the class is injectable or not in Magento 2 as per the comment.

2. In this answer get is used by object manager, can any one tell me how to convert below class into constructor as a dependency class.

3. How can we treat get and create into constructor as a dependency classes.

   $stockItemFactory = $this->objectManager->get('Magento\CatalogInventory\Api\Data\StockItemInterfaceFactory');
   $stockItem = $stockItemFactory->create();
   

Answer 1

Usually everything is injectable except models that have a table (or more) associated to it. For example the product model, the order model...
These should not be injected in the constructor because you will end up with the same class instance everywhere you inject it. And this could lead to problems.
When dealing with such classes you should inject the factory into the constructor and instantiate your class using the $factory->create() method.
You don't need to write the factories yourself. They will be autogenerated.

You should never use the object manager to instantiate the classes. Even if it works, it's not a good practice and it will make it harder for you to write tests.

get from the object manager acts as Mage::getSingleton from magento 1 and create acts as Mage::getModel.

And you can convert your code to use DI like this.

protected $stockItemFactory;
public function __construct(
   ....
   \Magento\CatalogInventory\Api\Data\StockItemInterfaceFactory $stockItemFactory,
   ....
) {
    ...
    $this->stockItemFactory = $stockItemFactory;
    ....
}

Having this set in your constructor, you can use this in your class methods (even in the constructor but below the line I mentioned) in order to instantiate your class:

$stockItem = $this->stockItemFactory->create();

I hope I covered all your questions.