Injecting interfaces, singleton, factory, models in Magento 2

singleton_instances_magento2.php

<?php
class SomeClass
{
    /**
     * @var \Magento\Framework\App\Config\ScopeConfigInterface
     */
    private $configInterface;
    /**
     * @var \Magento\Framework\App\Config
     */
    private $concreteConfig;
    /**
     * @var \Magento\Framework\App\ConfigFactory
     */
    private $configFactory;

    public function __construct(
        // Preferred for singleton => Inject a singleton but let Magento chose which concrete class thanks to DI
        \Magento\Framework\App\Config\ScopeConfigInterface $configInterface, // Resolves to \Magento\Framework\App\Config
        // Inject a concrete singleton
        \Magento\Framework\App\Config $concreteConfig,

        // Inject the factory of the model for later instanciation. Magento will automatically create the class in the /generated dir when required
        \Magento\Framework\App\ConfigFactory $configFactory
    )
    {
        $this->configInterface = $configInterface;
        $this->concreteConfig = $concreteConfig;
        $this->configFactory = $configFactory;
    }

    public function someMethod()
    {
        // Singleton: first way
        $firstSingleton = $this->configInterface->getValue('some/path');

        // Singleton: second way
        $secondSingleton = $this->concreteConfig->getValue('some/path');

        // New instance
        /** @var \Magento\Framework\App\Config $newConfig */
        $newConfig = $this->configFactory->create();
        $newValue = $newConfig->getValue('some/path');
    }
}
?>