Overriding Block, Model, Controller in Magento2

Overriding Block

Overriding catalog product ListProduct block. 

#1 – First create di.xml file in Folder Webkul/Hello/etc

<?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="Magento\Catalog\Block\Product\ListProduct" type="Webkul\Hello\Block\Rewrite\Product\ListProduct" />
</config>

#2 – Now create ListProduct.php Block file in Folder Webkul/Hello/Block/Rewrite/Product

<?php
	/**
	 * Hello Rewrite Product ListProduct Block
	 *
	 * @category    Webkul
	 * @package     Webkul_Hello
	 * @author      Webkul Software Private Limited
	 *
	 */
	namespace Webkul\Hello\Block\Rewrite\Product;

	class ListProduct extends Magento\Catalog\Block\Product\ListProduct
	{
		public function _getProductCollection()
		{
			// Do your stuff here
		}
	}

you can override other blocks using same approach.
 

Overriding Model

Overriding catalog product model. 

#1 – First create di.xml file in Folder Webkul/Hello/etc

<?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="Magento\Catalog\Model\Product" type="Webkul\Hello\Model\Rewrite\Catalog\Product" />
</config>

#2 – Now create Product.php Model file in Folder Webkul/Hello/Model/Rewrite/Catalog

<?php
	/**
	 * Hello Catalog Product Rewrite Model
	 *
	 * @category    Webkul
	 * @package     Webkul_Hello
	 * @author      Webkul Software Private Limited
	 *
	 */
	namespace Webkul\Hello\Model\Rewrite\Catalog;

	class Product extends \Magento\Catalog\Model\Product
	{
		public function isSalable()
		{
			// Do your stuff here
			return parent::isSalable();
		}

	}

you can override other models using same approach.

Overriding Controller

Overriding product view controller. 

#1 – First create di.xml file in Folder Webkul/Hello/etc

<?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="Magento\Catalog\Controller\Product\View" type="Webkul\Hello\Controller\Rewrite\Product\View" />
</config>

#2 – Now create View.php Controller file in Folder Webkul/Hello/Controller/Rewrite/Product

<?php
	/**
	 * Hello Rewrite Product View Controller
	 *
	 * @category    Webkul
	 * @package     Webkul_Hello
	 * @author      Webkul Software Private Limited
	 *
	 */
	namespace Webkul\Hello\Controller\Rewrite\Product;

	class View extends Magento\Catalog\Controller\Product\View
	{
		/**
		 * @return \Magento\Framework\Controller\Result\Redirect|\Magento\Framework\View\Result\Page
		 */
		public function execute()
		{
			// Do your stuff here
			return parent::execute();
		}
	}

you can override other controllers using same approach.