reason of end to end testing code example

Example 1: what is end to end testing

End to end testing (E2E testing) refers to a software testing method 
that involves testing an application's workflow from beginning to end. 
This method basically aims to replicate real user scenarios so that 
the system can be validated for integration and data integrity.

Example 2: what is end to end testing

I do UI testing with Selenium Webdriver/java/cucumber/junit/testng,
API: Postman for manual, RestAssured java library 
for automation Database: SQL Programming for 
SQL developer for manual, JDBC Library for Automation. 
For example; 1. With user input a data is created in UI
2. I send request from API and verify data 
is matching with UI (you reach your database through API,
it is credible as much as your API software is credible)
3. Send SQL queries using JDBC get request to
database and verify data is matching with
post request JDBC is a Java-based data access 
technology used for Java database connectivity. 
It provides classes and interfaces to connect or
communicate Java application with database. 
JDBC API is a Java API that can access any 
kind of data stored in a Relational Database.
It enables Java programs to execute SQL statements.