Java is purely object oriented programming. Everything in Java is an object. 

# for update the system

sudo apt update 

# for upgrading the system

sudo apt upgrade 

# for showing present working directory

pwd  

# for showing the folders and files of current working directory

ls 

to remove the directory

rm -r directory

Python is fast growing language and easy to learn. It is used for web development, machine learning, and AI.

Spring boot is Java framework. Which comes with a lot of features like Auto configuration. 

Spring security

Spring jdbc

Spring hibernate

Etc. 

Linux is better of servers. Almost all the websites are hosted on Linux machines. It is very safe and secure. It comes with both GUI and CLI but CLI works better with Linux machines.

Type script is just a superset of javascript. If you know javascript, then you know typescript.

There are many data types in Java few of them are as follows

Integer

Long

Float

Double

Char

String

Boolean 

Have you learnt about HttpClient. If not then I would love to tell you that, HttpClient is used for sending request to backend.