Trouble initializing a java project in vscode

(Note: to anyone downvoting, kindly tell me the reason so I can improve and not repeat it)

I followed the instructions given in Java Getting Started inside vscode, but it resulted in failure. I do not just want a, perhaps hacky, fix for this. I want to understand what is causing the problem.

I am on Ubuntu 19.04 64bit.
vscode version 1.38.1
I have all the basic java extensions included in the pack installed in vscode.
java --version returns :

java 12.0.2 2019-07-16
Java(TM) SE Runtime Environment (build 12.0.2+10)
Java HotSpot(TM) 64-Bit Server VM (build 12.0.2+10, mixed mode, sharing)

This is what I followed.

It first took me to the newly generated launch.json instead of directly running the code as stated in the procedure.
Here is my launch.json

{
    // Use IntelliSense to learn about possible attributes.
    // Hover to view descriptions of existing attributes.
    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    "configurations": [
        {
            "type": "java",
            "name": "Debug (Launch) - Current File",
            "request": "launch",
            "mainClass": "${file}"
        },
        {
            "type": "java",
            "name": "Debug (Launch)-QuickStart",
            "request": "launch",
            "mainClass": "QuickStart"
        }
    ]
}

I went ahead and hit F5 on my file again and this is what I was met with

user@machine:$ cd /home/user/test/java/proj ; /usr/lib/jvm/java-12-oracle/bin/java -agentlib:jdwp=transport=dt_socket,server=n,suspend=y,address=localhost:42535 --enable-preview -Dfile.encoding=UTF-8 -cp /home/user/.config/Code/User/workspaceStorage/db4b3a82ff379b93685473884410762c/redhat.java/jdt_ws/jdt.ls-java-project/bin QuickStart 
Error: Could not find or load main class QuickStart
Caused by: java.lang.ClassNotFoundException: QuickStart

Please help me understand this and thereby fix it. If I have missed out any crucial information, please ask in the comments and I will promptly add it.

Thank you