Answer 1

Comments are ignored by the compiler and their sole purpose is for you to leave a comment for yourself or others to read when they view the source code. You can use them for whatever purpose you want, but often they're used to describe the current code and its function (maybe explain the arguments, vars, methods, etc..), remind yourself on things to finish or fix, post your copyright info, etc...

Basically, any information you want to leave behind to help yourself and others out later, post it as comments. You may think that you'll always remember what you were thinking in this exact moment, until it's a year later and you're reading the code for the first time and wonder, "wtf was I even doing?" Your comments will come in handy and maybe even save you endless hours of re-reading code you wrote.

Also, it's worth looking into the JAVADOC system with Java. It allows you to write your comment and document your code at the exact same time. This is how many people generate those fancy HTML documents that maps out all of their code/functions/etc..

http://www.oracle.com/technetwork/java/javase/tech/index-137868.html

^Check that out. I don't remember if SoloLearn covers JAVADOC, but I consider it a must for anyone dealing with Java.