This Java naming conventions tutorial explains how and when to use snake, kebab, Pascal and camel case with variables, methods, functions and classes.

You use this library to generate a random number and insert the number into an array variable index. You can add one or several random numbers into your array variables, but Java does not ...

$1 is the name of a scratch variable that jshell creates to store 52. (Scratch variables are created whenever literals are entered.) Execute System.out.println($1) and you’ll see 52 as the output.