Effective Java by Joshua Bloch, Chapter 4: Classes and Interfaces, Item 19: Use interfaces only to define types "The constant interface pattern is a poor use of interfaces." This book is worth reading! :) 2013-04-15T20:37:23.99Z+00:00 ... Additionally from JDK 1.5+ you can use static import for your global variables... More on stackoverflow.com

There is no better example of why global variables are "problematic" than this tale from Rinkworks I was asked about taking on a contract to maintain a piece of software. Something about the way it was presented made me wary. I asked to look over it first. What a sight! I use it as an example of why not to use global variables. Among other things, there were files with suites of functions on the following order: adjust_alpha() { alpha = gamma + offset * 3; } adjust_beta() { beta = gamma + offset * 3; } Dozens of functions that differed only by the global variable they modified. Just picture it: a multi-thousand line program with a graphical interface and a database that never used function parameters. The original programmer painted himself into a corner with his variable names. Clearly if you need variables "up," "down," "left," and "right," you name them as such. When he found himself needing those direction names in different parts of his program but was stuck because global variable names had to be unique, his solution was to use names like: up, _up, up_, Up, uP, UP, _Up, _UP down, _down, down_, Down, dOWN, DOWN, _Down, _DOWN ...and so on. Even the densest of my students comprehended immediately why that was bad. Needless to say, I turned down the job. Using global constants (static final) is generally OK. Edit: Escape underscores More on reddit.com

A variable limited in scope to a single function has something less than a couple hundred lines in which it might be affected. A variable limited in scope to a single class only has less than a few thousand lines in which it might be affected. A variable limited in scope to a package or class heirarchy has something less than 10k lines of code in which it might be affected. A variable that is not limited in scope at all could have literally millions of lines of code that could possibly affect it. It's hard to reason about millions of lines of code. It gets much much worse when multiple threads may be acting on it. More on reddit.com

Your professor is telling you not to use a global variable for a reason. Why you need to add values to an array list. Design your classes and instances around that why.

Professionally, we avoid global variables because they are bad design that can have nasty side effects.

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