How To Find Out Minimum JRE Requirement For Java Program

I finished a game in Java and sent it to a friend. Launching the program in my computer worked just fine.

But he got this error : "Could not find the main classs: Main. program will exit"

My JRE version is the most updated one. His JRE was version 1.6. He updated his JRE, and the problem was solved.

This is a bit worrying for me, because as far as I know, 1.6 isn't a very old version of the JRE. It's not the most recent one, but not that old.

This is worrying because I'm planning on sending my game to a lot of friends, and trying to distribute it on the internet.

A lot of people don't have the most updated JRE. And they are mostly non-programmers, so I can't expect them to update to the newest version of Java upon downloading my game. They might not know what Java is, even though they got it on their computer, and upon receiving an error, they'll just give up on the game.

If my game wouldn't work with a significantly old JRE, that would be reasonable. It's part of the nature of working with Java. But the fact that a relatively updated JRE, 1.6, doesn't work with my game, is worrying.

*(Please note: My game isn't implementing anything "special". Swing and KeyBindings are the 'newest' additions to Java that I can think of inside my game)*.

In short, I'd like to know that my game works on most of the computers it tries to run on. Knowing that it doesn't work on a relatively new JRE, is worrying.

So I have two questions:

1. Is it normal, for a Java program, to have such "high" demands for the JRE version? Do a lot of Java games demand at least version 1.6 of the JRE? Is this common?

2. How can I find out the minimum JRE version requirement for my program? Is there a methodical way to do this, or do I just have to go through all the libraries I use in my game and figure out what's the JRE release version for each one?