To the game itself:
Very nice! I really liked hovering over those buttons!

The routing of the enemies was nicely done, I saw no bugs, it worked flawlessly

I'd like more content, but I'm sure you know that

To the way you ship it:
Please add some startup scripts. Double-clicking jars doesn't work easily with natives, I'm afraid. But for that reason we have command line scripting:
Add a linux file "startLinux.sh":
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| #!/bin/sh java -Djava.library.path="nat/linux" -jar galaga.jar |
mac is a little more complicated, afaik... App bundles and such. I have never worked with a mac in my life, so I can't tell you how that works...
And for windows add a file "startWindows.bat":
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| java -Djava.library.path="nat/windows" -jar galaga.jar |
So that would help others trying out your game ^^