This sounds really weird....
btw. which editors did you use ? and was it really just a plain java file ?
my guess could be that your opening in Galileo triggered another default editor which gets persisted into the resource properties thus when you come back to Helios its not that content assists starts to work but that you are now actually using the right editor.
just a hunch.
btw. you can see what editor you use in right click on the file and what Open With.. says as the default.