There is an option in: Code Style > Java > Code Generation > Comment Code
With "Line comment at first column" you can switch between:
//System.out.println("");
// System.out.println("");
So, you don't have a 'space' after comment (in first line), but its close enough :)
Idea 2017.2+
Recent versions of IntelliJ IDEA support adding a space after the line comment. The setting is at the same location.
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