Is there any way to change the letter-spacing of text in Eclipse's code editor?
Maybe you can try changing from a fixed width font to the variable width font like Verdana or Tahoma. Window->Preferences->Appearance->Colors and Fonts->Basic->Text Font
If you mean the java code editor in Eclipse this is not possible. The editor is not a word processor. You can only change the font setings (typeface, style, color, size).
If you are referring to this kind of letter spacing, then no, I do not think so.
Not in the sense that a typography system allows you to tweak the appearance of text on a printed page.
The default for me is Courier New Regular 10. You can change the size to 12 or some other size.
Are you trying to change the kerning rules? Kerning is positioning different letters in a variable-width font. For instance in the word "We", the "e" is tucked in a little bit under the "W". The page-layout software that magazine publishers use can control this.
Fonts are opaque to Eclipse; it doesn't give you a way to change the rules within the font. Unfortunately the best you can do is try the different fonts and sizes until you find one that has kerning rules that work, more or less.