Since Eclipse Juno (v4.2), there is a Code Recommenders code assist plugin integrated which can do exactly what you want.
By default, only the smart completion is enabled, but in Window -> Preferences -> Code Recommenders
menu, you can enable Chains and, in your case, Subwords.
Make sure you also enable Subwords to be shown (Window -> Preferences -> Java -> Editor -> Content Assist -> Advanced
).
Here's a simple example with System.setOut()
method that was shown just for out
substring: