In order to make Java Content Assist work in CQ 5.6/ CQ 5.5, download the file crxde-missing-libs-1.0 from : https://dev.day.com/packageshare/packages/public/day/crxde/crxde-missing-libs.html and install it.
Once installed, go to /etc/crxde/profiles/default/libs/ in your Editor and refresh it. Once it is refreshed, right click and select Build:Refresh Classpath.