I believe you are looking for “word completion” Left Alt + / though you can change it in “Preferences->General->Keys” to be whatever key combo. Pressing / multiple times cycles through the different suggestions. It’s not very intelligent. For example if you have String apple; and double apps; and you type double x = a and then Alt + / eclipse might suggest apple instead of apps, even though apple is an String.
Edit: Looks like it’s ctrl + / on some computers.