Write in class source code next sting ( without
doing completion or adding manually import
statemnts ):
File file = new File();

After saving ( or after some time ) editor will 
discover that you have errors in your code.
Editor will underline as error this string.
You can see in left rule error icon and will
see error information when place cursor of mouse
above this icon.

But you don't get any assist about how this
problem can be solved.

It would be very useful to have assist pane (
something like completion ) (it can be called
through clicking on error icon ) when you can
choose way to solve your problem.
F.e. in our case this assist can suggest to you 
add import statement , change name for class, 
create new class,..... ( such possibility exist in 
eclipse ).