How to view all implementors of a given interface in NetBeans for Java?

Try this, select the type e.g. "String", then click with the right mouse button -> Navigate -> Inspect Hierarchy. On the right side you will see a window called "Hierarchy" or maybe just a vertical text with "Hierarchy" which is a minimized window (click to open).

If opened you see a combobox with "Supertypes". Change this to subtypes, there you see all classes or interfaces implementing/extending your interfaces or extending your classes.

Try this,

ctrl + alt + click

this will list all implementations of same interface

UPDATE: This bug has been marked as fixed, so you should be able to use Find Usages for this.

Original post: You cannot do this without some sort of plugin or other program. The inability to find usages in libraries has been under discussion for 10 years and is currently listed as a Priority 1 enhancement, having no easy workaround. No planned fix date has been announced.