I think this is the main difference between an IDE and an editor. Eclipse adds value by having some sense of your overall project. When you go through the process of setting up a project (for new or existing source), it parses all the files, gets info about all jars in classpath, etc. It can use this information to do things like:
- Change class name and every other java file which uses that class will also update correctly.
- Show errors when a dependency is missing.
-Change type of a method in one class and every error caused by that change, in every file is immediately highlighted.
-Right click on a method and ask eclipse to show you every method which calls that method...recursively.
-...and so on
You CAN just open a java file without creating a project, but then you don't get much more than syntax highlighting or formatting help.