You're most likely right about the flickering. You most likely wouldn't like the Microsoft Word style underlining of things (red for error, yellow for warning) and the menu of suggestions it offers to fix things ((a) import java.util.regex.* (b) remove Matcher (c) define new class Matcher etc) either. :)

I've taken a look at dabbrev-expand. I think it doesn't check a foreign namespace unless you point it at the file. This is what you mean by low-level. I guess in Eclipse/Java it compiles the code continuously and autocompletes methods based on the current class or identifier at the cursor (and only them).

When you say jump to the declaration? You mean in a different buffer it opens the file that declares the symbol? I think if the buffer doesn't compile in eclipse it will only work for class names (take you to their entry in the Type Hierarchy browser window or the original source) - otherwise it can't know which member is from which class.