Is external-src.zip necessary for intelliJ?

I don't know why IntelliJ works that way. I don't use IntelliJ.

But I can make a guess based on how it works in general.

Java byte code can be decompiled, but there are some limitations to decompilation. Some information is lost.

That includes:
* comments
* names of local variables
* names of arguments

Especially the last can make a big difference for IDE help.

You can see that you need to call the foobar method on someobj, but what are the arguments.

There is a huge difference between seeing:

void foobar(int paramInt, String paramString)

and:

void foobar(int numberThreads, String nameTest)

If the IDE is able to use the java doc comments in the source it can even provide more info about the parameters. Which is very nice.

But in my experience usually just known the real parameter names is sufficient.

Thanks Arne, for that nice summery. Yes sometimes browsing the real code is of course outstanding.
Best regards
Fredrik