How to retrieve a list of directories QUICKLY in Java?

Suppose a very simple program that lists out all the subdirectories of a given directory. Sound simple enough? Except the only way to list all subdirectories in Java is to use FilenameFilter combined with File.list().

This works for the trivial case, but when the folder has say 150,000 files and 2 sub folders, it's silly waiting there for 45 seconds iterating through all the files and testing for file.isDirectory(). Is there a better way to list sub directories??

PS. Sorry, please save the lectures on having too many files in the same directory. Our live environment has this as part of the requirement.