The I/O Package - The Java Programming Language - page 809

returns true . For our accept method, true means that the named file is a
directory.
The FileFilter interface is analogous to FilenameFilter , but works with a
single File object:
boolean accept(File pathname)
Returns true if the file represented by pathname should be part
of the filtered output.
Exercise 20.11 : Using FilenameFilter or FileFilter , write a program
that takes a directory and a suffix as parameters and prints all files it
can find that have that suffix.