RANDOM and I suspect that assert logic or message
is wrong for particular key impl:

    /**
     * FileKey is more robust that plain
FileObject, it's immutable.
     */
    private static class FileKey {
        private final String name;
        private final boolean folder;

        FileKey(FileObject fo) {
            name = fo.getNameExt();
            folder = fo.isFolder();
        }

        public String getNameExt() {
            return name;
        }

        public boolean isFolder() {
            return folder;
        }

        public boolean equals(Object o) {
            if (this == o) return true;
            if (!(o instanceof FileKey)) return false;

            final FileKey fileKey = (FileKey) o;

            if (folder != fileKey.folder) return
false;
            if (name != null ?
!name.equals(fileKey.name) : fileKey.name != null)
return false;

            return true;
        }

        public int hashCode() {
            int result;
            result = (name != null ?
name.hashCode() : 0);
            result = 29 * result + (folder ? 1 : 0);
            return result;
        }
    }