How to check if a folder exists

Using java.nio.file.Files:

Path path = ...;

if (Files.exists(path)) {
    // ...
}

You can optionally pass this method LinkOption values:

if (Files.exists(path, LinkOption.NOFOLLOW_LINKS)) {

There's also a method notExists:

if (Files.notExists(path)) {

Generate a file from the string of your folder directory

String path="Folder directory";    
File file = new File(path);

and use method exist.
If you want to generate the folder you sould use mkdir()

if (!file.exists()) {
            System.out.print("No Folder");
            file.mkdir();
            System.out.print("Folder created");
        }

Quite simple:

new File("/Path/To/File/or/Directory").exists();

And if you want to be certain it is a directory:

File f = new File("/Path/To/File/or/Directory");
if (f.exists() && f.isDirectory()) {
   ...
}

To check if a directory exists with the new IO:

if (Files.isDirectory(Paths.get("directory"))) {
  ...
}

isDirectory returns true if the file is a directory; false if the file does not exist, is not a directory, or it cannot be determined if the file is a directory or not.

See: documentation.