What is the best way to generate a unique and short file name in Java

I'd use Apache Commons Lang library (http://commons.apache.org/lang).

There is a class org.apache.commons.lang.RandomStringUtils that can be used to generate random strings of given length. Very handy not only for filename generation!

Here is the example:

String ext = "dat";
File dir = new File("/home/pregzt");
String name = String.format("%s.%s", RandomStringUtils.randomAlphanumeric(8), ext);
File file = new File(dir, name);

This works for me:

String generateUniqueFileName() {
    String filename = "";
    long millis = System.currentTimeMillis();
    String datetime = new Date().toGMTString();
    datetime = datetime.replace(" ", "");
    datetime = datetime.replace(":", "");
    String rndchars = RandomStringUtils.randomAlphanumeric(16);
    filename = rndchars + "_" + datetime + "_" + millis;
    return filename;
}

// USE:

String newFile;
do{
newFile=generateUniqueFileName() + "." + FileExt;
}
while(new File(basePath+newFile).exists());

Output filenames should look like :

2OoBwH8OwYGKW2QE_4Sep2013061732GMT_1378275452253.Ext

Well, you could use the 3-argument version: File.createTempFile(String prefix, String suffix, File directory) which will let you put it where you'd like. Unless you tell it to, Java won't treat it differently than any other file. The only drawback is that the filename is guaranteed to be at least 8 characters long (minimum of 3 characters for the prefix, plus 5 or more characters generated by the function).

If that's too long for you, I suppose you could always just start with the filename "a", and loop through "b", "c", etc until you find one that doesn't already exist.

I use the timestamp

i.e

new File( simpleDateFormat.format( new Date() ) );

And have the simpleDateFormat initialized to something like as:

new SimpleDateFormat("File-ddMMyy-hhmmss.SSS.txt");

EDIT

What about

new File(String.format("%s.%s", sdf.format( new Date() ),
                                random.nextInt(9)));

Unless the number of files created in the same second is too high.

If that's the case and the name doesn't matters

 new File( "file."+count++ );

:P