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

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'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);

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

Look at the File javadoc, the method createNewFile will create the file only if it doesn't exist, and will return a boolean to say if the file was created.

You may also use the exists() method:

int i = 0;
String filename = Integer.toString(i);
File f = new File(filename);
while (f.exists()) {
    i++;
    filename = Integer.toString(i);
    f = new File(filename);
}
f.createNewFile();
System.out.println("File in use: " + f);

If you have access to a database, you can create and use a sequence in the file name.

select mySequence.nextval from dual;

It will be guaranteed to be unique and shouldn't get too large (unless you are pumping out a ton of files).

I use current milliseconds with random numbers

i.e

Random random=new Random();
String ext = ".jpeg";
File dir = new File("/home/pregzt");
String name = String.format("%s%s",System.currentTimeMillis(),random.nextInt(100000)+ext);
File file = new File(dir, name);

Combining other answers, why not use the ms timestamp with a random value appended; repeat until no conflict, which in practice will be almost never.

For example: File-ccyymmdd-hhmmss-mmm-rrrrrr.txt

Why not just use something based on a timestamp..?

Problem is synchronization. Separate out regions of conflict.

Name the file as : (server-name)_(thread/process-name)_(millisecond/timestamp).(extension)
example : aws1_t1_1447402821007.png

How about generate based on time stamp rounded to the nearest millisecond, or whatever accuracy you need... then use a lock to synchronize access to the function.

If you store the last generated file name, you can append sequential letters or further digits to it as needed to make it unique.

Or if you'd rather do it without locks, use a time step plus a thread ID, and make sure that the function takes longer than a millisecond, or waits so that it does.

It looks like you've got a handful of solutions for creating a unique filename, so I'll leave that alone. I would test the filename this way:

    String filePath;
    boolean fileNotFound = true;
    while (fileNotFound) {
        String testPath = generateFilename();

        try {
            RandomAccessFile f = new RandomAccessFile(
                new File(testPath), "r");
        } catch (Exception e) {
            // exception thrown by RandomAccessFile if 
            // testPath doesn't exist (ie: it can't be read)

            filePath = testPath;
            fileNotFound = false;
        }
    }
    //now create your file with filePath