How expensive is File.exists in Java

Measure the necessary time and see yourself. As you say it is absolutely file system dependent.

        long t1 = System.currentTimeMillis();
        ...Your File.exists call
        long t2 = System.currentTimeMillis();
        System.out.println("time: " + (t2 - t1) + " ms");

You will see that it will always give you different results, since it depends also on the way your OS caches data, on its load etc.