[java] Likelihood of collision using most significant bits of a UUID in Java

If I'm using Long uuid = UUID.randomUUID().getMostSignificantBits() how likely is it to get a collision. It cuts off the least significant bits, so there is a possibility that you run into a collision, right?

You are better off just generating a random long value, then all the bits are random. In Java 6, new Random() uses the System.nanoTime() plus a counter as a seed.

There are different levels of uniqueness.

If you need uniqueness across many machines, you could have a central database table for allocating unique ids, or even batches of unique ids.

If you just need to have uniqueness in one app you can just have a counter (or a counter which starts from the currentTimeMillis()*1000 or nanoTime() depending on your requirements)

I thinks this is the best example for using randomUUID :

http://www.javapractices.com/topic/TopicAction.do?Id=56

Raymond Chen has a really excellent blog post on this:

GUIDs are globally unique, but substrings of GUIDs aren't

You are better off just generating a random long value, then all the bits are random. In Java 6, new Random() uses the System.nanoTime() plus a counter as a seed.

There are different levels of uniqueness.

If you need uniqueness across many machines, you could have a central database table for allocating unique ids, or even batches of unique ids.

If you just need to have uniqueness in one app you can just have a counter (or a counter which starts from the currentTimeMillis()*1000 or nanoTime() depending on your requirements)

Raymond Chen has a really excellent blog post on this:

GUIDs are globally unique, but substrings of GUIDs aren't

Raymond Chen has a really excellent blog post on this:

GUIDs are globally unique, but substrings of GUIDs aren't

Use Time YYYYDDDD (Year + Day of Year) as prefix. This decreases database fragmentation in tables and indexes. This method returns byte[40]. I used it in a hybrid environment where the Active Directory SID (varbinary(85)) is the key for LDAP users and an application auto-generated ID is used for non-LDAP Users. Also the large number of transactions per day in transactional tables (Banking Industry) cannot use standard Int types for Keys

private static final DecimalFormat timeFormat4 = new DecimalFormat("0000;0000");

public static byte[] getSidWithCalendar() {
    Calendar cal = Calendar.getInstance();
    String val = String.valueOf(cal.get(Calendar.YEAR));
    val += timeFormat4.format(cal.get(Calendar.DAY_OF_YEAR));
    val += UUID.randomUUID().toString().replaceAll("-", "");
    return val.getBytes();
}

I thinks this is the best example for using randomUUID :

http://www.javapractices.com/topic/TopicAction.do?Id=56