Likelihood of collision using most significant bits of a UUID in Java

Question

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

User · Answer

Raymond Chen has a really excellent blog post on this  GUIDs are globally unique  but substrings of GUIDs aren t

User · Answer

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

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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

User · Answer

Raymond Chen has a really excellent blog post on this  GUIDs are globally unique  but substrings of GUIDs aren t

User · Answer

Raymond Chen has a really excellent blog post on this  GUIDs are globally unique  but substrings of GUIDs aren t

User · Answer

According to the documentation  the static method UUID randomUUID   generates a type 4 UUID   This means that six bits are used for some type information and the remaining 122 bits are assigned randomly   The six non-random bits are distributed with four in the most significant half of the UUID and two in the least significant half  So the most significant half of your UUID contains 60 bits of randomness  which means you on average need to generate 2 30 UUIDs to get a collision  compared to 2 61 for the full UUID    So I would say that you are rather safe  Note  however that this is absolutely not true for other types of UUIDs  as Carl Seleborg mentions   Incidentally  you would be slightly better off by using the least significant half of the UUID  or just generating a random long using SecureRandom

User · Answer

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

User · Answer

According to the documentation  the static method UUID randomUUID   generates a type 4 UUID   This means that six bits are used for some type information and the remaining 122 bits are assigned randomly   The six non-random bits are distributed with four in the most significant half of the UUID and two in the least significant half  So the most significant half of your UUID contains 60 bits of randomness  which means you on average need to generate 2 30 UUIDs to get a collision  compared to 2 61 for the full UUID    So I would say that you are rather safe  Note  however that this is absolutely not true for other types of UUIDs  as Carl Seleborg mentions   Incidentally  you would be slightly better off by using the least significant half of the UUID  or just generating a random long using SecureRandom

User · Answer

According to the documentation  the static method UUID randomUUID   generates a type 4 UUID   This means that six bits are used for some type information and the remaining 122 bits are assigned randomly   The six non-random bits are distributed with four in the most significant half of the UUID and two in the least significant half  So the most significant half of your UUID contains 60 bits of randomness  which means you on average need to generate 2 30 UUIDs to get a collision  compared to 2 61 for the full UUID    So I would say that you are rather safe  Note  however that this is absolutely not true for other types of UUIDs  as Carl Seleborg mentions   Incidentally  you would be slightly better off by using the least significant half of the UUID  or just generating a random long using SecureRandom

User · Answer

Raymond Chen has a really excellent blog post on this  GUIDs are globally unique  but substrings of GUIDs aren t

User · Answer

According to the documentation  the static method UUID randomUUID   generates a type 4 UUID   This means that six bits are used for some type information and the remaining 122 bits are assigned randomly   The six non-random bits are distributed with four in the most significant half of the UUID and two in the least significant half  So the most significant half of your UUID contains 60 bits of randomness  which means you on average need to generate 2 30 UUIDs to get a collision  compared to 2 61 for the full UUID    So I would say that you are rather safe  Note  however that this is absolutely not true for other types of UUIDs  as Carl Seleborg mentions   Incidentally  you would be slightly better off by using the least significant half of the UUID  or just generating a random long using SecureRandom

User · Answer

I thinks this is the best example for using randomUUID    http   www javapractices com topic TopicAction do Id 56

User · Answer

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

User · Answer

I thinks this is the best example for using randomUUID    http   www javapractices com topic TopicAction do Id 56

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

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