how to generate a unique random number?

Question

when storing to student may happen to have the same id, so how can i avoid the duplication, I have tried the while loop and the for each loop but it did not work

public void addStudent(Student student)

{

student.setId(ganerateNewRandom("aa", 2));

students.add(student);

}






  public String ganerateNewRandom(String prefix, int number)
 {

  Random random = new Random();

    StringBuffer ret = new StringBuffer(prefix);


    for(int i=0;i<number;i++) ret.append(random.nextInt(10));


    return ret.toString();


}

There are hundreds of duplicates of this topic here on SO; please use the search facility before posting... — Oliver Charlesworth, Mar 28 '13 at 16:00
Use an `int seed` variable which value would be unique for the application and it will increment every time you use it. If you're in a multithreaded environment, you should use `AtomicInteger`. — Luiggi Mendoza, Mar 28 '13 at 16:01
There is a realy good example. Check this: "Generate 1M unique random keys with alpanumeric subset" http://stackoverflow.com/questions/11053377/generate-1m-unique-random-keys-with-alpanumeric-subset — signon, Mar 28 '13 at 16:07

score 5 · Answer 1 · answered Mar 28 '13 at 16:01

5

I suggest you just imitate the way it is done in databases: Start with id=1 and each time enlarge it by 1: id++. That way each student will have unique id and you don't have to generate random numbers.

answered Mar 28 '13 at 16:01

Martinsos

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Problem with that it is not taking into account multi-threading and multiple JVMs. – user1697575 Mar 28 '13 at 16:29
Nobody mentioned multiple JVMs or threading. – Martinsos Mar 28 '13 at 20:55

user1697575 · Answer 2 · 2013-03-28T16:16:47.417

3

Use java.util.UUID to generate proper unique ID.

example:

UUID myID = UUID.randomUUID();

It uses RFC 4122: A Universally Unique IDentifier (UUID) URN Namespace" methodology.

edited Mar 28 '13 at 16:16

answered Mar 28 '13 at 16:10

user1697575

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Nice tips! well i suppose they still exists a tiny chance to get a duplicate isn't it ? – Gab Mar 28 '13 at 16:15
@Gab Its pretty solid and standard practice for UID generation in Java. Refer to RFC 4122 to get more information. – user1697575 Mar 28 '13 at 16:19
Wikipedia: "In other words, only after generating 1 billion UUIDs every second for the next 100 years, the probability of creating just one duplicate would be about 50%." – Louis Wasserman Mar 28 '13 at 16:47

score 2 · Answer 3 · answered Mar 28 '13 at 16:11

2

Just tell me all of this is not used to generate a database id. if it's the case you are going the worst way, id generation MUST be delegated to the database

answered Mar 28 '13 at 16:11

Gab

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score 1 · Answer 4 · answered Mar 28 '13 at 16:01

1

To make sure your random numbers are unique you must store all previous random numbers.

How you store them depends on how many you plan to generate.

answered Mar 28 '13 at 16:01

OldCurmudgeon

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score 1 · Answer 5 · answered Mar 28 '13 at 16:02

1

I would recommend checking out Google's repository with regards to random number generation here

answered Mar 28 '13 at 16:02

david99world

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how to generate a unique random number?

5 Answers5