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Generate small UID with uniqueness

I need to generate the UID (alphanumeric) for my use case but that should be a maximum of 7 characters long as we want UID to be random but manageable, like a PNR (CYB6KL) for example.
Now if I am not wrong, I can generate a random UID that is small, but uniqueness might be compromised because of collisions (birthday paradox), so for 32 bits, 50% collision probability would be around 77k UID generations.
So in essence, I need a way to generate UIDs that are:
Small (max 7 character)
Random
Unique
Don't require lookups for the previous existance.
I will be storing this UID in a database column and it's imperative that the UID is unique. It will NOT be the table's primary key which right now is an autogenerated ID.
I am thinking of something along the lines, but I am not sure about uniqueness.
BigInteger big = new BigInteger(32, new SecureRandom());
return big.toString(32).toUpperCase();
Really appreciate any thoughts that might help on this. Generation must be unique.
Thanks in advance.

You can use a library like hashids for this purpose which implements a bimorphic translation that can encode a numeric value into a string code with a custom alphabet. This should do exactly what you want. If you need this to be traversal-secure, you should use some kind of SecureRandom as source for the underlying numeric value. If not, you could even base this on the auto increment value you already have. The benefit of reusing the primary key is that you can just translate the string code and do a lookup by primary key.

How to produce a True UUID in Java?

I was under the impression that the UUID spec required a guaranteed, true, globally unique result, not unique 99.99999999999% of the time, but truly 100% of the time. From the spec:
A UUID is 128 bits long, and can guarantee
uniqueness across space and time.
It looks like java only support V3 and V4 of the UUID spec. V4 isn't truly unique. With the V3 implementation using nameUUIDFromBytes, the following results in duplicates, because the computer is too fast (edit: looping to 10 and called new Date().getTime() will produce duplicates because the computer loops faster than new Date().getTime() can produce a different value on each iteration):
String seed;
for (int i = 0; i < 10; i++) {
seed = "<hostname>" + new Date().getTime();
System.out.println(java.util.UUID.nameUUIDFromBytes(seed.getBytes()));
}
Am I mistaken in assuming that a UUID is 100% unique, and that it is only practically unique but not perfectly so? Is there anyway to do this in Java?

There are different methods of UUID generation. The kind you're using is behaving exactly as it should. You're using nameUUIDFromBytes, a "Static factory to retrieve a type 3 (name based) UUID based on the specified byte array."
This generates the same UUID if given the same name. As you've discovered, your loop is passing-in the same name every time, so you get the same UUID.
Have a look at Gabe's advice here: Which UUID version to use?
He recommends you use V4, which as others have pointed out is good enough for any realistic use case.

Because your entropy is limited to your memory, you can never ensure a UUID is "guaranteed, true, globally unique result". However, 99.99999999999% is already pretty good.
If you want to ensure unique values in your database, you could use a simple integer that's incremented to be sure it's unique. If you want to use UUIDs and be really sure they're unique, you just have to check that upon creation. If there's a duplicate, just create another one until it's unique.
Duplicates can happen, but IIRC, part of them is created dependent on your current time, so if you're just creating one every 5 minutes, you should be safe.

As others have pointed out, the type-4 UUID returned by UUID.randomUUID() is likely to be unique enough for any practical application. Cases where it's not are likely to be pathological: for example, rolling back a VM to a live snapshot, without restarting the Java process, so that the random-number generator goes back to an exact prior state.
By contrast, a type-3 or type-5 UUID is only as unique as what you put into it.
A type-1 UUID (time-based) should be very slightly "more" unique, under certain constraints. The Java platform does not include support for generating a type-1 UUID, but I've written code (possibly not published) to call a UUID generating library via JNI. It was 18 lines of C and 11 lines of Java.

How to create a 64 bit Unique Integer in Java

I need to create a 64 bit unique integer in Java so that collision chances are low. The system is not distributed, so collisions between different computers are not a problem.
Is there any way, we can create a 64 bit integer in Java which is always Unique?
As of now I am using -
long number = System.nanoTime();
Is this the right way to generate 64 bit Unique Integer in Java or is there anything else I can try?
UPDATE:-
How about doing this way? Will this be unique?
UUID number = UUID.randomUUID();
long uniqueNumber = number.timestamp();

If you need the numbers to be unique in one process, robust between restarts, you can use a simple AtomicLong and a timer.
private static final AtomicLong TS = new AtomicLong();
public static long getUniqueTimestamp() {
long micros = System.currentTimeMillis() * 1000;
for ( ; ; ) {
long value = TS.get();
if (micros <= value)
micros = value + 1;
if (TS.compareAndSet(value, micros))
return micros;
}
}
This will give you a unique "timestamp" with a millisecond accuracy but can only handle 1000 ids per millisecond without getting ahead of the actual time. This works fine on restart as the time will jump past previous values (again assuming you have less than one million per second on average)

Use a HashSet in order to ensure uniqueness of the values you're storing. You can then check whether the insert was successful by checking what add returns. If the values have to be 'randomised' you can use your own algorithm, or check out SecureRandom.
Long getUniqueNumber(HashSet uniqueNumberSet) {
Long unique = generateUniqueNumber();
if(!uniqueNumberSet.add(unique)) { // handle collision }
return unique;
}

As Marc B said, the best approach is a simple long which is initialized with zero and incremented every time you need a new value.
If you need concurrency, or if performance is not an issue at all, then you can use AtomicLong as suggested by Loc Ha; however, if you really need it to be a long and not an int, then I suspect you are going to be generating lots of them, so you should probably avoid the extra overhead of AtomicLong unless you are sure you also need concurrency.
System.nanoTime() is not a good idea, as you have no guarantee that two consecutive calls to it will always yield different values.
EDIT (to cover update in question)
No, the timestamp part of the UUID is not guaranteed to be unique, for precisely the same reasons that System.nanoTime() is not guaranteed to be unique. If the timestamp of the UUID was unique, then there would be no need to have a UUID type, we would just always use that timestamp part. Time is always a bad way to go about guaranteeing uniqueness.

You want to get unique ID, the appropriate way(although 128 bit):
UUID.randomUUID();
A bit less appropriate(collisions* are possible) with 64 bits:
UUID.getLeastSignificantBits();
UUID.getMostSignificantBits();
To really get unique ID(if they are critical to your operation):
Use centralised storage of all IDs
When you need an ID, let this centralised system handle it -> DB and auto incremented values are usually the easiest way
*collisions => 2 or more equal values

Is a generated UUID from a String reversible?

If I generate a UUID from a "seed" string as follows, is there any way for someone to re-generate the original string?
UUID uuid = null;
try {
uuid = UUID.nameUUIDFromBytes(("seedString").getBytes("utf8"));
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
}
System.out.println("UUID: " + uuid.toString());
I would assume it isn't possible, as I believe this person found here: Convert UUID to bytes
However, I see that the same UUID is generated every time from a certain String/bytes, and since it has to be unique, simple "seed" values could just be guessed? For example, UUID of f is 8fa14cdd-754f-31cc-a554-c9e71929cce7 so if I see that I know it was generated from "f".

Since you are getting the UUID by casting bytes to a UUID, and you are always using the same starting bytes to cast from, the uuid would always be the same UUID across multiple runs.
I think you've confused a random seed with the "from bytes" method in the UUID routines. It is more like a cast than a seed initialization. And even if it was like a seed initialization, initializing with a constant seed would only mean that you always walk the "same" pseudo-random path (meaning that after walking it once, you can know the next step(s)).
aug also makes an excellent point, which I'll elaborate a bit on here. A UUID is an identifier, which is assumed to be unique only by virtue of there being so many to choose from; however, if you create a routine that returns the same one(s) repeatedly, it's not going to be unique due to your selection mechanism. The actual mechanism doesn't assure uniqueness; even less so when using a routine guaranteed to return identical values.
As they are not guaranteed to be unique (UUIDs have a fixed number of bits and eventually all combinations can be exhausted), one can imagine that there are more inputs than UUIDs (although there's a lot of UUIDs) so UUID collision is inevitable (even if it would theoretically take more time than the heat death of the universe). From a practical side of things, you probably have little to worry about; but, it could still (minuscule chance) happen.
This also means that one can (in theory) guarantee that some two inputs out there can wind up with the same UUID, and as a result, UUIDs are not generally reversible (however, in specific (limited) cases, perhaps they could be made reversible).

There are an infinite number of strings that may generate a given UUID, so even if somebody guesses the string you used to create a given UUID, they may never be sure.

How to get a unique alphanumeric based on a unique integer

My webapplication has a table in the database with an id column which will always be unique for each row. In addition to this I want to have another column called code that will have a 6 digit unique Alphanumeric code with numbers 0-9 and alphabets A-Z. Alphabets and number can be duplicate in a code. i.e. FFQ77J. I understand the uniqueness of this 6 digit alphanumeric code reduces over time as more rows are added but for now I am ok with this.
Requirement (update)
- The code should be at least of length 6
- Each code should be Alphanumeric
So I want to generate this Alphanumeric code.
Question
What is a good way to do this?
Should I generate the code and after the generation, run a query to the database and check if it already exists, and if so then generate a new one? To ensure the uniqueness, does this piece of code need to be synchronized so that only one thread runs it?
Is there something built-in to the database that will let me do this?
For the generation I will be using something like this which I saw in this answer
char[] symbols = new char[36];
char[] buf;
for (int idx = 0; idx < 10; ++idx)
symbols[idx] = (char) ('0' + idx);
for (int idx = 10; idx < 36; ++idx)
symbols[idx] = (char) ('A' + idx - 10);
public String nextString()
{
for (int idx = 0; idx < buf.length; ++idx)
buf[idx] = symbols[random.nextInt(symbols.length)];
return new String(buf);
}

Since it's a requirement for the shortcode to not be guessable, you don't want to tie it to your uniqueID row ID. Otherwise that means your rowID needs to be random, in addition to unique. Starting with a counter 0, and incrementing, makes it pretty obvious when your codes are: 000001, 000002, 000003, and so forth.
For your short code, generate a random 32bit int, omit the sign and convert to base36. Make a call to your database, to ensure it's available.
You haven't explicitly called out scalability, but I think it's important to understand the limitations of your design wrt to scale.
At 2^31 possible 6 char base36 values, you will have collisions at ~65k rows (see Birthday Paradox questions)
From your comment, modify your code:
public String nextString()
{
return Integer.toString(random.nextInt(),36);
}

I would simply do this:
String s = Integer.toString(i, 36).toUpperCase();
Choosing base-36 will use characters 0-9a-z for the digits. To get a string that uses uppercase letters (as per your question) you would need to fold the result to upper case.
If you use an auto increment column for your id, set the next value to at least 60,466,176, which when rendered to base 36 is 100000 - always giving you a 6 digit number.

I would start with 0 for an empty table and do a
SELECT MAX(ID) FROM table
to find the largest id so far. Store it in an AtmoicInteger and convert it using toString
AtomicInteger counter = new AtomicInteger(maxSoFar);
String nextId = Integer.toString(counter.incrementAndGet(), 36);
or for padding. 36 ^^ 6 = 2176782336L
String nextId = Long.toString(2176782336L + counter.incrementAndGet(), 36).substring(1);
This will give you uniqueness and no duplicates to worry about. (it's not random either)

Simply, you can use Integer.toString(int i, int radix). Since you have base 36(26 letters+10 digits) you set the radix to 36 and i to your integer. For example, to use 16501, do:
String identifier=Integer.toString(16501, 36);
You can uppercase it with .toUpperCase()
Now onto your other questions, yes, you should query the database first to ensure it doesn't exist. If depending on the database, it may need to be synchronized, or it may not be as it'll use its own locking system. In any case, you'd need to tell us which database.
On the question of whether there's a builtin, we'd need to know the DB type as well.

To create a random but unique value within a small range here are some ideas I know of:
Create a new random value and try to insert it.
Let a database constraint catch violations. This column should also likely be indexed. The DML may need to be tried several times until a unique ID is found. This will lead to more collisions as time progresses, as noted (see the birthday problem).
Create a "free IDs" table ahead of time and on usage mark the ID as being used (or delete it from the "free IDs" table). This is similar to #1 but shifts when the work is done.
This allows the work of finding "free IDs" to be done at another time, perhaps during a cron job, so that there will not be a contraint violation during the insert keeping the insert itself the "same speed" throughout the usage of said domain. Make sure to use transactions.
Create a 1-to-1/injective "mixer" function such that the output "appears random". The point is this function must be 1-to-1 to inherently avoid duplicates.
This output number would then be "base 36 encoded" (which is also injective); but it would be guaranteed unique as long as the input (say, an auto-increment PK) was unique. This would likely be less random than the other approaches, but should still create a nice-looking non-linear output.
A custom injective function can be created around an 8-bit lookup table fairly trivially - just process a byte at a time and shuffle the map appropriately. I really like this idea, but it can still lead to somewhat predictable output
To find free IDs, approaches #1 and #2 above can use "probing with IN" to minimize the number of SQL statements used. That is, generate a bunch of random values and query for them using IN (keeping in mind what sizes of IN your database likes) and then see which values were free (as having no results).
To create a unique ID not constained to such a small space, a GUID or even hashing (e.g. SHA1) might be useful. However, these only guarantee uniqueness because they have 126/160-bit spaces so that the chance of collision (for different input/time-space) is currently accepted as improbable.
I actually really like the idea of using an injective function. Bearing in mind that it is not good "random" output, consider this pseudo-code:
byte_map = [0..255]
map[0] = shuffle(byte_map, seed[0])
..
map[n] = shuffle(byte_map, seed[1])
output[0] = map[0][input[0]]
..
output[n] = map[n][input[n]]
output_str = base36_encode(output[0] .. output[n])
While a very simple setup, numbers like 0x200012 and 0x200054 will still share common output - e.g. 0x1942fe and 0x1942a9 - although the lines will be changed a bit due to the later application of the base-36 encoding. This could probably be further improved to "make it look more random".

For efficient usage, try caching generated code in a HashSet<String> in your application:
HashSet<String> codes = new HashSet<String>();
This way you don't have to make a db call every time to check whether the generated code is unique or not. All you have to do is:
codes.contains(newCode);
And, yes, you should synchronize your method which updates the cache
public synchronize String getCode ()
{
String newCode = "";
do {
newCode = nextString();
}
while(codes.contains(newCode));
codes.put(newCode);
}

You mentioned in your comments that the relationship between id and code should not be easily guessable. For this you basically need encryption; there are plenty of encryption programs and modules out there that will perform encryption for you, given a secret key that you initially generate. To employ this approach, I would recommend converting your id into ascii (i.e., representing as base-256, and then interpreting each base-256 digit as a character) and then running the encryption, and then converting the encrypted ascii (base-256) into base 36 so you get your alpha-numeric, and then using 6 randomly chosen locations in the base 36 representation to get your code. You can resolve collisions e.g. by just choosing the nearest unused 6-digit alpha-numeric code when a collision occurs, and noting the re-assigned alpha-numeric code for the id in a (code <-> id) table that you will have to maintain anyway since you cannot decrypt directly if you only store 6 base-36 digits of the encrypted id.