How to generate a random String in Java

Question

I have an object called Student  and it has studentName  studentId  studentAddress  etc  For the studentId  I have to generate random string consist of seven numeric charaters  eg   studentId   getRandomId    studentId    1234567   lt -- from the random generator    And I have to make sure that there is no duplicate id

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Random ran   new Random    int top   3  char data        String dat        for  int i 0  i lt  top  i        data    char  ran nextInt 25  97     dat   data   dat     System out println dat

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This is very nice    http   commons apache org proper commons-lang apidocs org apache commons lang3 RandomStringUtils html - something like RandomStringUtils randomNumeric 7     There are 10 7 equiprobable  if java util Random is not broken  distinct values so uniqueness may be a concern

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The first question you need to ask is whether you really need the ID to be random  Sometime  sequential IDs are good enough   Now  if you do need it to be random  we first note a generated sequence of numbers that contain no duplicates can not be called random   p Now that we get that out of the way  the fastest way to do this is to have a Hashtable or HashMap containing all the IDs already generated  Whenever a new ID is generated  check it against the hashtable  re-generate if the ID already occurs  This will generally work well if the number of students is much less than the range of the IDs  If not  you re in deeper trouble as the probability of needing to regenerate an ID increases  P generate new ID    number of id already generated   number of all possible ids  In this case  check back the first paragraph  do you need the ID to be random     Hope this helps

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You can also use UUID class from java util package  which returns random uuid of 32bit characters String    java util UUID randomUUID   toString    http   java sun com j2se 1 5 0 docs api java util UUID html

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Many possibilities     You know how to generate randomly an integer right  You can thus generate a char from it     ex 65 -  A   It depends what you need  the level of randomness  the security involved    but for a school project i guess getting UUID substring would fit

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I think the following class code will help you  It supports multithreading but you can do some improvement like remove sync block and and sync to getRandomId   method   public class RandomNumberGenerator    private static final Set lt String gt  generatedNumbers   new HashSet lt String gt      public RandomNumberGenerator        public static void main String   args        final int maxLength   7      final int maxTry   10       for  int i   0  i  lt  10  i              System out println i      studentId     RandomNumberGenerator getRandomId maxLength  maxTry             public static String getRandomId final int maxLength  final int maxTry        final Random random   new Random System nanoTime         final int max    int  Math pow 10  maxLength       final int maxMin    int  Math pow 10  maxLength-1       int i   0      boolean unique   false      int randomId   -1      while  i  lt  maxTry            randomId   random nextInt max - maxMin - 1    maxMin           synchronized  generatedNumbers                if  generatedNumbers contains randomId     false                    unique   true                  break                                  i              if  unique    false            throw new RuntimeException  Cannot generate unique id                synchronized  generatedNumbers            generatedNumbers add String valueOf randomId               return String valueOf randomId

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Generating a random string of characters is easy - just use java util Random and a string containing all the characters you want to be available  e g   public static String generateString Random rng  String characters  int length        char   text   new char length       for  int i   0  i  lt  length  i                  text i    characters charAt rng nextInt characters length                return new String text       Now  for uniqueness you ll need to store the generated strings somewhere  How you do that will really depend on the rest of your application

[java] How to generate a random String in Java

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