Java random long number in 0 x n range

Question

Random class has a method to generate random int in a given range  For example   Random r   new Random     int x   r nextInt 100     This would generate an int number more or equal to 0 and less than 100  I d like to do exactly the same with long number   long y   magicRandomLongGenerator 100     Random class has only nextLong    but it doesn t allow to set range

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The below Method will Return you a value between 10000000000 to 9999999999   long min   1000000000L long max   9999999999L      public static long getRandomNumber long min  long max        Random random   new Random                 return random nextLong      max - min    max

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How about this   public static long nextLong  NonNull Random r  long min  long max        if  min  gt  max          throw new IllegalArgumentException  min gt max        if  min    max          return min      long n   r nextLong          abs  use instead of Math abs  which might return min value        n   n    Long MIN VALUE   0   n  lt  0   -n   n        limit to range      n   n    max - min       return min   n

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Starting from Java 7  or Android API Level 21   5 0   you could directly use ThreadLocalRandom current   nextLong n   for 0   x  lt  n  and ThreadLocalRandom current   nextLong m  n   for m   x  lt  n   See  Alex s answer for detail     If you are stuck with Java 6  or Android 4 x  you need to use an external library  e g  org apache commons math3 random RandomDataGenerator getRandomGenerator   nextLong 0  n-1   see  mawaldne s answer   or implement your own nextLong n    According to https   docs oracle com javase 1 5 0 docs api java util Random html nextInt is implemented as   public int nextInt int n         if  n lt  0                  throw new IllegalArgumentException  n must be positive          if   n  amp  -n     n      i e   n is a power of 2          return  int   n    long next 31    gt  gt  31         int bits  val       do            bits   next 31            val   bits   n         while bits - val    n-1   lt  0        return val       So we may modify this to perform nextLong   long nextLong Random rng  long n          error checking and 2 x checking removed for simplicity     long bits  val     do         bits    rng nextLong    lt  lt  1   gt  gt  gt  1        val   bits   n       while  bits-val  n-1   lt  0L      return val

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From the page on Random      The method nextLong is implemented by class Random  as if by   public long nextLong        return   long next 32   lt  lt  32    next 32           Because class Random uses a seed with only 48 bits  this algorithm will not return all possible long values    So if you want to get a Long  you re already not going to get the full 64 bit range   I would suggest that if you have a range that falls near a power of 2  you build up the Long as in that snippet  like this   next 32      long nextInt 8   lt  lt  3    to get a 35 bit range  for example

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Further improving kennytm s answer  A subclass implementation taking the actual implementation in Java 8 into account would be   public class MyRandom extends Random     public long nextLong long bound        if  bound  lt   0          throw new IllegalArgumentException  bound must be positive               long r   nextLong    amp  Long MAX VALUE      long m   bound - 1L      if   bound  amp  m     0       i e   bound is a power of 2       r    bound   r   gt  gt   Long SIZE - 1         else         for  long u   r  u -  r   u   bound    m  lt  0L  u   nextLong    amp  Long MAX VALUE             return r

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The methods above work great  If you re using apache commons  org apache commons math random   check out RandomData  It has a method  nextLong long lower  long upper    http   commons apache org math userguide random html  http   commons apache org math api-1 1 org apache commons math random RandomData html nextLong long  20long

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use  system time as seed value to get a good random number                 Random random   new Random System currentTimeMillis                   long x               do                  x random nextLong                  while x lt 0  amp  amp  x  gt  n        Loop until get a number greater or equal to 0 and smaller than n

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If you want a uniformly distributed pseudorandom long in the range of  0 m   try using the modulo operator and the absolute value method combined with the nextLong   method as seen below   Math abs rand nextLong      m    Where rand is your Random object   The modulo operator divides two numbers and outputs the remainder of those numbers  For example  3   2 is 1 because the remainder of 3 and 2 is 1   Since nextLong   generates a uniformly distributed pseudorandom long in the range of  - 2 48  2 48   or somewhere in that range   you will need to take the absolute value of it  If you don t  the modulo of the nextLong   method has a 50  chance of returning a negative value  which is out of the range  0 m    What you initially requested was a uniformly distributed pseudorandom long in the range of  0 100   The following code does so   Math abs rand nextLong      100

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From Java 8 API   It could be easier to take actual implementation from API doc https   docs oracle com javase 8 docs api java util Random html longs-long-long-long- they are using it to generate longs stream  And your origin can be  0  like in the question   long nextLong long origin  long bound      long r   nextLong      long n   bound - origin  m   n - 1    if   n  amp  m     0L      power of two     r    r  amp  m    origin    else if  n  gt  0L        reject over-represented candidates     for  long u   r  gt  gt  gt  1                ensure nonnegative          u   m -  r   u   n   lt  0L        rejection check          u   nextLong    gt  gt  gt  1     retry               r    origin        else                   range not representable as long     while  r  lt  origin    r  gt   bound        r   nextLong          return r

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The standard method to generate a number  without a utility method  in a range is to just use the double with the range   long range   1234567L  Random r   new Random   long number    long  r nextDouble   range     will give you a long between 0  inclusive  and range  exclusive   Similarly if you want a number between x and y   long x   1234567L  long y   23456789L  Random r   new Random   long number   x   long  r nextDouble    y-x       will give you a long from 1234567  inclusive  through 123456789  exclusive   Note  check parentheses  because casting to long has higher priority than multiplication

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public static long randomLong long min  long max        try               Random  random    new Random            long    result    min    long   random nextDouble      max - min            return  result            catch  Throwable t   t printStackTrace         return 0L

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If you can use java streams  you can try the following   Random randomizeTimestamp   new Random    Long min   ZonedDateTime parse  2018-01-01T00 00 00 000Z   toInstant   toEpochMilli    Long max   ZonedDateTime parse  2019-01-01T00 00 00 000Z   toInstant   toEpochMilli    randomizeTimestamp longs generatedEventListSize  min  max  forEach timestamp - gt      System out println timestamp         This will generate numbers in the given range for longs

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ThreadLocalRandom  ThreadLocalRandom has a nextLong long bound  method   long v   ThreadLocalRandom current   nextLong 100     It also has nextLong long origin  long bound  if you need an origin other than 0  Pass the origin  inclusive  and the bound  exclusive    long v   ThreadLocalRandom current   nextLong 10 100      For 2-digit integers  10-99 inclusive    SplittableRandom has the same nextLong methods and allows you to choose a seed if you want a reproducible sequence of numbers

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import java util        Random rnd   new Random         long name   Math abs rnd nextLong       This should work

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The methods using the r nextDouble   should use   long number    long   rand nextDouble   max     long number   x    long r nextDouble     y-x

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Use the     operator  resultingNumber    r nextLong      maximum - minimum     minimum    By using the     operator  we take the remainder when divided by your maximum value  This leaves us with only numbers from 0  inclusive  to the divisor  exclusive    For example   public long randLong long min  long max        return  new java util Random   nextLong      max - min     min

[java] Java: random long number in 0 <= x < n range

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