Java generating non-repeating random numbers

Question

I want to create a set of random numbers without duplicates in Java   For example I have an array to store 10 000 random integers from 0 to 9999   Here is what I have so far   import java util Random  public class Sort       public static void main String   args            int   nums   new int 10000            Random randomGenerator   new Random             for  int i   0  i  lt  nums length    i               nums i    randomGenerator nextInt 10000                       But the above code creates duplicates   How can I make sure the random numbers do not repeat

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public class Randoms    static int z  a   1111  b   9999  r   public static void main String     args             rand           public static void rand          Random ran   new Random        for  int i   1  i    1  i              z   ran nextInt b - a   1    a          System out println z           randcheck             private static void randcheck          for  int i   3  i  gt   0  i--            if  z    0                r   z   10              arr i    r              z   z   10                      for  int i   0  i  lt   3  i              for  int j   i   1  j  lt   3  j                  if  arr i     arr j                     rand

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In Java 8  if you want to have a list of non-repeating N random integers in range  a  b   where b is exclusive  you can use something like this   Random random   new Random    List lt Integer gt  randomNumbers   random ints a  b  distinct   limit N  boxed   collect Collectors toList

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public class RandomNum       public static void main String   args            Random rn   new Random            HashSet lt Integer gt  hSet   new HashSet lt  gt             while hSet size      1000                hSet add rn nextInt 1000                      System out println hSet

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HashSet lt Integer gt hashSet new HashSet lt  gt     Random random   new Random      now add random number to this set while true        hashSet add random nextInt 1000        if hashSet size    1000          break

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Integer   arr          Collections shuffle Arrays asList arr      For example   public static void main String   args        Integer   arr   new Integer 1000       for  int i   0  i  lt  arr length  i              arr i    i            Collections shuffle Arrays asList arr        System out println Arrays toString arr

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Achintya Jha has the right idea here  Instead of thinking about how to remove duplicates  you remove the ability for duplicates to be created in the first place   If you want to stick with an array of ints and want to randomize their order  manually  which is quite simple  follow these steps    create array of size n  loop through and initialize each value at index i to the value i  or i 1 if you wish to have the numbers 1 to n rather than 0 to n-1   finally  loop through the array again swapping each value for a value at a random index    Your code could be modified to look like this   import java util Random   public class Sort          use a constant rather than having the  magic number  10000 scattered about     public static final int N   10000       public static void main String   args                  array to store N random integers  0 - N-1          int   nums   new int N               initialize each value at index i to the value i          for  int i   0  i  lt  nums length    i                        nums i    i                     Random randomGenerator   new Random            int randomIndex     the randomly selected index each time through the loop         int randomValue     the value at nums randomIndex  each time through the loop             randomize order of values         for int i   0  i  lt  nums length    i                            select a random index              randomIndex   randomGenerator nextInt nums length                    swap values              randomValue   nums randomIndex                nums randomIndex    nums i                nums i    randomValue                      And if I were you I would likely break each of these blocks into separate  smaller methods rather than having one large main method   Hope this helps

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If you re using JAVA 8 or more than use stream functionality following way  Stream generate    - gt   new Random    nextInt 10000   distinct   limit 10000

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A simple algorithm that gives you random numbers without duplicates can be found in the book Programming Pearls p  127   Attention  The resulting array contains the numbers in order  If you want them in random order  you have to shuffle the array  either with Fisher   Yates shuffle or by using a List and call Collections shuffle     The benefit of this algorithm is that you do not need to create an array with all the possible numbers and the runtime complexity is still linear O n    public static int   sampleRandomNumbersWithoutRepetition int start  int end  int count        Random rng   new Random         int   result   new int count       int cur   0      int remaining   end - start      for  int i   start  i  lt  end  amp  amp  count  gt  0  i              double probability   rng nextDouble            if  probability  lt    double  count     double  remaining                count--              result cur      i                    remaining--            return result

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Here we Go   public static int getRandomInt int lower  int upper        if lower  gt  upper  return 0      if lower    upper  return lower      int difference   upper - lower      int start   getRandomInt           nonneg int in the range 0  difference - 1     start   Math abs start     difference 1        start    lower      return start     public static void main String   args        List lt Integer gt  a  new ArrayList         int i      int c 0      for               c            i  getRandomInt 100  500000           if   a contains i                  a add i               if  c    10000  break              System out println i                          for int rand   a            System out println rand                Get Random number  Returns a random integer x satisfying lower  lt   x  lt   upper  If lower   upper  returns 0   param lower  param upper  return  In the main method I created list then i check if the random number exist on the list if it doesn t exist i will add the random number to the list

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If you need generate numbers with intervals  it can be just like that   Integer   arr   new Integer   int   Math random      16 - 30    30     for  int i   0  i  lt  arr length  i      arr i    i    Collections shuffle Arrays asList arr    System out println Arrays toString arr       The result    1  10  2  4  9  8  7  13  18  17  5  21  12  16  23  20  6  0  22  14  24            15  3  11  19   Note    If you need that the zero does not leave you could put an  if

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How about this   LinkedHashSet lt Integer gt  test   new LinkedHashSet lt Integer gt     Random random   new Random    do      test add random nextInt 1000    1    while test size      1000     The user can then iterate through the Set using a for loop

[java] Java generating non-repeating random numbers

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