How to Get Random Elements from Java HashSet?

07 Jan, 2021
Unlike List classes, the HashSet class does not provide any methods using which we can get the elements using their index. It makes it difficult to get random elements from it using the index.
We need to get random elements from HashSet, which can be done by either of the two ways:
By converting it to an arrayUsing an Iterator or a for loop
By converting it to an array
Using an Iterator or a for loop
Example:
Input:


hs.add(11);
hs.add(24);
hs.add(34);
hs.add(43);
hs.add(55);
hs.add(66);
hs.add(72);
hs.add(80);
hs.add(99);


Output:

Random element: 99
Method 1: By converting to an array.
Firstly convert HashSet into an array and then access the random element from it.
Then we will create an object of Random class and will call the nextInt() method of that class which will give us any random number less than or equal to the size of the HashSet.
And then using an array we will simply print the element present at that index.
Java
// Java program to get random elements from HashSet// using an array  import java.io.*;import java.util.*;  class GFG {    public static void main(String[] args)    {        // creating the HashSet        Set<Integer> hs = new HashSet<Integer>();          hs.add(11);        hs.add(24);        hs.add(34);        hs.add(43);        hs.add(55);        hs.add(66);        hs.add(72);        hs.add(80);        hs.add(99);          // convert HashSet to an array        Integer[] arrayNumbers = hs.toArray(new Integer[hs.size()]);          // generate a random number        Random rndm = new Random();          // this will generate a random number between 0 and        // HashSet.size - 1        int rndmNumber = rndm.nextInt(hs.size());          // get the element at random number index        System.out.println("Random element: "                           + arrayNumbers[rndmNumber]);    }}
Random element: 11
Method 2: Using an Iterator or a for loop
In order to get random elements from the HashSet object, we need to generate a random number between 0 (inclusive) and the size of the HashSet (exclusive).
And then iterate through the set till we reach the element located at the random number position as given below.
In this approach, we will get the element at a random index using an Iterator.
Java
// Java program to get random elements from HashSet// using an Iterator  import java.io.*;import java.util.*;  class GFG {    public static void main(String[] args)    {        Set<Integer> hs = new HashSet<Integer>();          hs.add(11);        hs.add(24);        hs.add(34);        hs.add(43);        hs.add(55);        hs.add(66);        hs.add(72);        hs.add(80);        hs.add(99);          System.out.println("Random element: "                           + getRandomElement(hs));    }      private static <E>        E getRandomElement(Set<? extends E> set)    {          Random random = new Random();          // Generate a random number using nextInt        // method of the Random class.        int randomNumber = random.nextInt(set.size());          Iterator<? extends E> iterator = set.iterator();          int currentIndex = 0;        E randomElement = null;          // iterate the HashSet        while (iterator.hasNext()) {              randomElement = iterator.next();              // if current index is equal to random number            if (currentIndex == randomNumber)                return randomElement;              // increase the current index            currentIndex++;        }          return randomElement;    }}
Random element: 99
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