Core Java

Java Set Draw Example

In Java, when dealing with collections, it’s often useful to retrieve a random element – draw – from a Set. Java provides the java.util.Random class and, ThreadLocalRandom class in the java.util.concurrent. In this article, we’ll explore different methods to get a random element from a Java Set.

1. The Set Interface in Java

A Set is a collection of elements without duplicates. The Set interface is part of the Java collections library and is implemented by classes such as HashSet, LinkedHashSet, and TreeSet. A Set is suitable for scenarios where you need to store and manage unique values.

2. Getting a Random Element Using the Random Class

To get a random element from a Set in Java, we can use the java.util.Random class to generate random numbers and then use that to retrieve a random element from the Set. Below is an example:

public class RandomElementFromSet {

    public static void main(String[] args) {
        
       // Create a HashSet
        Set<String> fruits = new HashSet<>();
        fruits.add("orange");
        fruits.add("banana");
        fruits.add("apple");
        fruits.add("guava");
        fruits.add("pineapple");

        // Get a random element from the set
        String randomFruit = getRandomElement(fruits);

        // Print the random element
        System.out.println("Random Fruit: " + randomFruit);
    }

    private static <T> T getRandomElement(Set<T> set) {
        if (set.isEmpty()) {
            return null;
        }

        // Convert the set to an array
        T[] array = (T[]) set.toArray();

        // Use Random to get a random index
        Random random = new Random();
        int randomIndex = random.nextInt(array.length);

        // Return the random element
        return array[randomIndex];
    }
}

In this example, we first create a HashSet and then provide a helper method named getRandomElement that takes a Set as an argument. This method first checks if the set is not empty, converts it to an array, and then generates a random index within the range of the set size. Finally, it returns the element at the randomly selected index of the array.

Now, if we run the main class, the result should be a random output as shown below:

Fig 1: output from getting a Random Element from a Set in Java using a Random class
Fig 1: output from getting a Random Element from a Set in Java using a Random class

3. Getting a Random Element Using the ThreadLocalRandom Class

This method is similar to the previous ones but employs the ThreadLocalRandom class to obtain a random index. The ThreadLocalRandom class provides a more efficient and thread-safe alternative to Random.

The code listing below shows how to get a random element from a Set using the ThreadLocalRandom class.

public class RandomSetElementThreadLocal {

    public static void main(String[] args) {
        // Create a Set (e.g., HashSet)
        Set<String> fruits = new HashSet<>();
        fruits.add("orange");
        fruits.add("banana");
        fruits.add("apple");
        fruits.add("guava");
        fruits.add("pineapple");

        // Use ThreadLocalRandom to get a random element
        int randomIndex = ThreadLocalRandom.current().nextInt(fruits.size());

        // Use an iterator to get the element at the random index
        String randomFruit = getRandomElement(fruits, randomIndex);

        // Print the random element
        System.out.println("Random Fruit: " + randomFruit);
    }

    // Helper method to get the element at a specific index in a Set
    private static <T> T getRandomElement(Set<T> set, int index) {
        int currentIndex = 0;
        for (T element : set) {
            if (currentIndex == index) {
                return element;
            }
            currentIndex++;
        }
        throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + set.size());
    }
}

In this example:

  • First, we create a Set and in this example, we use a HashSet.
  • Next, we use the ThreadLocalRandom.current().nextInt(size) method to generate a random index within the size of the set. This index will be used to access a random element later.
  • Next, we created a helper method getRandomElement(Set<T> set, int index) which is used to iterate through the set and retrieve the element at the specified index.
  • And, Finally, in our main class, we use the helper method to retrieve the random element with: String randomElement = getRandomElement(stringSet, randomIndex);.

4. Conclusion

In this article, we explored how to leverage the Random and ThreadLocalRandom classes in Java to retrieve a random element from a Set. We can choose between the traditional Random class and the more modern and thread-safe ThreadLocalRandom class, providing us with solutions for integrating randomness into our applications.

5. Download the Source Code

This was an example of how to retrieve a random element from a draw Set sample in Java.

Download
You can download the full source code of this example here: Retrieve a Random Element from a Draw Set Sample in Java

Omozegie Aziegbe

Omos holds a Master degree in Information Engineering with Network Management from the Robert Gordon University, Aberdeen. Omos is currently a freelance web/application developer who is currently focused on developing Java enterprise applications with the Jakarta EE framework.
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