How to generate a random alpha-numeric string in Java ?

To generate a random alphanumeric string in Java, you can use a combination of characters and a random number generator. Here are two common approaches:

1. Using Random or SecureRandom (Core Java)

This method uses a predefined set of alphanumeric characters and selects characters randomly using Random or SecureRandom:

import java.security.SecureRandom;
import java.util.Random;

public class RandomString {
    // Define alphanumeric characters (62 total)
    private static final String ALPHA_NUMERIC = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

    // Generate a random string of length `length`
    public static String generate(int length) {
        if (length <= 0) throw new IllegalArgumentException("Length must be positive");

        StringBuilder sb = new StringBuilder(length);
        Random random = new SecureRandom(); // Use SecureRandom for security

        for (int i = 0; i < length; i++) {
            // Pick a random index from the ALPHA_NUMERIC string
            int index = random.nextInt(ALPHA_NUMERIC.length());
            sb.append(ALPHA_NUMERIC.charAt(index));
        }

        return sb.toString();
    }

    public static void main(String[] args) {
        System.out.println(generate(10)); // Example: "X8sL3aQ9pZ"
    }
}

2. Using Java 8+ Streams (Concise)

For a more concise approach, use Java 8’s IntStream and Random:

import java.security.SecureRandom;
import java.util.stream.IntStream;

public class RandomString {
    private static final String ALPHA_NUMERIC = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

    public static String generate(int length) {
        if (length <= 0) throw new IllegalArgumentException("Length must be positive");

        SecureRandom random = new SecureRandom();
        return IntStream.range(0, length)
                .map(i -> random.nextInt(ALPHA_NUMERIC.length()))
                .mapToObj(index -> String.valueOf(ALPHA_NUMERIC.charAt(index)))
                .collect(Collectors.joining());
    }

    public static void main(String[] args) {
        System.out.println(generate(10)); // Example: "7GgF2kL9qY"
    }
}

Key Notes

• Security: Use SecureRandom instead of Random for cryptographic safety (e.g., session IDs, tokens).
• Character Set: The ALPHA_NUMERIC string contains 62 characters (uppercase, lowercase, digits).
• Efficiency: Both methods use StringBuilder (or streams) for efficient string concatenation.

Alternative: Apache Commons Lang

If you’re using Apache Commons Lang, simplify with RandomStringUtils:

import org.apache.commons.lang3.RandomStringUtils;

public class Example {
    public static void main(String[] args) {
        String randomString = RandomStringUtils.randomAlphanumeric(10); // Example: "x4Tp9sLm2A"
        System.out.println(randomString);
    }
}

Choose the method that best fits your use case!