There are a few ways to generate a random alphanumeric string in Java. Here are three methods:

Method 1: Using the Random class

The Random class provides a method called nextInt() that can be used to generate a random integer. To generate a random alphanumeric string, you can first generate a random integer between 0 and 99. Then, you can use the integer to index into a character array that contains all the alphanumeric characters. For example, the following code generates a random alphanumeric string of length 10:

Java

import java.util.Random;

public class RandomAlphanumericStringGenerator {

  public static void main(String[] args) {
    Random random = new Random();
    char[] characters = "abcdefghijklmnopqrstuvwxyz0123456789".toCharArray();
    String randomString = "";
    for (int i = 0; i < 10; i++) {
      randomString += characters[random.nextInt(99)];
    }
    System.out.println(randomString);
  }
}

Method 2: Using the UUID class

The UUID class provides a method called randomUUID() that generates a random UUID. A UUID is a 128-bit number that is guaranteed to be unique. To generate a random alphanumeric string, you can simply convert the UUID to a string. For example, the following code generates a random alphanumeric string of length 32:

Java

import java.util.UUID;

public class RandomAlphanumericStringGenerator {

  public static void main(String[] args) {
    UUID uuid = UUID.randomUUID();
    String randomString = uuid.toString();
    System.out.println(randomString);
  }
}

Method 3: Using the Apache Commons Lang library

The Apache Commons Lang library provides a class called RandomStringUtils that can be used to generate random strings. The RandomStringUtils class has a method called randomAlphanumeric() that can be used to generate a random alphanumeric string of the specified length. For example, the following code generates a random alphanumeric string of length 10 using the Apache Commons Lang library:

Java

import org.apache.commons.lang3.RandomStringUtils;

public class RandomAlphanumericStringGenerator {

  public static void main(String[] args) {
    String randomString = RandomStringUtils.randomAlphanumeric(10);
    System.out.println(randomString);
  }
}

Which method you use to generate a random alphanumeric string depends on your specific needs. If you need a random string that is guaranteed to be unique, then you should use the UUID class. If you need a random string that can be any length, then you should use the RandomStringUtils class. If you need a random string that is compatible with a specific character set, then you can use the Random class and specify the character set.