import java.util.UUID;
public class RandomStringGenerator {
    public static void main(String[] args) {
        String randomString = usingUUID();
        System.out.println("Random string is: " + randomString);
        System.out.println("Random string of 8 characters is: "
            + randomString.substring(0, 8));
    }
    static String usingUUID() {
        UUID randomUUID = UUID.randomUUID();
        return randomUUID.toString().replaceAll("-", "");
    }
}

import java.util.Math;
public class RandomStringGenerator {
    public static void main(String[] args) {
        String randomString = usingMath(10);
        System.out.println("Random string is: " + randomString);
    }
    static String usingMath(int length) {
        String alphabetsInUpperCase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        String alphabetsInLowerCase = "abcdefghijklmnopqrstuvwxyz";
        String numbers = "0123456789";
        // create a super set of all characters
        String allCharacters = alphabetsInLowerCase + alphabetsInUpperCase + numbers;
        // initialize a string to hold result
        StringBuffer randomString = new StringBuffer();
        // loop for 10 times
        for (int i = 0; i < length; i++) {
            // generate a random number between 0 and length of all characters
            int randomIndex = (int)(Math.random() * allCharacters.length());
            // retrieve character at index and add it to result
            randomString.append(allCharacters.charAt(randomIndex));
        }
        return randomString.toString();
    }
}

import java.util.Random;
public class RandomStringGenerator {
    public static void main(String[] args) {
        String randomString = usingRandom(10);
        System.out.println("Random string is: " + randomString);
    }
    static String usingRandom(int length) {
        String alphabetsInUpperCase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        String alphabetsInLowerCase = "abcdefghijklmnopqrstuvwxyz";
        String numbers = "0123456789";
        // create a super set of all characters
        String allCharacters = alphabetsInLowerCase + alphabetsInUpperCase + numbers;
        // initialize a string to hold result
        StringBuffer randomString = new StringBuffer();
        // loop for 10 times
        for (int i = 0; i < length; i++) {
            // generate a random number between 0 and length of all characters 
            int randomIndex = random.nextInt(allCharacters.length());
            // retrieve character at index and add it to result
            randomString.append(allCharacters.charAt(randomIndex));
        }
        return randomString.toString();
   }
}

import org.apache.commons.lang.RandomStringUtils;
public class RandomStringGenerator {
    public static void main(String[] args) {
        // generate a random string of 10 alphabets
        String randomString = RandomStringUtils.randomAlphabetic(10);
        System.out.println("Random string of 10 alphabets: " + randomString);
        randomString = RandomStringUtils.randomNumeric(10);
        System.out.println("Random string of 10 numbers: " + randomString);
        randomString = RandomStringUtils.randomAlphanumeric(10);
        System.out.println("Random string of 10 alphanumeric characters: " + randomString);
   }
}

<dependency>
   <groupId>org.apache.commons</groupId>
   <artifactId>commons-lang3</artifactId>
   <version>3.9</version>
</dependency>