Generate a random alpha numeric string

import java.security.SecureRandom;
import java.util.Locale;
import java.util.Objects;
import java.util.Random;
 
public class TestJava {
 
	public static void main(String[] args) {
		RandomString randomString = new RandomString();
 
	    System.out.println(randomString.nextString());
	}
 
}
 
class RandomString {
 
    //Generate a random string.
    public String nextString() {
        for (int idx = 0; idx < buf.length; ++idx)
            buf[idx] = symbols[random.nextInt(symbols.length)];
        return new String(buf);
    }
 
    public static final String upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
 
    public static final String lower = upper.toLowerCase(Locale.ROOT);
 
    public static final String digits = "0123456789";
 
    public static final String alphanum = upper + lower + digits;
 
    private final Random random;
 
    private final char[] symbols;
 
    private final char[] buf;
 
    public RandomString(int length, Random random, String symbols) {
        if (length < 1) throw new IllegalArgumentException();
        if (symbols.length() < 2) throw new IllegalArgumentException();
        this.random = Objects.requireNonNull(random);
        this.symbols = alphanum.toCharArray();
        this.buf = new char[length];
    }
 
    //Create an alphanumeric string generator.
    public RandomString(int length, Random random) {
        this(length, random, alphanum);
    }
 
    //Create an alphanumeric strings from a secure generator.
    public RandomString(int length) {
        this(length, new SecureRandom());
    }
 
    //Create session identifiers.
    public RandomString() {
        this(21);
    }
 
}

import java.util.UUID;
 
public class TestJava {
 
	public static void main(String[] args) {
	    String uuid = UUID.randomUUID().toString();	    
	    System.out.println(uuid);
	}
 
}