Create an random string in Java

RandomString.java

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

/**
 * Random String Generator.
 *
 * <code>new RandomString().nextString()</code>
 *
 * from https://stackoverflow.com/questions/41107/how-to-generate-a-random-alpha-numeric-string 
 */
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 = symbols.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);
  }

}