Java generate sequence of random characters or numbers or strings

import java.util.Random;

/**
 * Created by Pritom K Mondal.
 */
public class RandomString {
    public static String formatAsLength(Long number, Integer minLength = 0) {
        return String.format("%0${minLength}d%n".toString(), number).trim();
    }

    private static final char[] symbols = new char[62];
    private static final char[] chars = new char[52];
    private static final char[] numbers = new char[10];

    static {
        initialize();
    }

    public static void initialize() {
        Integer symIdx = 0, numIdx = 0, charIdx = 0;
        for (int idx = 48; idx <= 57; ++idx) {
            symbols[symIdx++] = (char) (idx);
            numbers[numIdx++] = (char) (idx);
        }
        for (int idx = 97; idx < 26 + 97; ++idx) {
            symbols[symIdx++] = (char) (idx);
            chars[charIdx++] = (char) (idx);
        }
        for (int idx = 65; idx < 26 + 65; ++idx) {
            symbols[symIdx++] = (char) (idx);
            chars[charIdx++] = (char) (idx);
        }
    }

    private static Random random = new Random();

    public static String nextString(int length) {
        if (length < 1) {
            throw new IllegalArgumentException("length < 1: " + length);
        }
        if (symbols[0] == (char) 0) {
            initialize();
        }
        String returnString = "";
        for (int idx = 0; idx < length; ++idx) {
            returnString += symbols[random.nextInt(symbols.length)].toString()
        }
        return returnString;
    }

    public static String nextNumber(int length) {
        if (length < 1) {
            throw new IllegalArgumentException("length < 1: " + length);
        }
        if (symbols[0] == (char) 0) {
            initialize();
        }
        char[] buf = new char[length];
        for (int idx = 0; idx < buf.length; ++idx) {
            buf[idx] = numbers[random.nextInt(numbers.length)];
        }
        return new String(buf);
    }

    public static String nextCharacter(int length) {
        if (length < 1) {
            throw new IllegalArgumentException("length < 1: " + length);
        }
        if (symbols[0] == (char) 0) {
            initialize();
        }
        char[] buf = new char[length];
        for (int idx = 0; idx < buf.length; ++idx) {
            buf[idx] = chars[random.nextInt(chars.length)];
        }
        return new String(buf);
    }
}

Usage:

String mixing = RandomString.nextString(40);
String number = RandomString.nextNumber(40);
String chars = RandomString.nextCharacter(40);
System.out.println("MIXING: " + mixing);
System.out.println("NUMBER: " + number);
System.out.println("CHARS:  " + chars);

Output:

MIXING: ZP8RQCpEF7Kr9veJDlbUWafqNxjqh4xwJr3bvw9J
NUMBER: 5578864807912590082652398553220665093932
CHARS:  nZvejdXyiMEzzbeuJyVTpaGwmGykqNVMQLAckAfP