如何生成不重复的订单号？这里提供一个不重复订单号生成方法

import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

/**
 * 订单号生成器
 */
public class OrderNoGenerator {
    private int size;
    private int length;
    private Set<String> orderNos;

    /**
     * 订单号生成器
     * 
     * 为了保证生成性能需满足条件size<10^length/4
     *
     * @param size   保证连续不重读的数
     * @param length 生成随机数的长度
     */
    public OrderNoGenerator(int size, int length) {
        this.size = size;
        this.length = length;
        AssertUtils.requireTrue(size < Math.pow(10, length) / 4, "参数不符合要求");
        orderNos = Collections.synchronizedSet(new HashSet());
    }


    /**
     * 获取不重复的随机数
     *
     * @return
     */
    public String generatorOrderNo() {
        if (orderNos.size() < size) {
            more();
        }
        String next = orderNos.iterator().next();
        orderNos.remove(next);
        return next;
    }

    private void more() {
        for (int i = 0; i < size * 4; i++) {
            orderNos.add(RandomUtils.randomNumber(length));
        }
    }

}

/**
 * 随机数工具
 */
public class RandomUtils {

    public static int[] nums = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    public static Random random = new Random();

    /**
     * 生成随机数字
     *
     * @param length 随机数长度
     * @return
     */
    public static String randomNumber(int length) {
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < length; i++) {
            int index = random.nextInt(10);
            builder.append(nums[index]);
        }
        return builder.toString();
    }
}

public class AssertUtils {

    public static void requireTrue(boolean value, String msg) {
        if (!value) {
            throw new RuntimeException(msg);
        }
    }

}