java calculate probability

rates is the probability of winning, for example: 0.3, 0.5, 0.2

package com.ddb.activity.base.util;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class LotteryProbabilityUtil {
	
	public static void main(String[] args) {
		List<BigDecimal> rates=new ArrayList<BigDecimal>();
		rates.add(new BigDecimal("0.01"));
		rates.add(new BigDecimal("0.2"));
		//rates.add(new BigDecimal("0.3"));
		//rates.add(new BigDecimal("0.2"));
		rates.add(new BigDecimal("0.2"));
		rates.add(new BigDecimal("0.09"));
//		int j=0;
		for(int i=0;i<10;i++){
			int a=LotteryProbabilityUtil.getRandomIndex(rates);
			System.out.println(a);
			if(a==0){
//				System.out.println(++j);
			}
//			
		}
	}
	/**
	 * Calculate the probability
	 * @param rates
	 * @return the drawn position
	 */
	public static int getRandomIndex(List<BigDecimal> rates) {
		Random random = new Random();
		BigDecimal ranNum = new BigDecimal(random.nextDouble());
		BigDecimal sumRate = new BigDecimal("0.0");
		for(BigDecimal rate:rates) {
			sumRate = sumRate.add(rate);
		}
		BigDecimal minVal = new BigDecimal("0.0");
		BigDecimal maxVal = new BigDecimal("0.0");
		for (int i = 0; i < rates.size(); i++) {
			BigDecimal rangeVal = rates.get(i).divide(sumRate, BigDecimal.ROUND_HALF_EVEN);
			maxVal = minVal.add(rangeVal);
			boolean exp1 = false;
			if(i == 0) {
				exp1 = (minVal.compareTo (ranNum) <= 0);
			} else {
				exp1 = (minVal.compareTo (ranNum) <0);
			}
			if(exp1&&(ranNum.compareTo(maxVal)<=0)) {
				return i;
			}
			minVal = minVal.add (rangeVal);
		}
		throw new RuntimeException("Get Random Index Error");
	}
}