有一组优惠 A－20优惠， B－30元优惠， C－5元优惠  当用户满足条件时随机返回一个优惠；在活动预算一定的情况下需要保证C类优惠券中奖概率最高，B类优惠券最低；以下是权重的实现；    

/**
 * Copyright 2014-2015, , Inc. All Rights Reserved.
 * 
 * Date: 2016年1月23日
 */

package org.demo.core;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;

/***
 * 
 * Desc:TODO
 * 
 * @author zhangwei<[email protected]>
 * @since 2016年1月23日 下午1:35:16
 * @version v 0.1
 */
public class WeightRandomTest {
	private static Map<Double, String> weightMapping = new HashMap<>();

	static {
		weightMapping.put(30d, "A");
		weightMapping.put(20D, "B");
		weightMapping.put(50D, "C");
	}

	public static void main(String[] args) {
		int c = 0;
		int a = 0;
		int b = 0;
		for (int i = 0; i < 2000000; i++) {
			String str = WeightRandomUtil.getWeightRandom(weightMapping);
			if ("A".equals(str)) {
				a++;
			} else if ("B".equals(str)) {
				b++;
			} else if ("C".equals(str)) {
				c++;
			}
		}

		System.out.println(c / 2000000d);
		System.out.println(b / 2000000d);
		System.out.println(a / 2000000d);
	}
}

class WeightRandomUtil {

	/***
	 * 计算权重总和
	 * 
	 * @param weightArrays
	 * @return
	 * @author zhangwei<[email protected]>
	 */
	private static double weightSum(Set<Double> weights) {
		double weightSum = 0;
		for (double weightValue : weights) {
			weightSum += weightValue;
		}
		return weightSum;
	}

	/***
	 * 
	 * 
	 * @param weightArrays
	 *            权重数组
	 * @return 返回数据
	 * @author zhangwei<[email protected]>
	 * @param <T>
	 */
	public static <T> T getWeightRandom(Map<Double, T> weightValueMapping) {
		double weightSum = weightSum(weightValueMapping.keySet());
		double stepWeightSum = 0;
		List<Double> list = new ArrayList<>(weightValueMapping.keySet());
		Collections.sort(list, new Comparator<Double>() {

			@Override
			public int compare(Double o1, Double o2) {
				return (int) (o2 - o1);
			}

		});
		double r = Math.random();
		for (double weight : list) {
			// 计算权重值
			stepWeightSum += weight;
			// 如果随机数落在了权重区间则返回索引值
			if (r <= stepWeightSum / weightSum) {
				return weightValueMapping.get(weight);
			}
		}
		return null;
	}
}