java抽奖工具类：按概率抽奖

public class LotteryUtil {

    private List<Continuous> lotteryList;
    private double maxElement;

    /**
     * 得到一个抽奖对象
     * @param map 抽奖概率，key:奖品id，value:概率
     * @param <T>
     * @return
     */
    public static<T> LotteryUtil getInstance(Map<T, Double> map) {
        return new LotteryUtil(map);
    }

    /**
     * 构造抽奖集合
     * @param map 为奖品的概率
     */
    private <T> LotteryUtil(Map<T, Double> map) {
        if(map.size() == 0){
            throw new IllegalArgumentException("集合不能为空！");
        }
        double minElement = 0d;
        lotteryList = new ArrayList<>(map.size());
        for(Map.Entry<T, Double> entry : map.entrySet()) {
            if(null == entry.getValue() || entry.getValue() <= 0) {
                continue;
            }
            minElement = maxElement;
            maxElement = maxElement + entry.getValue();
            Continuous continuous = new Continuous(entry.getKey(), minElement, maxElement);
            lotteryList.add(continuous);
        }
    }

    /**
     * 抽奖操作
     *
     * @return
     */
    public <T>T lottery() {
        Random r = new Random();
        //生成随机数
        double d = r.nextDouble() * maxElement;
        if(d == 0d) {
            // 如果生成了0，就再生成一次
            d = r.nextDouble() * maxElement;
        }
        int size = lotteryList.size();
        for(int i = 0; i < size; i++){
            Continuous continuous = lotteryList.get(i);
            if(continuous.isContainKey(d)){
                return (T)continuous.getVal();
            }
        }
        return null;
    }

    /**
     * 定义一个连续区间
     * 集合中元素x满足:(minElement, maxElement]
     * 数学表达式为：minElement < x <= maxElement
     *
     */
    class Continuous {

        private Object val;
        private double minElement;
        private double maxElement;

        public Continuous(Object val, double minElement, double maxElement) {
            this.val = val;
            this.minElement = minElement;
            this.maxElement = maxElement;
        }

        /**
         * 判断当前集合是否包含特定元素
         * @param element
         * @return
         */
        public boolean isContainKey(double element){
            boolean flag = false;
            if(element > minElement && element <= maxElement){
                flag = true;
            }
            return flag;
        }

        public Object getVal() {
            return val;
        }

        public void setVal(Object val) {
            this.val = val;
        }

        public double getMinElement() {
            return minElement;
        }

        public void setMinElement(double minElement) {
            this.minElement = minElement;
        }

        public double getMaxElement() {
            return maxElement;
        }

        public void setMaxElement(double maxElement) {
            this.maxElement = maxElement;
        }
    }
}

public class LotteryUtilTest {

    @Test
    public void testLottery() {
        Map<Integer, Double> map = new HashMap<>();
        map.put(1, 0.10d);
        map.put(2, 0.10d);
        map.put(3, 0.20d);
        map.put(4, 0.20d);

        Map<Integer, Integer> retMap = new HashMap<>();
        int times = 100000;
        LotteryUtil lotteryUtil = LotteryUtil.getInstance(map);
        for(int i = 0; i < times; i++) {
            Integer result = lotteryUtil.lottery();
            Integer num = retMap.get(result);
            retMap.put(result, null == num ? 1 : num + 1);
        }
        System.out.println(retMap);

        Map<Integer, Double> setRate = new HashMap<>();
        double total = 0d;
        for(Map.Entry<Integer, Double> entry : map.entrySet()) {
            total += entry.getValue();
        }
        for(Map.Entry<Integer, Double> entry : map.entrySet()) {
            setRate.put(entry.getKey(), entry.getValue() / total);
        }
        Map<Integer, Double> realRate = new HashMap<>();
        for(Map.Entry<Integer, Integer> entry : retMap.entrySet()) {
            realRate.put(entry.getKey(), (double)(entry.getValue()) / times);
        }
        for(Map.Entry<Integer, Double> entry : realRate.entrySet()) {
            Integer key = entry.getKey();
            System.out.println(String.format("奖品id为：%s, 抽奖次数为:%s, 设定的概率为:%s，实际的概率为:%s",
                    key, times, setRate.get(key), realRate.get(key)));
        }
    }
}