package org.usc.usc.lottery;

public class Gift {
    private int index;
    private String gitfId;
    private String giftName;
    private double probability;

    public Gift(int index, String gitfId, String giftName, double probability) {
        this.index = index;
        this.gitfId = gitfId;
        this.giftName = giftName;
        this.probability = probability;
    }

    public int getIndex() {
        return index;
    }

    public void setIndex(int index) {
        this.index = index;
    }

    public String getGitfId() {
        return gitfId;
    }

    public void setGitfId(String gitfId) {
        this.gitfId = gitfId;
    }

    public String getGiftName() {
        return giftName;
    }

    public void setGiftName(String giftName) {
        this.giftName = giftName;
    }

    public double getProbability() {
        return probability;
    }

    public void setProbability(double probability) {
        this.probability = probability;
    }

    @Override
    public String toString() {
        return "Gift [index=" + index + ", gitfId=" + gitfId + ", giftName=" + giftName + ", probability=" + probability + "]";
    }

}

package org.usc.usc.lottery;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/**
 * 不同概率抽奖工具包
 *
 * @author Shunli
 */
public class LotteryUtil {
    /**
     * 抽奖
     *
     * @param orignalRates
     *            原始的概率列表，保证顺序和实际物品对应
     * @return
     *         物品的索引
     */
    public static int lottery(List<Double> orignalRates) {
        if (orignalRates == null || orignalRates.isEmpty()) {
            return -1;
        }

        int size = orignalRates.size();

        // 计算总概率，这样可以保证不一定总概率是1
        double sumRate = 0d;
        for (double rate : orignalRates) {
            sumRate += rate;
        }

        // 计算每个物品在总概率的基础下的概率情况
        List<Double> sortOrignalRates = new ArrayList<Double>(size);
        Double tempSumRate = 0d;
        for (double rate : orignalRates) {
            tempSumRate += rate;
            sortOrignalRates.add(tempSumRate / sumRate);
        }

        // 根据区块值来获取抽取到的物品索引
        double nextDouble = Math.random();
        sortOrignalRates.add(nextDouble);
        Collections.sort(sortOrignalRates);

        return sortOrignalRates.indexOf(nextDouble);
    }
}

package org.usc.usc.lottery;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

/**
 * 不同概率抽奖
 *
 * @author ShunLi
 */
public class LotteryTest {
    public static void main(String[] args) {
        List<Gift> gifts = new ArrayList<Gift>();
        // 序号==物品Id==物品名称==概率
        gifts.add(new Gift(1, "P1", "物品1", 0.2d));
        gifts.add(new Gift(2, "P2", "物品2", 0.2d));
        gifts.add(new Gift(3, "P3", "物品3", 0.4d));
        gifts.add(new Gift(4, "P4", "物品4", 0.3d));
        gifts.add(new Gift(5, "P5", "物品5", 0d));
        gifts.add(new Gift(6, "P6", "物品6", -0.1d));
        gifts.add(new Gift(7, "P7", "物品7", 0.008d));

        List<Double> orignalRates = new ArrayList<Double>(gifts.size());
        for (Gift gift : gifts) {
            double probability = gift.getProbability();
            if (probability < 0) {
                probability = 0;
            }
            orignalRates.add(probability);
        }

        // // test
        // for (int i = 0; i < 10000; i++) {
        // try {
        // Gift tuple = gifts.get(LotteryUtil.lottery(orignalRates));
        // System.out.println(tuple);
        // } catch (Exception e) {
        // System.out.println("lottery failed, please check it!");
        // }
        // }

        // statistics
        Map<Integer, Integer> count = new HashMap<Integer, Integer>();
        double num = 1000000;
        for (int i = 0; i < num; i++) {
            int orignalIndex = LotteryUtil.lottery(orignalRates);

            Integer value = count.get(orignalIndex);
            count.put(orignalIndex, value == null ? 1 : value + 1);
        }

        for (Entry<Integer, Integer> entry : count.entrySet()) {
            System.out.println(gifts.get(entry.getKey()) + ", count=" + entry.getValue() + ", probability=" + entry.getValue() / num);
        }
    }

}

Gift [index=1, gitfId=P1, giftName=物品1, probability=0.2], count=199139, probability=0.199139
Gift [index=2, gitfId=P2, giftName=物品2, probability=0.1], count=99328, probability=0.099328
Gift [index=3, gitfId=P3, giftName=物品3, probability=0.4], count=396575, probability=0.396575
Gift [index=4, gitfId=P4, giftName=物品4, probability=0.3], count=296997, probability=0.296997
Gift [index=7, gitfId=P7, giftName=物品7, probability=0.0080], count=7961, probability=0.007961