咖啡汪日志——stream常用的分组处理

根据优惠券 id, 对优惠券进行分组

List<Coupon> userUsableCoupons = findCouponsByStatus(
                userId, CouponStatus.USABLE.getCode()
        );
 // 通过模板 id , 获取可用优惠券
        Map<Integer, List<Coupon>> templateId2Coupons = userUsableCoupons.stream()
                .collect(Collectors.groupingBy(Coupon::getTemplateId));

自建分组类型进行分组

public enum ValueLevel{
    
    GGS,AGE,TTS}
Map<ValueLevel,List<Transaction>> map = transactions.stream().collect(Collectors.groupingBy(t -> {
    
    
        if(t.getValue() <= 700) return ValueLevel.AGE;
        else if (t.getValue() <=1000) return ValueLevel.GGS;
        else return ValueLevel.TTS;
    } ));
    

先分组再去重

 Map<Integer,Set<Transaction>> map1 = transactions.stream().collect(groupingBy(Transaction::getYear,toSet()));
 

先分组，再计算每组的元素个数

 Map<Integer,Long> map2 = transactions.stream().collect(groupingBy(Transaction::getYear,counting()));
 

每个子组内，交易额最大的

Map<Integer,Transaction> map3 = transactions.stream().collect(groupingBy(Transaction::getYear,collectingAndThen(maxBy(comparingInt(Transaction::getValue)),Optional::get)));

每个子组，取其交易额的总和

 Map<Integer,Integer> map4 = transactions.stream().collect(groupingBy(Transaction::getYear,summingInt(Transaction::getValue)));

每个子组，进行另外条件筛选分层映射

Map<Integer,Set<ValueLevel>> map5 = transactions.stream().collect(groupingBy(Transaction::getYear,mapping( transaction -> {
    
    
        if(transaction.getValue() <= 700) return ValueLevel.AGE;
        else if(transaction.getValue() <= 1000) return ValueLevel.GGS;
        else return ValueLevel.TTS;
    },toCollection(HashSet::new))));