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Java8 lambda表达式常见用法

Java8的lambda用于操作集合非常好用，这里记录一些常用API

1.从实体集合中提取某字段：字符串类型、double类型、BigDecimal类型，关键字：map、mapToDouble

//BigDecimal，提取并聚合
BigDecimal amount = orders.stream().map(JDOrder::getOrderFee).reduce(BigDecimal.ZERO,BigDecimal::add);


//String
List<String> voucherIds = voucherAssigns.stream().map(v -> v.getVoucherId()).collect(Collectors.toList());
List<String> giftIds = itemGifts.stream().map( ItemGifts::getGiftId).collect(Collectors.toList());

//double，提取并聚合
Double totalAmount = soldSums.stream().filter(s -> !ObjectUtils.isEmpty(s.getSum())).mapToDouble(s -> s.getSum()).sum();
//Integer
List<Integer> couponIds = itemCoupons.stream().map(i -> i.getCouponId()).collect(Collectors.toList());

2.根据某个字段过滤：可以是=、 >= ，关键字:filter

items = items.stream().filter(i -> i.getPrice()>=couponPrice).collect(Collectors.toList());

3. 分组

Map<String, List<UserGroupMemberVo>> userGroupMemberGroupBy = memberByGroupIdResult.stream().collect(Collectors.groupingBy(UserGroupMemberVo::getMemberType));

4. 分组，并且每组只保留一个对象

# 根据userId分组，出现重复对象保留前者
Map<Long, UserRelationship> userRelationshipMap = userRelationships.stream().collect(Collectors.toMap(UserRelationship::getUserId, Function.identity(), (k1, k2) -> k1));