Stream的 ::

Stream的 :: 用来取出实体类中的属性值：

小例子：

@Override
public List<AttrEntity> getRelationAttr(Long attrgroupId) {
    List<AttrAttrgroupRelationEntity> entities = relationDao.selectList(new QueryWrapper<AttrAttrgroupRelationEntity>().eq("attr_group_id", attrgroupId));

    List<Long> attrIds = entities.stream().map((attr) -> {
        return attr.getAttrId();
    }).collect(Collectors.toList());

    //上面的map可以优化成下面这样：
    List<Long> attrIds = entities.stream().map(AttrAttrgroupRelationEntity::getAttrId).collect(Collectors.toList());

    Collection<AttrEntity> attrEntities = this.listByIds(attrIds);
    return (List<AttrEntity>) attrEntities;
}