当前位置:网站首页>关于Stream和Map的巧用
关于Stream和Map的巧用
2022-07-06 09:30:00 【机智的爆爆哥】
题目是这样的
//假设db中有两个这样的表 数据有100w 最终要返回的是Provice表的List集合 如果status为fasle 则不返回 如何在java层面解决呢?
public class Mydemo {
static ArrayList<Province> provinces = new ArrayList<>();
static ArrayList<ProvinceStatus> provinceStatusList = new ArrayList<>();
static {
for (int i = 0; i < 10000; i++) {
Province province = new Province();
ProvinceStatus provinceStatus = new ProvinceStatus();
province.setName(UUID.randomUUID().toString());
province.setProvinceId(i);
provinceStatus.setProvinceId(i);
provinceStatus.setStatus(new Random().nextBoolean());
//添加
provinces.add(province);
provinceStatusList.add(provinceStatus);
}
System.out.println("显示的列表大小为:" + provinceStatusList.stream().filter(ProvinceStatus::getStatus).count());
}
public static void main(String[] args) {
ArrayList<Province> resList = new ArrayList<>();
System.out.println("最终显示的个数:" + resList.size());
}
}
@Data
class Province {
private Integer provinceId;
private String name;
}
@Data
class ProvinceStatus {
private Integer provinceId;
private Boolean status;
}
答案如下,有更好的想法请留言哦!
public static void main(String[] args) {
//双重for循环? 不 用hash是个好办法
long start1 = System.currentTimeMillis();
System.out.println();
Map<Integer, Boolean> map = provinceStatusList.stream()
.collect(Collectors.toMap(ProvinceStatus::getProvinceId, ProvinceStatus::getStatus));
ArrayList<Province> resList = new ArrayList<>();
for (Province province : Mydemo.provinces) {
if (map.get(province.getProvinceId())) {
//显示这些
resList.add(province);
}
}
long start2 = System.currentTimeMillis();
System.out.println("用时:" + (start2 - start1));
System.out.println("最终显示的个数:" + resList.size());
}
}
实测在数据量大的时候,并行流反而速度更慢,应该是因为没有经过什么计算,毕竟并行流适合CPU密集型的任务,即包含大量计算的时候使用。
边栏推荐
猜你喜欢
Error occurred during initialization of VM Could not reserve enough space for object heap
~76 sprite map
Solr standalone installation
ByteDance open source Gan model compression framework, saving up to 97.8% of computing power - iccv 2021
数据传送指令
Alibaba cloud server docker installation mysql5.5
Shell_ 06_ Judgment and circulation
Shell_ 01_ data processing
Shell_ 05_ operator
J'ai traversé le chemin le plus fou, le circuit cérébral d'un programmeur de saut d'octets
随机推荐
MySQL数字函数
Design of DS18B20 digital thermometer system
Activit零零碎碎要人命的坑
[unsolved] 7-15 shout mountain
Mongodb learning notes
8086 CPU 内部结构
~87 animation
GCC error: terminate called after throwing an instance of 'std:: regex_ error‘ what(): regex
Ruoyi-Cloud 踩坑的BUG
8086 segmentation technology
字节跳动开源GAN模型压缩框架,算力最高节省97.8%丨ICCV 2021
Fdog series (V): use QT to imitate QQ to realize login interface to main interface, function chapter.
唯有学C不负众望 TOP2 p1变量
@RequestMapping、@GetMapping
唯有學C不負眾望 TOP5 S1E8|S1E9:字符和字符串&&算術運算符
Activiti目录(四)查询代办/已办、审核
Von Neumann architecture
Fdog series (VI): use QT to communicate between the client and the client through the server (less information, recommended Collection)
"One year after graduation, I won ACL best paper"
~69 other ways to use icon fonts