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關於Stream和Map的巧用
2022-07-06 17:04:00 【機智的爆爆哥】
題目是這樣的
//假設db中有兩個這樣的錶 數據有100w 最終要返回的是Provice錶的List集合 如果status為fasle 則不返回 如何在java層面解决呢?
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("顯示的列錶大小為:" + provinceStatusList.stream().filter(ProvinceStatus::getStatus).count());
}
public static void main(String[] args) {
ArrayList<Province> resList = new ArrayList<>();
System.out.println("最終顯示的個數:" + resList.size());
}
}
@Data
class Province {
private Integer provinceId;
private String name;
}
@Data
class ProvinceStatus {
private Integer provinceId;
private Boolean status;
}
答案如下,有更好的想法請留言哦!
public static void main(String[] args) {
//雙重for循環? 不 用hash是個好辦法
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())) {
//顯示這些
resList.add(province);
}
}
long start2 = System.currentTimeMillis();
System.out.println("用時:" + (start2 - start1));
System.out.println("最終顯示的個數:" + resList.size());
}
}
實測在數據量大的時候,並行流反而速度更慢,應該是因為沒有經過什麼計算,畢竟並行流適合CPU密集型的任務,即包含大量計算的時候使用。
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