java 从几十万条数据的list集合中随机取出1000个元素重新生成集合？

Posted 2023-03-27

java 从几十万条数据的list集合中随机取出1000个元素重新生成集合，要求速度比较快。

参考技术A List<String> list = new ArrayList<String>();
List<String> new_list = new ArrayList<String>();
for (int i = 0 ; i < 1200000;i++)

list.add(i+"");

long time1 = System.currentTimeMillis();
Collections.shuffle(list);
new_list = list.subList(0,1000);//方法1耗时65~130
new_list = createRandomList(list,1000);//方法2耗时2~5
long time2 = System.currentTimeMillis();
System.out.println(time2 - time1);

private static List createRandomList(List list, int n)

Map map = new HashMap();
List listNew = new ArrayList();
if (list.size() <= n)

return list;

else

while (map.size() < n)

int random = (int) (Math.random() * list.size());
if (!map.containsKey(random))

map.put(random, "");
listNew.add(list.get(random));


return listNew;



这个方法测试 参考技术B package test2;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;

public class test2_test1
static HashSet<Object> hashSet1 = new HashSet<>();
static List<Object> list = new ArrayList<>();

static HashSet<Integer> getObject(int i)
HashSet<Integer> hashSet = new HashSet<>(i);
int random = (int)Math.random() * list.size();
while (hashSet.size() < i)
hashSet.add(random);
random = (int)Math.random() * list.size();

return hashSet;

//获取指定范围内无重复数字(list索引)

static void addObject(int i1)
for (int i : getObject(i1))
hashSet1.add(list.get(i));


//获取hashset的值以获取list的指定索引位置值,将该值添加到新的集合中

public static void main(String[] args)
addObject(1000);