集合（六）集合分片

在某些场景中，需要批量执行数据，可以通过数据库分页查询，也可以通过java来对集合进行分页。将list（或其他集合）拆分成多份，java传统的集合操作中并没有这样的方法。常见方法有：

1、手动分片：

    public static  List> averageAssign(List source, int n) {
        List> result = new ArrayList>();
        int remaider = source.size() % n;  //(先计算出余数)
        int number = source.size() / n;  //然后是商
        int offset = 0;//偏移量
        for (int i = 0; i < n; i++) {
            List value = null;
            if (remaider > 0) {
                value = source.subList(i * number + offset, (i + 1) * number + offset + 1);
                remaider--;
                offset++;
            } else {
                value = source.subList(i * number + offset, (i + 1) * number + offset);
            }
            result.add(value);
        }
        return result;
    }

2、google guava分片： 

import com.google.common.collect.Lists;
public class ListsPartitionTest1 {
    public static void main(String[] args) {
        List ls = Arrays.asList("1,2,3,4,5,6,7,8,9,1,2,4,5,6,7,7,6,6,6,6,6,66".split(","));
        System.out.println(Lists.partition(ls, 20));
    }
}

再如：

import com.google.common.collect.Lists;

import java.util.Arrays;
import java.util.List;

public class ListsTest {

    public static void main(String[] args) {
        List list = Arrays.asList(1, 2, 3, 4, 5);
        // 按大小 2 拆分
        List> listPartitionList = Lists.partition(list, 2);

        System.out.println("原 List:");
        System.out.println(list);

        System.out.println("拆分后:");
        for (List partition: listPartitionList) {
            System.out.println(partition);
        }
    }

}



原 List:
[1, 2, 3, 4, 5]
拆分后:
[1, 2]
[3, 4]
[5]

3、apache commons collections分片：

import org.apache.commons.collections4.ListUtils;
public class ListsPartitionTest2 {
    public static void main(String[] args) {
      List intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);
      System.out.println(ListUtils.partition(intList, 3));
    }
}

4、使用Java8 stream流 partition by，partitioningBy是一种特殊的分组，只会分成两组

 List nums = Lists.newArrayList(1,1,8,2,3,4,5,6,7,9,10);
Map> numMap= numList.stream().collect(Collectors.partitioningBy(num -> num > 5));
        System.out.println(numMap);

{false=[2, 3, 4, 5], true=[8, 6, 7, 9, 10]}