有时候，我们需要将大的集合按指定的数量分割成若干个小集合。（比如：集合作为SQL中IN的参数，而SQL又有长度限制，所以需要分批分几次进行查询）

虽然此需求感觉不常见，但偶也写过几次类似的方法，故记录之。

更新于2017年：其实Guava库有个已有的方法实现此需求：Lists.partition(List<T> list, int size)

v2，更新于2016-01-20

v1的代码使用后发现有问题，如果对分组后的一子集作删除操作，其他子集用迭代器遍历时会出现ConcurrentModificationException。

修改后的代码如下:

import java.util.ArrayList;

import java.util.List;

public class CollectionGroupUtil {

    public static List groupListByQuantity(List list, int quantity) {

        if (list == null || list.size() == 0) {

            return list;

        }

        if (quantity <= 0) {

            new IllegalArgumentException("Wrong quantity.");

        }

        List wrapList = new ArrayList();

        int count = 0;

        while (count < list.size()) {

            wrapList.add(new ArrayList(list.subList(count, (count + quantity) > list.size() ? list.size() : count + quantity)));

            count += quantity;

        }

        return wrapList;

    }

}
import java.util.ArrayList;

import java.util.List;

import org.junit.Test;

public class CollectionGroupUtilTest {

    /**

     * 大于分组数量的情况

     */

    @Test

    public void test() {

        List<String> allList = new ArrayList<String>();

        for (int i = 1; i <= 504; i++) {

            allList.add(i  + "");

        }

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(allList, 50);

        int i = 0;

        List<String> list = null;

        for (int c = 0; c < groupList.size(); c++) {

            list = groupList.get(c);

            System.out.println("第" + (c + 1) + "组： ");

            for (String temp : list) {

                System.out.print(temp + ", ");

            }

            System.out.println();

        }

    }

    /**

     * 小于分组数量的情况

     */

    @Test

    public void test2() {

        List<String> allList = new ArrayList<String>();

        for (int i = 1; i <= 45; i++) {

            allList.add(i  + "");

        }

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(allList, 50);

        int i = 0;

        List<String> list = null;

        for (int c = 0; c < groupList.size(); c++) {

            list = groupList.get(c);

            System.out.println("第" + (c + 1) + "组： ");

            for (String temp : list) {

                System.out.print(temp + ", ");

            }

            System.out.println();

        }

    }

    /**

     * 集合只有一个记录的情况

     */

    @Test

    public void test3() {

        List<String> allList = new ArrayList<String>();

        for (int i = 1; i <= 1; i++) {

            allList.add(i  + "");

        }

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(allList, 50);

        int i = 0;

        List<String> list = null;

        for (int c = 0; c < groupList.size(); c++) {

            list = groupList.get(c);

            System.out.println("第" + (c + 1) + "组： ");

            for (String temp : list) {

                System.out.print(temp + ", ");

            }

            System.out.println();

        }

    }

    /**

     * 空集合的情况

     */

    @Test

    public void test4() {

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(null, 50);

        System.out.println(groupList);

        groupList = CollectionGroupUtil.groupListByQuantity(new ArrayList(), 50);

        System.out.println(groupList);

    }

    /**

     * 集合刚满一个分组的情况

     */

    @Test

    public void test5() {

        List<String> allList = new ArrayList<String>();

        for (int i = 1; i <= 50; i++) {

            allList.add(i  + "");

        }

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(allList, 50);

        int i = 0;

        List<String> list = null;

        for (int c = 0; c < groupList.size(); c++) {

            list = groupList.get(c);

            System.out.println("第" + (c + 1) + "组： ");

            for (String temp : list) {

                System.out.print(temp + ", ");

            }

            System.out.println();

        }

    }

    /**

     * 出现ConcurrentModificationException的情况

     */

    @Test

    public void test6() {

        List<String> allList = new ArrayList<String>();

        for (int i = 1; i <= 55; i++) {

            allList.add(i  + "");

        }

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(allList, 50);

        groupList.get(0).remove(0);

        int i = 0;

        List<String> list = null;

        for (int c = 0; c < groupList.size(); c++) {

            list = groupList.get(c);

            System.out.println("第" + (c + 1) + "组： ");

            for (String temp : list) {

                System.out.print(temp + ", ");

            }

            System.out.println();

        }

    }

}

以简单的方式重现ConcurrentModificationException异常：

import java.util.ArrayList;

import java.util.List;

public class SeeConcurrentModificationExceptionInSubList {

    public static void main(String[] args) {

        List<String> originalList = new ArrayList<String>();

        for (int i = 1; i <= 5; i++) {

            originalList.add(i + "");

        }

        List<String> subList1 = originalList.subList(0, 3);

        List<String> subList2 = originalList.subList(3, originalList.size());

        /* 按坐标删除 */

        subList1.remove(0);

        /* 用迭代器删除 */

        /*

        Iterator<String> i = subList1.iterator();

        i.next();

        i.remove();

        */

        System.out.println("subList1 -> ");

        for (String tempStr : subList1) {

            System.out.println(tempStr);

        }

        System.out.println("subList2 -> ");

        for (String tempStr : subList2) {

            System.out.println(tempStr);

        }

    }

}

v1，更新于2016-01-18

工具类

import java.util.ArrayList;

import java.util.List;

public class CollectionGroupUtil {

    public static List groupListByQuantity(List list, int quantity) {

        if (list == null || list.size() == 0) {

            return list;

        }

        if (quantity <= 0) {

            new IllegalArgumentException("Wrong quantity.");

        }

        List wrapList = new ArrayList();

        int count = 0;

        while (count < list.size()) {

            wrapList.add(list.subList(count, (count + quantity) > list.size() ? list.size() : count + quantity));

            count += quantity;

        }

        return wrapList;

    }

}

测试类

import java.util.ArrayList;

import java.util.List;

import org.junit.Test;

public class CollectionGroupUtilTest {

    /**

     * 大于分组数量的情况

     */

    @Test

    public void test() {

        List<String> allList = new ArrayList<String>();

        for (int i = 1; i <= 504; i++) {

            allList.add(i  + "");

        }

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(allList, 50);

        int i = 0;

        List<String> list = null;

        for (int c = 0; c < groupList.size(); c++) {

            list = groupList.get(c);

            System.out.println("第" + (c + 1) + "组： ");

            for (String temp : list) {

                System.out.print(temp + ", ");

            }

            System.out.println();

        }

    }

    /**

     * 小于分组数量的情况

     */

    @Test

    public void test2() {

        List<String> allList = new ArrayList<String>();

        for (int i = 1; i <= 45; i++) {

            allList.add(i  + "");

        }

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(allList, 50);

        int i = 0;

        List<String> list = null;

        for (int c = 0; c < groupList.size(); c++) {

            list = groupList.get(c);

            System.out.println("第" + (c + 1) + "组： ");

            for (String temp : list) {

                System.out.print(temp + ", ");

            }

            System.out.println();

        }

    }

    /**

     * 集合只有一个记录的情况

     */

    @Test

    public void test3() {

        List<String> allList = new ArrayList<String>();

        for (int i = 1; i <= 1; i++) {

            allList.add(i  + "");

        }

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(allList, 50);

        int i = 0;

        List<String> list = null;

        for (int c = 0; c < groupList.size(); c++) {

            list = groupList.get(c);

            System.out.println("第" + (c + 1) + "组： ");

            for (String temp : list) {

                System.out.print(temp + ", ");

            }

            System.out.println();

        }

    }

    /**

     * 空集合的情况

     */

    @Test

    public void test4() {

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(null, 50);

        System.out.println(groupList);

        groupList = CollectionGroupUtil.groupListByQuantity(new ArrayList(), 50);

        System.out.println(groupList);

    }

    /**

     * 集合刚满一个分组的情况

     */

    @Test

    public void test5() {

        List<String> allList = new ArrayList<String>();

        for (int i = 1; i <= 50; i++) {

            allList.add(i  + "");

        }

        List<List<String>> groupList = CollectionGroupUtil.groupListByQuantity(allList, 50);

        int i = 0;

        List<String> list = null;

        for (int c = 0; c < groupList.size(); c++) {

            list = groupList.get(c);

            System.out.println("第" + (c + 1) + "组： ");

            for (String temp : list) {

                System.out.print(temp + ", ");

            }

            System.out.println();

        }

    }

}

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