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();
}
}}
