java如何将list按照指定数量分成小list
作者：赶路人儿
本文介绍了四种不同的方法对集合进行分区操作,包括手动编写代码、使用Guava库、Apache Commons Collection库以及Java 8的流操作,每种方法都有其特点和适用场景,需要注意的是,部分方法返回的是原集合的视图,而部分则返回的是新的集合
java将list按照指定数量分成小list
自己编写相关代码：
使用guava
import java.util.ArrayList;
import java.util.List;
import com.google.common.collect.Lists;
public class Test4 {
public static void main(String[] args) {
List<Long> list = new ArrayList<>();
list.add(1L);
list.add(2L);
list.add(3L);
list.add(4L);
list.add(5L);
list.add(6L);
list.add(7L);
list.add(8L);
list.add(9L);
//test1(list);
test2(list);
}
private static void test1(List<Long> list) {
int size = 2;
List<List<Long>> listArr = new ArrayList<>();
int arrSize = list.size()%size==0?list.size()/size:list.size()/size+1;
for(int i=0;i<arrSize;i++) {
List<Long> sub = new ArrayList<>();
for(int j=i*size;j<=size*(i+1)-1;j++) {
if(j<=list.size()-1) {
sub.add(list.get(j));
}
}
listArr.add(sub);
}
System.out.println(listArr.toString());
}
private static void test2(List<Long> list) {
int size = 16;
List<List<Long>> subSets = Lists.partition(list, size);
System.out.println(subSets.toString());
}
} 补充：guava分割其他collectionsiew plain
@Test
public void givenCollection_whenParitioningIntoNSublists_thenCorrect() {
Collection<Integer> intCollection = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);
Iterable<List<Integer>> subSets = Iterables.partition(intCollection, 3);
List<Integer> firstPartition = subSets.iterator().next();
List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(1, 2, 3);
assertThat(firstPartition, equalTo(expectedLastPartition));
} 以上需要注意的是：
partition返回的是原list的subview.视图，也即，原list改变后，partition之后的结果也会随着改变。
@Test
public void givenListPartitioned_whenOriginalListIsModified_thenPartitionsChangeAsWell() {
// Given
List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);
List<List<Integer>> subSets = Lists.partition(intList, 3);
// When
intList.add(9);
// Then
List<Integer> lastPartition = subSets.get(2);
List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8, 9);
assertThat(lastPartition, equalTo(expectedLastPartition));
} 使用apache commons collection
@Test
public void givenList_whenParitioningIntoNSublists_thenCorrect() {
List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);
List<List<Integer>> subSets = ListUtils.partition(intList, 3);
List<Integer> lastPartition = subSets.get(2);
List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8);
assertThat(subSets.size(), equalTo(3));
assertThat(lastPartition, equalTo(expectedLastPartition));
} - 没有对应的Iterable.partions方法，类似guava那样
- partition后的结果同样是原集合的视图。
Java8方法
1）通过grouping by：
@Test
public final void givenList_whenParitioningIntoNSublistsUsingGroupingBy_thenCorrect() {
List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);
Map<Integer, List<Integer>> groups =
intList.stream().collect(Collectors.groupingBy(s -> (s - 1) / 3));
List<List<Integer>> subSets = new ArrayList<List<Integer>>(groups.values());
List<Integer> lastPartition = subSets.get(2);
List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8);
assertThat(subSets.size(), equalTo(3));
assertThat(lastPartition, equalTo(expectedLastPartition));
} 按年龄分组：
Map<Integer, List<Person>> personGroups = Stream.generate(new PersonSupplier()).
limit(100).
collect(Collectors.groupingBy(Person::getAge));
Iterator it = personGroups.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<Integer, List<Person>> persons = (Map.Entry) it.next();
System.out.println("Age " + persons.getKey() + " = " + persons.getValue().size());
} 2）通过partition by：
@Test
public void givenList_whenParitioningIntoSublistsUsingPartitionBy_thenCorrect() {
List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);
Map<Boolean, List<Integer>> groups =
intList.stream().collect(Collectors.partitioningBy(s -> s > 6));
List<List<Integer>> subSets = new ArrayList<List<Integer>>(groups.values());
List<Integer> lastPartition = subSets.get(1);
List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8);
assertThat(subSets.size(), equalTo(2));
assertThat(lastPartition, equalTo(expectedLastPartition));
} 按照成年未成年人分组：
Map<Boolean, List<Person>> children = Stream.generate(new PersonSupplier()).
limit(100).
collect(Collectors.partitioningBy(p -> p.getAge() < 18));
System.out.println("Children number: " + children.get(true).size());
System.out.println("Adult number: " + children.get(false).size()); 注意：
1.Java8方式，分组后的list不再是原list的视图。所以，原list的改变不会影响分组后的结果。
2partitioningBy 其实是一种特殊的 groupingBy，它依照条件测试的是否两种结果来构造返回的数据结构，get(true) 和 get(false) 能即为全部的元素对象。
总结
以上为个人经验，希望能给大家一个参考，也希望大家多多支持脚本之家。