将一个list分成多个list的Java实现。

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本文介绍如何使用Guava和Apache Commons Collections库将Java集合按需分割成子集合的方法，并展示了如何利用Java 8 Stream API进行分组和条件分割的具体实现。

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将一个list或其他集合按需分成多份，例如，分批发送等。是很常见的需求。但是Java传统的集合操作似乎没有提供这一支持。幸好，google guava 和apache commons collections都提供了相应的实现。

方法一：guava实现，将一个list按三个一组分成N个小的list

@Test
public void givenList_whenParitioningIntoNSublists_thenCorrect() {
    List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);
    List<List<Integer>> subSets = Lists.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));
}

方法二：guava分割其他collections

@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));
}

方法三：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));
}

1.没有对应的Iterable.partions方法，类似guava那样

2.partition后的结果同样是原集合的视图。

方法四：Java8方法。(在Java stream 中有讲。)

a.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());
}

b.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));
}

按照成年人与未成年人分组：

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