将一个list分红多个list

时间 2019-11-13

标签一个 list 分红多个繁體版

原文原文链接

将一个list或其余集合按需分红多份，例如，分批发送等，是很常见的需求，可是Java传统的集合操做彷佛没有提供这一支持。apache

幸亏，google guava 和apache commons collections都提供了相应的实现。数据结构

方法一：guava实现，将一个list按三个一组分红N个小的list

代码块测试

[@Test](https://my.oschina.net/azibug)

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分割其余collections

代码块google

[@Test](https://my.oschina.net/azibug)

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));

}

以上须要注意的是，partition返回的是原list的subview.视图，也即，原list改变后，partition以后的结果也会随着改变。.net

代码块code

[@Test](https://my.oschina.net/azibug)

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

代码块对象

[@Test](https://my.oschina.net/azibug)

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，相似guava那样
partition后的结果一样是原集合的视图。

方法四：Java8方法。(在Java stream 中有讲。)

a.经过grouping byget

代码块it

[@Test](https://my.oschina.net/azibug)

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：io

代码块

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.经过partition by

代码块

@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：

代码块

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：

Java8方式，分组后的list再也不是原list的视图。因此，原list的改变不会影响分组后的结果。
partitioningBy 实际上是一种特殊的 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) 能即为所有的元素对象