1.概述
本章中介绍如何分开几个子list在list中，对于简单操作，java标准collection api不支持，庆幸是
Guava 和Apache Commons Collections 都实现操作
2.使用Guava分开list
Guava从list中分开指定大小sublist，使用Lists.partition操作
@Test
public void givenList_whenParitioningIntoNSublists_thenCorrect() {
List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);
//list被分成3个list subSets[0]->1，2，3 subSets[1]->4,5,6 subSets[2]->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));
}
3.使用Guava分区集合
集合分开也可以使用Guava
@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.newArrayList(1, 2, 3);
assertThat(firstPartition, equalTo(expectedLastPartition));
}
记住这个分区是原集合子视图，意味着更改原集合子视图也会改变
@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.newArrayList(7, 8, 9);
assertThat(lastPartition, equalTo(expectedLastPartition));
}
4.使用Apache Commons Collections 分区list
最新版本Apache Commons Collections 也支持list分区
@Test
public void givenList_whenParitioningIntoNSublists_thenCorrectCommon() {
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.newArrayList(7, 8);
assertThat(subSets.size(), equalTo(3));
assertThat(lastPartition, equalTo(expectedLastPartition));
}
最后也有相同tips，分区结果集合是原集合子视图
5.使用java8分区list
现在看下如何使用java8分区list
5.1 Collectors partitioningBy
使用 Collectors.partitioningBy() 分开list变成2个子list
@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 (groups.values());
List<Integer> lastPartition = subSets.get(1);
List<Integer> expectedLastPartition = Lists.newArrayList(7, 8);
assertThat(subSets.size(), equalTo(2));
assertThat(lastPartition, equalTo(expectedLastPartition));
}
注意:分区结果不是原list子视图，因此任何原list更改不会影响分区视图
5.2 Collectors groupingBy
可以使用Collectors.groupingBy() 分开多个分区
@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(groups.values());
List<Integer> lastPartition = subSets.get(2);
List<Integer> expectedLastPartition = Lists.newArrayList(7, 8);
assertThat(subSets.size(), equalTo(3));
assertThat(lastPartition, equalTo(expectedLastPartition));
}
Tips:Collectors.groupingBy()分区结果不会被原list影响
5.3 分开list使用split
@Test
public void givenList_whenSplittingBySeparator_thenCorrect() {
List<Integer> intList = Lists.newArrayList(1, 2, 3, 0, 4, 5, 6, 0, 7, 8);
int[] indexes =
Stream.of(IntStream.of(-1), IntStream.range(0, intList.size())
.filter(i -> intList.get(i) == 0), IntStream.of(intList.size()))
.flatMapToInt(s -> s).toArray();
List<List<Integer>> subSets =
IntStream.range(0, indexes.length - 1)
.mapToObj(i -> intList.subList(indexes[i] + 1, indexes[i + 1]))
.collect(Collectors.toList());
List<Integer> lastPartition = subSets.get(2);
List<Integer> expectedLastPartition = Lists. newArrayList(7, 8);
assertThat(subSets.size(), equalTo(3));
assertThat(lastPartition, equalTo(expectedLastPartition));
}
tips：我们使用0作为分区点，第一获取索引为零元素，然后分开这些索引 list
6.总结
上面解决方案是引入第三方包（Guava or the Apache Commons Collections ），两种非常轻量级和很好用
项目中在classpath中引入其中一个包
