Java 将列表沿元素拆分为子列表
我有这个列表(Java 将列表沿元素拆分为子列表,java,list,java-8,collectors,Java,List,Java 8,Collectors,我有这个列表(list): 我想要这样的东西: [["a", "b"], ["c"], ["d", "e"]] String als = Arrays.toString(new String[]{"a", "b", null, "c", null, "d", "e"}); String[] sa = als.substring(1, als.length() - 1).split("null, "); List<List<String>> res = Stream.of
list
):
我想要这样的东西:
[["a", "b"], ["c"], ["d", "e"]]
String als = Arrays.toString(new String[]{"a", "b", null, "c", null, "d", "e"});
String[] sa = als.substring(1, als.length() - 1).split("null, ");
List<List<String>> res = Stream.of(sa).map(s -> Arrays.asList(s.split(", "))).collect(Collectors.toList());
换句话说,我想使用
null
值作为分隔符将列表拆分为子列表,以获得列表列表(list
)。我正在寻找一个Java8解决方案。我试过使用收集器。Partitioning by
,但我不确定它是否是我想要的。谢谢 目前我提出的唯一解决方案是实现您自己的定制收集器
在阅读解决方案之前,我想补充一些关于这方面的注意事项。我把这个问题更多地看作是一个编程练习,我不确定它是否可以用并行流来完成
因此,您必须意识到,如果管道以并行方式运行,它将无声地中断
这不是一种理想的行为,应该避免。这就是为什么我在组合器部分抛出一个异常(而不是(l1,l2)->{l1.addAll(l2);return l1;}
),因为它在组合两个列表时并行使用,所以会产生一个异常而不是错误的结果
此外,由于列表复制(尽管它使用本机方法来复制底层数组),这也不是很有效
下面是收集器的实现:
private static Collector<String, List<List<String>>, List<List<String>>> splitBySeparator(Predicate<String> sep) {
final List<String> current = new ArrayList<>();
return Collector.of(() -> new ArrayList<List<String>>(),
(l, elem) -> {
if (sep.test(elem)) {
l.add(new ArrayList<>(current));
current.clear();
}
else {
current.add(elem);
}
},
(l1, l2) -> {
throw new RuntimeException("Should not run this in parallel");
},
l -> {
if (current.size() != 0) {
l.add(current);
return l;
}
);
}
作为一个例子,这似乎可以并行完成(这要归功于他!)。因此,它将自定义收集器实现简化为:
private static Collector<String, List<List<String>>, List<List<String>>> splitBySeparator(Predicate<String> sep) {
return Collector.of(() -> new ArrayList<List<String>>(Arrays.asList(new ArrayList<>())),
(l, elem) -> {if(sep.test(elem)){l.add(new ArrayList<>());} else l.get(l.size()-1).add(elem);},
(l1, l2) -> {l1.get(l1.size() - 1).addAll(l2.remove(0)); l1.addAll(l2); return l1;});
}
专用静态收集器splitBySeparator(谓词sep){
返回Collector.of(()->new ArrayList(Arrays.asList(new ArrayList())),
(l,elem)->{if(sep.test(elem)){l.add(newarraylist());}else l.get(l.size()-1).add(elem);},
(l1,l2)->{l1.get(l1.size()-1).addAll(l2.remove(0));l1.addAll(l2);返回l1;});
}
这让关于并行性的段落有点过时,但是我让它成为一个很好的提醒
请注意,流API并不总是一个替代品。有些任务更容易、更适合使用流,有些任务则不然。在您的情况下,您还可以为此创建实用程序方法:
private static <T> List<List<T>> splitBySeparator(List<T> list, Predicate<? super T> predicate) {
final List<List<T>> finalList = new ArrayList<>();
int fromIndex = 0;
int toIndex = 0;
for(T elem : list) {
if(predicate.test(elem)) {
finalList.add(list.subList(fromIndex, toIndex));
fromIndex = toIndex + 1;
}
toIndex++;
}
if(fromIndex != toIndex) {
finalList.add(list.subList(fromIndex, toIndex));
}
return finalList;
}
private static List splitBySeparator(List List,Predicate这里有另一种方法,它使用分组函数,利用列表索引进行分组
在这里,我通过元素后面的第一个索引对元素进行分组,值为null
。因此,在您的示例中,“a”
和“b”
将映射到2
。此外,我还将null
值映射到-1
索引,稍后应该将其删除
List<String> list = Arrays.asList("a", "b", null, "c", null, "d", "e");
Function<String, Integer> indexGroupingFunc = (str) -> {
if (str == null) {
return -1;
}
int index = list.indexOf(str) + 1;
while (index < list.size() && list.get(index) != null) {
index++;
}
return index;
};
Map<Integer, List<String>> grouped = list.stream()
.collect(Collectors.groupingBy(indexGroupingFunc));
grouped.remove(-1); // Remove null elements grouped under -1
System.out.println(grouped.values()); // [[a, b], [c], [d, e]]
请不要投票。我没有足够的地方在评论中解释这一点
这是一个带有流
和foreach
的解决方案,但它严格等同于Alexis的解决方案或foreach
循环(不太清晰,我无法摆脱复制构造函数):
List result=new ArrayList();
最终列表当前=新的ArrayList();
list.stream().forEach(s->{
如果(s==null){
结果.添加(新ArrayList(当前));
current.clear();
}否则{
当前。添加;
}
}
);
结果。添加(当前);
系统输出打印项次(结果);
我知道你想用Java 8找到一个更优雅的解决方案,但我真的认为它并不是为这种情况设计的。正如spoon先生所说,在这种情况下,我们更喜欢天真的方式。解决方案是使用流。collect
。使用其构建器模式创建收集器已经作为解决方案给出了native是另一个重载的collect
,稍微原始一点
List<String> strings = Arrays.asList("a", "b", null, "c", null, "d", "e");
List<List<String>> groups = strings.stream()
.collect(() -> {
List<List<String>> list = new ArrayList<>();
list.add(new ArrayList<>());
return list;
},
(list, s) -> {
if (s == null) {
list.add(new ArrayList<>());
} else {
list.get(list.size() - 1).add(s);
}
},
(list1, list2) -> {
// Simple merging of partial sublists would
// introduce a false level-break at the beginning.
list1.get(list1.size() - 1).addAll(list2.remove(0));
list1.addAll(list2);
});
- 第一个参数是累积对象
- 第二个函数累积
- 第三个是前面提到的组合器
这是一个非常有趣的问题。我提出了一个单线解决方案。它可能性能不太好,但很有效
List<String> list = Arrays.asList("a", "b", null, "c", null, "d", "e");
Collection<List<String>> cl = IntStream.range(0, list.size())
.filter(i -> list.get(i) != null).boxed()
.collect(Collectors.groupingBy(
i -> IntStream.range(0, i).filter(j -> list.get(j) == null).count(),
Collectors.mapping(i -> list.get(i), Collectors.toList()))
).values();
List List=Arrays.asList(“a”、“b”、null、“c”、null、“d”、“e”);
集合cl=IntStream.range(0,list.size())
.filter(i->list.get(i)!=null).boxed()
.collect(收集器.groupingBy(
i->IntStream.range(0,i).filter(j->list.get(j)==null.count(),
Collectors.mapping(i->list.get(i),Collectors.toList())
).values();
这是@Rohit Jain提出的类似想法。我将空值之间的空间分组。
如果您确实需要列表
,您可以附加:
List<List<String>> ll = cl.stream().collect(Collectors.toList());
List ll=cl.stream().collect(Collectors.toList());
虽然已经有了几个答案,并且已经有了一个公认的答案,但是这个主题仍然缺少几点。首先,共识似乎是使用流解决这个问题只是一个练习,而传统的for-loop方法更可取。其次,到目前为止给出的答案忽略了一个认可的问题ach使用数组或向量样式的技术,我认为这大大改进了streams解决方案
首先,这里是一个常规解决方案,用于讨论和分析:
static List<List<String>> splitConventional(List<String> input) {
List<List<String>> result = new ArrayList<>();
int prev = 0;
for (int cur = 0; cur < input.size(); cur++) {
if (input.get(cur) == null) {
result.add(input.subList(prev, cur));
prev = cur + 1;
}
}
result.add(input.subList(prev, input.size()));
return result;
}
获取出现null
的索引非常容易。绊脚石是在左侧添加-1
,在右侧添加size
。我选择使用Stream.of
进行追加,然后使用flatMapToInt
将它们展平。(我尝试了其他几种方法,但这一种似乎是最干净的。)
在这里使用数组作为索引要方便一些。首先,访问数组的符号比列表更好:索引[i]
vs.索引。获取(i)
。其次,使用数组可以避免装箱
此时,数组中的每个索引值(最后一个除外)都比子列表的起始位置小一个。其右边的索引是子列表的结尾。我们只需在数组和映射上进行流式处理
List<String> strings = Arrays.asList("a", "b", null, "c", null, "d", "e");
List<List<String>> groups = strings.stream()
.collect(() -> {
List<List<String>> list = new ArrayList<>();
list.add(new ArrayList<>());
return list;
},
(list, s) -> {
if (s == null) {
list.add(new ArrayList<>());
} else {
list.get(list.size() - 1).add(s);
}
},
(list1, list2) -> {
// Simple merging of partial sublists would
// introduce a false level-break at the beginning.
list1.get(list1.size() - 1).addAll(list2.remove(0));
list1.addAll(list2);
});
List<List<String>> groups = strings.stream()
.reduce(new ArrayList<List<String>>(),
(list, s) -> {
if (list.isEmpty()) {
list.add(new ArrayList<>());
}
if (s == null) {
list.add(new ArrayList<>());
} else {
list.get(list.size() - 1).add(s);
}
return list;
},
(list1, list2) -> {
list1.addAll(list2);
return list1;
});
List<String> list = Arrays.asList("a", "b", null, "c", null, "d", "e");
Collection<List<String>> cl = IntStream.range(0, list.size())
.filter(i -> list.get(i) != null).boxed()
.collect(Collectors.groupingBy(
i -> IntStream.range(0, i).filter(j -> list.get(j) == null).count(),
Collectors.mapping(i -> list.get(i), Collectors.toList()))
).values();
List<List<String>> ll = cl.stream().collect(Collectors.toList());
static List<List<String>> splitConventional(List<String> input) {
List<List<String>> result = new ArrayList<>();
int prev = 0;
for (int cur = 0; cur < input.size(); cur++) {
if (input.get(cur) == null) {
result.add(input.subList(prev, cur));
prev = cur + 1;
}
}
result.add(input.subList(prev, input.size()));
return result;
}
static List<List<String>> splitStream(List<String> input) {
int[] indexes = Stream.of(IntStream.of(-1),
IntStream.range(0, input.size())
.filter(i -> input.get(i) == null),
IntStream.of(input.size()))
.flatMapToInt(s -> s)
.toArray();
return IntStream.range(0, indexes.length-1)
.mapToObj(i -> input.subList(indexes[i]+1, indexes[i+1]))
.collect(toList());
}
// Java plus pidgin Scala
int[] indexes =
[-1] ++ IntStream.range(0, input.size())
.filter(i -> input.get(i) == null) ++ [input.size()];
static List<List<String>> splitStream(List<String> input) {
int sz = input.size();
int[] indexes =
IntStream.rangeClosed(-1, sz)
.filter(i -> i == -1 || i == sz || input.get(i) == null)
.toArray();
return IntStream.range(0, indexes.length-1)
.mapToObj(i -> input.subList(indexes[i]+1, indexes[i+1]))
.collect(toList());
}
import java.util.*;
import java.util.function.*;
import java.util.stream.*;
import static java.util.Arrays.asList;
public class ListSplitting {
static final Map<List<String>, List<List<String>>> TESTCASES = new LinkedHashMap<>();
static {
TESTCASES.put(asList(),
asList(asList()));
TESTCASES.put(asList("a", "b", "c"),
asList(asList("a", "b", "c")));
TESTCASES.put(asList("a", "b", "#", "c", "#", "d", "e"),
asList(asList("a", "b"), asList("c"), asList("d", "e")));
TESTCASES.put(asList("#"),
asList(asList(), asList()));
TESTCASES.put(asList("#", "a", "b"),
asList(asList(), asList("a", "b")));
TESTCASES.put(asList("a", "b", "#"),
asList(asList("a", "b"), asList()));
TESTCASES.put(asList("#"),
asList(asList(), asList()));
TESTCASES.put(asList("a", "#", "b"),
asList(asList("a"), asList("b")));
TESTCASES.put(asList("a", "#", "#", "b"),
asList(asList("a"), asList(), asList("b")));
TESTCASES.put(asList("a", "#", "#", "#", "b"),
asList(asList("a"), asList(), asList(), asList("b")));
}
static final Predicate<String> TESTPRED = "#"::equals;
static void testAll(BiFunction<List<String>, Predicate<String>, List<List<String>>> f) {
TESTCASES.forEach((input, expected) -> {
List<List<String>> actual = f.apply(input, TESTPRED);
System.out.println(input + " => " + expected);
if (!expected.equals(actual)) {
System.out.println(" ERROR: actual was " + actual);
}
});
}
static <T> List<List<T>> splitStream(List<T> input, Predicate<? super T> pred) {
int[] edges = IntStream.range(-1, input.size()+1)
.filter(i -> i == -1 || i == input.size() ||
pred.test(input.get(i)))
.toArray();
return IntStream.range(0, edges.length-1)
.mapToObj(k -> input.subList(edges[k]+1, edges[k+1]))
.collect(Collectors.toList());
}
static <T> List<List<T>> splitLoop(List<T> input, Predicate<? super T> pred) {
List<List<T>> result = new ArrayList<>();
int start = 0;
for (int cur = 0; cur < input.size(); cur++) {
if (pred.test(input.get(cur))) {
result.add(input.subList(start, cur));
start = cur + 1;
}
}
result.add(input.subList(start, input.size()));
return result;
}
public static void main(String[] args) {
System.out.println("===== Loop =====");
testAll(ListSplitting::splitLoop);
System.out.println("===== Stream =====");
testAll(ListSplitting::splitStream);
}
}
List<List<String>> result = list.stream()
.map(Arrays::asList)
.map(x -> new LinkedList<String>(x))
.map(Arrays::asList)
.map(x -> new LinkedList<List<String>>(x))
.reduce( (a, b) -> {
if (b.getFirst().get(0) == null)
a.add(new LinkedList<String>());
else
a.getLast().addAll(b.getFirst());
return a;}).get();
System.out.println(result);
[[a, b], [c], [d, e]]
List<String> input = Arrays.asList("a", "b", null, "c", null, "d", "e");
List<List<String>> result = StreamEx.of(input)
.groupRuns((a, b) -> a != null && b != null)
.remove(list -> list.get(0) == null).toList();
split(L) :
- if (no null value found) -> return just the simple list
- else -> cut L around 'null' naming the resulting sublists L1 and L2
return split(L1) + split(L2)
public List<List<String>> split(List<String> input) {
OptionalInt index = IntStream.range(0, input.size())
.filter(i -> input.get(i) == null)
.findAny();
if (!index.isPresent())
return asList(input);
List<String> firstHalf = input.subList(0, index.getAsInt());
List<String> secondHalf = input.subList(index.getAsInt()+1, input.size());
return asList(firstHalf, secondHalf).stream()
.map(this::split)
.flatMap(List::stream)
.collect(toList());
}
public List<List<String>> split(List<String> input) {
int index = IntStream.range(0, input.size())
.filter(i -> input.get(i) == null)
.findAny().orElse(input.size());
return asList(input.subList(0, index), input.subList(index+1, input.size())).stream()
.map(this::split)
.flatMap(List::stream)
.filter(list -> !list.isEmpty())
.collect(toList());
}
List<String> list = N.asList(null, null, "a", "b", null, "c", null, null, "d", "e");
Stream.of(list).splitIntoList(null, (e, any) -> e == null, null).filter(e -> e.get(0) != null).forEach(N::println);
String s = ....;
String[] parts = s.split("sth");
List<T> l = ...
List<List<T>> parts = l.split(condition) (possibly with several overloaded variants)
String als = Arrays.toString(new String[]{"a", "b", null, "c", null, "d", "e"});
String[] sa = als.substring(1, als.length() - 1).split("null, ");
List<List<String>> res = Stream.of(sa).map(s -> Arrays.asList(s.split(", "))).collect(Collectors.toList());
List<List<String>> part(List<String> input, List<List<String>> acc, List<String> cur, int i) {
if (i == input.size()) return acc;
if (input.get(i) != null) {
cur.add(input.get(i));
} else if (!cur.isEmpty()) {
acc.add(cur);
cur = new ArrayList<>();
}
return part(input, acc, cur, i + 1);
}
part(input, new ArrayList<>(), new ArrayList<>(), 0)
AtomicInteger i = new AtomicInteger();
List<List<String>> x = Stream.of("A", "B", null, "C", "D", "E", null, "H", "K")
.collect(Collectors.groupingBy(s -> s == null ? i.incrementAndGet() : i.get()))
.entrySet().stream().map(e -> e.getValue().stream().filter(v -> v != null).collect(Collectors.toList()))
.collect(Collectors.toList());
System.out.println(x);
Arrays.asList(IntStream.range(0, abc.size()-1).
filter(index -> abc.get(index).equals("#") ).
map(index -> (index)).toArray()).
stream().forEach( index -> {for (int i = 0; i < index.length; i++) {
if(sublist.size()==0){
sublist.add(new ArrayList<String>(abc.subList(0, index[i])));
}else{
sublist.add(new ArrayList<String>(abc.subList(index[i]-1, index[i])));
}
}
sublist.add(new ArrayList<String>(abc.subList(index[index.length-1]+1, abc.size())));
});