[英]How to get a random element from a list with stream api?
What is the most effective way to get a random element from a list with Java8 stream api?使用 Java8 流 api 从列表中获取随机元素的最有效方法是什么?
Arrays.asList(new Obj1(), new Obj2(), new Obj3());
Thanks.谢谢。
Why with streams?为什么使用流? You just have to get a random number from 0 to the size of the list and then call get
on this index:您只需要获取一个从 0 到列表大小的随机数,然后在该索引上调用get
:
Random r = new Random();
ElementType e = list.get(r.nextInt(list.size()));
Stream will give you nothing interesting here, but you can try with: Stream 在这里不会给你任何有趣的东西,但你可以尝试:
Random r = new Random();
ElementType e = list.stream().skip(r.nextInt(list.size())).findFirst().get();
Idea is to skip an arbitrary number of elements (but not the last one!), then get the first element if it exists.想法是跳过任意数量的元素(但不是最后一个!),然后获取第一个元素(如果存在)。 As a result you will have an Optional<ElementType>
which will be non empty and then extract its value with get
.结果,您将有一个Optional<ElementType>
非空,然后使用get
提取其值。 You have a lot of options here after having skip.跳过后,您在这里有很多选择。
Using streams here is highly inefficient...在这里使用流是非常低效的......
Note: that none of these solutions take in account empty lists, but the problem is defined on non-empty lists.注意:这些解决方案都没有考虑空列表,但问题是在非空列表上定义的。
There are much more efficient ways to do it, but if this has to be Stream the easiest way is to create your own Comparator, which returns random result (-1, 0, 1) and sort your stream:有更有效的方法可以做到这一点,但如果这必须是 Stream,最简单的方法是创建自己的 Comparator,它返回随机结果 (-1, 0, 1) 并对流进行排序:
List<String> strings = Arrays.asList("a", "b", "c", "d", "e", "f");
String randomString = strings
.stream()
.sorted((o1, o2) -> ThreadLocalRandom.current().nextInt(-1, 2))
.findAny()
.get();
ThreadLocalRandom has ready "out of the box" method to get random number in your required range for comparator. ThreadLocalRandom 已经准备好“开箱即用”的方法来获取比较器所需范围内的随机数。
While all the given answers work, there is a simple one-liner that does the trick without having to check if the list is empty first:虽然所有给定的答案都有效,但有一个简单的单行代码可以解决问题,而无需先检查列表是否为空:
List<String> list = List.of("a", "b", "c");
list.stream().skip((int) (list.size() * Math.random())).findAny();
For an empty list this will return an Optional.empty
.对于一个空列表,这将返回一个Optional.empty
。
If you HAVE to use streams, I wrote an elegant, albeit very inefficient collector that does the job:如果你必须使用流,我写了一个优雅的,虽然效率很低的收集器来完成这项工作:
/**
* Returns a random item from the stream (or null in case of an empty stream).
* This operation can't be lazy and is inefficient, and therefore shouldn't
* be used on streams with a large number or items or in performance critical sections.
* @return a random item from the stream or null if the stream is empty.
*/
public static <T> Collector<T, List<T>, T> randomItem() {
final Random RANDOM = new Random();
return Collector.of(() -> (List<T>) new ArrayList<T>(),
(acc, elem) -> acc.add(elem),
(list1, list2) -> ListUtils.union(list1, list2), // Using a 3rd party for list union, could be done "purely"
list -> list.isEmpty() ? null : list.get(RANDOM.nextInt(list.size())));
}
Usage:用法:
@Test
public void standardRandomTest() {
assertThat(Stream.of(1, 2, 3, 4).collect(randomItem())).isBetween(1, 4);
}
In the last time I needed to do something like that I did that:在最后一次我需要做类似的事情时,我这样做了:
List<String> list = Arrays.asList("a", "b", "c");
Collections.shuffle(list);
String letter = list.stream().findAny().orElse(null);
System.out.println(letter);
If you don't know in advance the size of the your list, you could do something like that :如果您事先不知道列表的大小,则可以执行以下操作:
yourStream.collect(new RandomListCollector<>(randomSetSize));
I guess that you will have to write your own Collector implementation like this one to have an homogeneous randomization :我想您将不得不编写自己的 Collector 实现,就像这样才能实现均匀随机化:
public class RandomListCollector<T> implements Collector<T, RandomListCollector.ListAccumulator<T>, List<T>> {
private final Random rand;
private final int size;
public RandomListCollector(Random random , int size) {
super();
this.rand = random;
this.size = size;
}
public RandomListCollector(int size) {
this(new Random(System.nanoTime()), size);
}
@Override
public Supplier<ListAccumulator<T>> supplier() {
return () -> new ListAccumulator<T>();
}
@Override
public BiConsumer<ListAccumulator<T>, T> accumulator() {
return (l, t) -> {
if (l.size() < size) {
l.add(t);
} else if (rand.nextDouble() <= ((double) size) / (l.gSize() + 1)) {
l.add(t);
l.remove(rand.nextInt(size));
} else {
// in any case gSize needs to be incremented
l.gSizeInc();
}
};
}
@Override
public BinaryOperator<ListAccumulator<T>> combiner() {
return (l1, l2) -> {
int lgSize = l1.gSize() + l2.gSize();
ListAccumulator<T> l = new ListAccumulator<>();
if (l1.size() + l2.size()<size) {
l.addAll(l1);
l.addAll(l2);
} else {
while (l.size() < size) {
if (l1.size()==0 || l2.size()>0 && rand.nextDouble() < (double) l2.gSize() / (l1.gSize() + l2.gSize())) {
l.add(l2.remove(rand.nextInt(l2.size()), true));
} else {
l.add(l1.remove(rand.nextInt(l1.size()), true));
}
}
}
// set the gSize of l :
l.gSize(lgSize);
return l;
};
}
@Override
public Function<ListAccumulator<T>, List<T>> finisher() {
return (la) -> la.list;
}
@Override
public Set<Characteristics> characteristics() {
return Collections.singleton(Characteristics.CONCURRENT);
}
static class ListAccumulator<T> implements Iterable<T> {
List<T> list;
volatile int gSize;
public ListAccumulator() {
list = new ArrayList<>();
gSize = 0;
}
public void addAll(ListAccumulator<T> l) {
list.addAll(l.list);
gSize += l.gSize;
}
public T remove(int index) {
return remove(index, false);
}
public T remove(int index, boolean global) {
T t = list.remove(index);
if (t != null && global)
gSize--;
return t;
}
public void add(T t) {
list.add(t);
gSize++;
}
public int gSize() {
return gSize;
}
public void gSize(int gSize) {
this.gSize = gSize;
}
public void gSizeInc() {
gSize++;
}
public int size() {
return list.size();
}
@Override
public Iterator<T> iterator() {
return list.iterator();
}
}
}
If you want something easier and still don't want to load all your list in memory:如果您想要更简单的东西并且仍然不想将所有列表加载到内存中:
public <T> Stream<T> getRandomStreamSubset(Stream<T> stream, int subsetSize) {
int cnt = 0;
Random r = new Random(System.nanoTime());
Object[] tArr = new Object[subsetSize];
Iterator<T> iter = stream.iterator();
while (iter.hasNext() && cnt <subsetSize) {
tArr[cnt++] = iter.next();
}
while (iter.hasNext()) {
cnt++;
T t = iter.next();
if (r.nextDouble() <= (double) subsetSize / cnt) {
tArr[r.nextInt(subsetSize)] = t;
}
}
return Arrays.stream(tArr).map(o -> (T)o );
}
but you are then away from the stream api and could do the same with a basic iterator但是您随后离开了流 api,并且可以使用基本迭代器执行相同的操作
Another idea would be to implement your own Spliterator
and then use it as a source for Stream
:另一个想法是实现您自己的Spliterator
,然后将其用作Stream
的源:
import java.util.List;
import java.util.Random;
import java.util.Spliterator;
import java.util.function.Consumer;
import java.util.function.Supplier;
public class ImprovedRandomSpliterator<T> implements Spliterator<T> {
private final Random random;
private final T[] source;
private int size;
ImprovedRandomSpliterator(List<T> source, Supplier<? extends Random> random) {
if (source.isEmpty()) {
throw new IllegalArgumentException("RandomSpliterator can't be initialized with an empty collection");
}
this.source = (T[]) source.toArray();
this.random = random.get();
this.size = this.source.length;
}
@Override
public boolean tryAdvance(Consumer<? super T> action) {
if (size > 0) {
int nextIdx = random.nextInt(size);
int lastIdx = size - 1;
action.accept(source[nextIdx]);
source[nextIdx] = source[lastIdx];
source[lastIdx] = null; // let object be GCed
size--;
return true;
} else {
return false;
}
}
@Override
public Spliterator<T> trySplit() {
return null;
}
@Override
public long estimateSize() {
return source.length;
}
@Override
public int characteristics() {
return SIZED;
}
}
public static <T> Collector<T, ?, Stream<T>> toShuffledStream() {
return Collectors.collectingAndThen(
toCollection(ArrayList::new),
list -> !list.isEmpty()
? StreamSupport.stream(new ImprovedRandomSpliterator<>(list, Random::new), false)
: Stream.empty());
}
and then simply:然后简单地说:
list.stream()
.collect(toShuffledStream())
.findAny();
Details can be found here. 详细信息可以在这里找到。
...but it's definitely an overkill, so if you're looking for a pragmatic approach. ...但这绝对是一种矫枉过正,所以如果你正在寻找一种务实的方法。 Definitely go for Jean's solution .绝对去Jean的解决方案。
The selected answer has errors in its stream solution... You cannot use Random#nextInt with a non-positive long, "0" in this case.所选答案在其流解决方案中存在错误...在这种情况下,您不能将 Random#nextInt 与非正长“0”一起使用。 The stream solution will also never choose the last in the list Example:流解决方案也永远不会选择列表中的最后一个示例:
List<Integer> intList = Arrays.asList(0, 1, 2, 3, 4);
// #nextInt is exclusive, so here it means a returned value of 0-3
// if you have a list of size = 1, #next Int will throw an IllegalArgumentException (bound must be positive)
int skipIndex = new Random().nextInt(intList.size()-1);
// randomInt will only ever be 0, 1, 2, or 3. Never 4
int randomInt = intList.stream()
.skip(skipIndex) // max skip of list#size - 2
.findFirst()
.get();
My recommendation would be to go with the non-stream approach that Jean-Baptiste Yunès put forth, but if you must do a stream approach, you could do something like this (but it's a little ugly):我的建议是使用 Jean-Baptiste Yunès 提出的非流方法,但如果你必须使用流方法,你可以这样做(但它有点难看):
list.stream()
.skip(list.isEmpty ? 0 : new Random().nextInt(list.size()))
.findFirst();
您可以将以下部分添加到您的Stream
用于findAny
.sorted((f1, f2) -> (new Random().nextInt(1)) == 0 ? -1 : 1)
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