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本文主要研究一下Elasticsearch的Iterables

Iterables

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/common/util/iterable/Iterables.java

public class Iterables {public static <T> Iterable<T> concat(Iterable<T>... inputs) {Objects.requireNonNull(inputs);return new ConcatenatedIterable<>(inputs);}static class ConcatenatedIterable<T> implements Iterable<T> {private final Iterable<T>[] inputs;ConcatenatedIterable(Iterable<T>[] inputs) {this.inputs = Arrays.copyOf(inputs, inputs.length);}@Overridepublic Iterator<T> iterator() {return Stream.of(inputs).map(it -> StreamSupport.stream(it.spliterator(), false)).reduce(Stream::concat).orElseGet(Stream::empty).iterator();}}/** Flattens the two level {@code Iterable} into a single {@code Iterable}.  Note that this pre-caches the values from the outer {@code*  Iterable}, but not the values from the inner one. */public static <T> Iterable<T> flatten(Iterable<? extends Iterable<T>> inputs) {Objects.requireNonNull(inputs);return new FlattenedIterables<>(inputs);}static class FlattenedIterables<T> implements Iterable<T> {private final Iterable<? extends Iterable<T>> inputs;FlattenedIterables(Iterable<? extends Iterable<T>> inputs) {List<Iterable<T>> list = new ArrayList<>();for (Iterable<T> iterable : inputs) {list.add(iterable);}this.inputs = list;}@Overridepublic Iterator<T> iterator() {return StreamSupport.stream(inputs.spliterator(), false).flatMap(s -> StreamSupport.stream(s.spliterator(), false)).iterator();}}public static <T> T get(Iterable<T> iterable, int position) {Objects.requireNonNull(iterable);if (position < 0) {throw new IllegalArgumentException("position >= 0");}if (iterable instanceof List) {List<T> list = (List<T>)iterable;if (position >= list.size()) {throw new IndexOutOfBoundsException(Integer.toString(position));}return list.get(position);} else {Iterator<T> it = iterable.iterator();for (int index = 0; index < position; index++) {if (!it.hasNext()) {throw new IndexOutOfBoundsException(Integer.toString(position));}it.next();}if (!it.hasNext()) {throw new IndexOutOfBoundsException(Integer.toString(position));}return it.next();}}
}

• Iterables提供了concat、flatten、get三个静态方法，其中concat返回的是ConcatenatedIterable；flatten返回的是FlattenedIterables；get方法会先判断是否是List类型是的话直接通过position取值返回，不是则遍历iterable获取指定index的元素

实例

elasticsearch-7.0.1/server/src/test/java/org/elasticsearch/common/util/iterable/IterablesTests.java

public class IterablesTests extends ESTestCase {public void testGetOverList() {test(Arrays.asList("a", "b", "c"));}public void testGetOverIterable() {Iterable<String> iterable = () ->new Iterator<String>() {private int position = 0;@Overridepublic boolean hasNext() {return position < 3;}@Overridepublic String next() {if (position < 3) {String s = position == 0 ? "a" : position == 1 ? "b" : "c";position++;return s;} else {throw new NoSuchElementException();}}};test(iterable);}public void testFlatten() {List<List<Integer>> list = new ArrayList<>();list.add(new ArrayList<>());Iterable<Integer> allInts = Iterables.flatten(list);int count = 0;for(@SuppressWarnings("unused") int x : allInts) {count++;}assertEquals(0, count);list.add(new ArrayList<>());list.get(1).add(0);// changes to the outer list are not seen since flatten pre-caches outer list on init:count = 0;for(@SuppressWarnings("unused") int x : allInts) {count++;}assertEquals(0, count);// but changes to the original inner lists are seen:list.get(0).add(0);for(@SuppressWarnings("unused") int x : allInts) {count++;}assertEquals(1, count);}private void test(Iterable<String> iterable) {try {Iterables.get(iterable, -1);fail("expected IllegalArgumentException");} catch (IllegalArgumentException e) {assertThat(e, hasToString("java.lang.IllegalArgumentException: position >= 0"));}assertEquals("a", Iterables.get(iterable, 0));assertEquals("b", Iterables.get(iterable, 1));assertEquals("c", Iterables.get(iterable, 2));try {Iterables.get(iterable, 3);fail("expected IndexOutOfBoundsException");} catch (IndexOutOfBoundsException e) {assertThat(e, hasToString("java.lang.IndexOutOfBoundsException: 3"));}}
}

• flatten用于将Iterable类型的Iterable进行flat操作；get方法则获取iterable中指定index的元素

小结

Iterables提供了concat、flatten、get三个静态方法，其中concat返回的是ConcatenatedIterable；flatten返回的是FlattenedIterables；get方法会先判断是否是List类型是的话直接通过position取值返回，不是则遍历iterable获取指定index的元素

doc

• Iterables