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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 
    
      Iterable
     
       concat(Iterable
      
       ... inputs) {        Objects.requireNonNull(inputs);        return new ConcatenatedIterable<>(inputs);    }    static class ConcatenatedIterable
       
         implements Iterable
        
          {        private final Iterable
         
          [] inputs; ConcatenatedIterable(Iterable
          
           [] inputs) { this.inputs = Arrays.copyOf(inputs, inputs.length); } @Override public Iterator
           
             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 
            
              Iterable
             
               flatten(Iterable
              > inputs) { Objects.requireNonNull(inputs); return new FlattenedIterables<>(inputs); } static class FlattenedIterables
              
                implements Iterable
               
                 { private final Iterable
                > inputs; FlattenedIterables(Iterable
                > inputs) { List
                
                 
                  > list = new ArrayList<>(); for (Iterable
                  
                    iterable : inputs) { list.add(iterable); } this.inputs = list; } @Override public Iterator
                   
                     iterator() { return StreamSupport .stream(inputs.spliterator(), false) .flatMap(s -> StreamSupport.stream(s.spliterator(), false)).iterator(); } } public static 
                    
                      T get(Iterable
                     
                       iterable, int position) { Objects.requireNonNull(iterable); if (position < 0) { throw new IllegalArgumentException("position >= 0"); } if (iterable instanceof List) { List
                      
                        list = (List
                       
                        )iterable; if (position >= list.size()) { throw new IndexOutOfBoundsException(Integer.toString(position)); } return list.get(position); } else { Iterator
                        
                          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
    
      iterable = () ->                new Iterator
     
      () {                    private int position = 0;                    @Override                    public boolean hasNext() {                        return position < 3;                    }                    @Override                    public 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 = new ArrayList<>();        list.add(new ArrayList<>());        Iterable
        
          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
         
           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

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