Iterator
Java中有三種Iterator沸呐，Enumeration，Iterator呢燥，ListIterator
- Enumeration 崭添，一種用于遍歷舊式數(shù)據(jù)結(jié)構(gòu)的Iterator，比如Vector和Hashtable（一種同步哈希表）疮茄，也可以用于SequenceInputStream（用于合并輸入流滥朱，順序讀取）
//add 10 elements for test
Vector v = new Vector();
for (int i = 0; i < 10; i++) v.addElement(i);
Enumeration e = v.elements();
// Checking the next element availability
while (e.hasMoreElements())
{
// moving cursor to next element
int i = (Integer)e.nextElement();
System.out.print(i + " ");
}
- Iterator 通用的Iterator力试，可用于任何Collection對(duì)象徙邻，相比于Enumeration，在遍歷時(shí)可進(jìn)行讀取和刪除操作畸裳，下面列出了Iterator的基本方法
Iterator iterator = collection.iterator();
iterator.hasNext();//boolean
iterator.next();//Object
iterator.remove()//void
缺陷：不能新增或者替換元素缰犁，而且只能單向遍歷
- ListIterator
只適用于List集合，比如ArrayList怖糊，LinkedList帅容，不過(guò)該接口提供雙向遍歷：
listIterator.hasPrevious();//boolean
listIterator.previous();//Object
listIterator.previousIndex();//int
listIterator.set();//void replace the last element find by next() or previous()
listIterator.add()//void add element before the element which will be returned by next()
ListIterator沒(méi)有當(dāng)前元素，當(dāng)前指針的位置始終在next()和previous()兩個(gè)元素之間
Spliterators
Spliterators 和其他的 Iterators一樣伍伤，都是遍歷數(shù)據(jù)源的手段并徘，包括Collections，IO Channel扰魂，Generator function
它除了支持順序遍歷之外麦乞，添加了Java 8對(duì)于并行的支持
每一個(gè)Spliterator的特性都不同蕴茴，可能的值有
ORDERED(0x00000010), DISTINCT(0x00000001), SORTED(0x00000004), SIZED(0x00000040), NONNULL(0x00000100), IMMUTABLE(0x00000400), CONCURRENT(0x00001000), SUBSIZED(0x00004000).
針對(duì)不同的特性，characteristics()返回位編碼下的特性int值姐直，hasCharacteristics檢測(cè)是否擁有該特性
//simple bitwise operation
default boolean hasCharacteristics(int characteristics) {
return (characteristics() & characteristics) == characteristics;
}
其中tryAdvance() 方法將next()和hasNext()結(jié)合起來(lái)倦淀，并且可以通過(guò)forEachRemaining(Consumer)，簡(jiǎn)單的對(duì)Iterator的剩余元素進(jìn)行遍歷
除此之外声畏，還可以使用estimateSize和getExactSizeIfKnown來(lái)估計(jì)遍歷對(duì)象的大小
// Create an array list for doubles.
ArrayList<Integer> al = new ArrayList<>();
// Add values to the array list.
al.add(1);
al.add(2);
al.add(-3);
al.add(-3);
al.add(-4);
al.add(5);
Stream<Integer> str1 = al.stream();
Spliterator<Integer> splitr1 = str1.spliterator();
System.out.println(splitr1.characteristics());
//結(jié)合hasNext和next方法
splitr1.tryAdvance((a) -> System.out.println(a+"!"));
// trySplit() 將剩余元素分成兩部分撞叽，取出第一部分
Spliterator<Integer> splitr2 = splitr1.trySplit();
splitr2.forEachRemaining((a) -> System.out.println(a));
splitr1.forEachRemaining((a) -> System.out.println(a));
結(jié)果
16464
1!
2
-3
-3
-4
5
Tips
- 16464 = 0x4050 = 0x4000+0x0010+0x0040=SUBSIZED+SIZED+ORDERED
- Even an inexact estimate is often useful and inexpensive to compute. 估計(jì)遍歷器大小的操作并不昂貴
- The characteristics of a given spliterator before splitting may differ from the characteristics after splitting，F(xiàn)or specific examples see the characteristic values SIZED, SUBSIZED and CONCURRENT. spliterator的characteristics在split前后可能不同插龄，詳見(jiàn)上述幾個(gè)特性
- 任何Iterator的引用在初始化時(shí)撑刺，下標(biāo)都會(huì)指向首元素之前
- 上述的三個(gè)Iterator只是不同的接口纯命，需要在具體的類中透硝，通過(guò)諸如elements(), iterator(), listIterator()等方法實(shí)現(xiàn)数焊，一般是通過(guò)匿名內(nèi)部類來(lái)實(shí)現(xiàn)的
System.out.println(new java.util.Vector().elements().getClass().getName());//java.util.Vector$1
Ref
https://www.geeksforgeeks.org/java-util-interface-spliterator-java8/
https://blog.csdn.net/java4found/article/details/8656594
https://www.geeksforgeeks.org/iterators-in-java/
