怎么在Java中利用Set&List实现迭代器
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ArrayList
private class Itr implements Iterator{ int cursor; // index of next element to return int lastRet = -1; // index of last element returned; -1 if no such int expectedModCount = modCount; // prevent creating a synthetic constructor Itr() {} public boolean hasNext() { return cursor != size; } @SuppressWarnings("unchecked") public E next() { checkForComodification(); int i = cursor; if (i >= size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; return (E) elementData[lastRet = i]; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } }
从代码中我们不难看出迭代器维护上一次return的元素下边和下一个将要return的元素下标,并且迭代器在进行修改操作时会检查在本次操作与上次操作之间是否有迭代器以外的操作,并且适时抛出ConcurrentModificationException(并发修改异常)来阻止更多错误的发生
LinkedList
private class ListItr implements ListIterator{ private Node lastReturned; private Node next; private int nextIndex; private int expectedModCount = modCount; ListItr(int index) { // assert isPositionIndex(index); next = (index == size) ? null : node(index); nextIndex = index; } public boolean hasNext() { return nextIndex < size; } public E next() { checkForComodification(); if (!hasNext()) throw new NoSuchElementException(); lastReturned = next; next = next.next; nextIndex++; return lastReturned.item; } public boolean hasPrevious() { return nextIndex > 0; } public E previous() { checkForComodification(); if (!hasPrevious()) throw new NoSuchElementException(); lastReturned = next = (next == null) ? last : next.prev; nextIndex--; return lastReturned.item; } public int nextIndex() { return nextIndex; } public int previousIndex() { return nextIndex - 1; } public void remove() { checkForComodification(); if (lastReturned == null) throw new IllegalStateException(); Node lastNext = lastReturned.next; unlink(lastReturned); if (next == lastReturned) next = lastNext; else nextIndex--; lastReturned = null; expectedModCount++; } public void set(E e) { if (lastReturned == null) throw new IllegalStateException(); checkForComodification(); lastReturned.item = e; } public void add(E e) { checkForComodification(); lastReturned = null; if (next == null) linkLast(e); else linkBefore(e, next); nextIndex++; expectedModCount++; } public void forEachRemaining(Consumer super E> action) { Objects.requireNonNull(action); while (modCount == expectedModCount && nextIndex < size) { action.accept(next.item); lastReturned = next; next = next.next; nextIndex++; } checkForComodification(); } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } }
LinkedList的迭代器类的实现逻辑与ArrayList大致相近但是其访问元素的方式由原来的下标变为 "指针"(Java强引用)
Set
通过看Java源码可以知道Set全家桶基本上都包含了Map,相当于是一种组合的方式
HashSet
构造方法
HashSet有多个构造方法但都是初始化一个HashMap或其子类
/** * Constructs a new, empty set; the backing {@code HashMap} instance has * default initial capacity (16) and load factor (0.75). */ public HashSet() { map = new HashMap<>(); } /** * Constructs a new set containing the elements in the specified * collection. The {@code HashMap} is created with default load factor * (0.75) and an initial capacity sufficient to contain the elements in * the specified collection. * * @param c the collection whose elements are to be placed into this set * @throws NullPointerException if the specified collection is null */ public HashSet(Collection extends E> c) { map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16)); addAll(c); } /** * Constructs a new, empty set; the backing {@code HashMap} instance has * the specified initial capacity and the specified load factor. * * @param initialCapacity the initial capacity of the hash map * @param loadFactor the load factor of the hash map * @throws IllegalArgumentException if the initial capacity is less * than zero, or if the load factor is nonpositive */ public HashSet(int initialCapacity, float loadFactor) { map = new HashMap<>(initialCapacity, loadFactor); } /** * Constructs a new, empty set; the backing {@code HashMap} instance has * the specified initial capacity and default load factor (0.75). * * @param initialCapacity the initial capacity of the hash table * @throws IllegalArgumentException if the initial capacity is less * than zero */ public HashSet(int initialCapacity) { map = new HashMap<>(initialCapacity); } /** * Constructs a new, empty linked hash set. (This package private * constructor is only used by LinkedHashSet.) The backing * HashMap instance is a LinkedHashMap with the specified initial * capacity and the specified load factor. * * @param initialCapacity the initial capacity of the hash map * @param loadFactor the load factor of the hash map * @param dummy ignored (distinguishes this * constructor from other int, float constructor.) * @throws IllegalArgumentException if the initial capacity is less * than zero, or if the load factor is nonpositive */ HashSet(int initialCapacity, float loadFactor, boolean dummy) { map = new LinkedHashMap<>(initialCapacity, loadFactor); }
iterator方法
该接口在HashSet中的实现相当的简单,可以看到iterator返回了keySet().iterator()
public Iteratoriterator() { return map.keySet().iterator(); }
HashMap的KeySet
从这一处代码可以看到iterator()返回了对象 KeyIterator
final class KeySet extends AbstractSet{ public final int size() { return size; } public final void clear() { HashMap.this.clear(); } public final Iterator iterator() { return new KeyIterator(); } public final boolean contains(Object o) { return containsKey(o); } public final boolean remove(Object key) { return removeNode(hash(key), key, null, false, true) != null; } public final Spliterator spliterator() { return new KeySpliterator<>(HashMap.this, 0, -1, 0, 0); } public final void forEach(Consumer super K> action) { Node [] tab; if (action == null) throw new NullPointerException(); if (size > 0 && (tab = table) != null) { int mc = modCount; for (Node e : tab) { for (; e != null; e = e.next) action.accept(e.key); } if (modCount != mc) throw new ConcurrentModificationException(); } } }
HashMap的KeyIterator
KeyIterator是HashIterator一个子类在此一并展示了,这个类从字段结构上跟LinkedList的ListItr还是很像的
获取next的机制 Node
abstract class HashIterator { Nodenext; // next entry to return Node current; // current entry int expectedModCount; // for fast-fail int index; // current slot HashIterator() { expectedModCount = modCount; Node [] t = table; current = next = null; index = 0; if (t != null && size > 0) { // advance to first entry do {} while (index < t.length && (next = t[index++]) == null); } } public final boolean hasNext() { return next != null; } final Node nextNode() { Node [] t; Node e = next; if (modCount != expectedModCount) throw new ConcurrentModificationException(); if (e == null) throw new NoSuchElementException(); if ((next = (current = e).next) == null && (t = table) != null) { do {} while (index < t.length && (next = t[index++]) == null); } return e; } public final void remove() { Node p = current; if (p == null) throw new IllegalStateException(); if (modCount != expectedModCount) throw new ConcurrentModificationException(); current = null; removeNode(p.hash, p.key, null, false, false); expectedModCount = modCount; } } final class KeyIterator extends HashIterator implements Iterator { public final K next() { return nextNode().key; } }
以上就是怎么在Java中利用Set&List实现迭代器,小编相信有部分知识点可能是我们日常工作会见到或用到的。希望你能通过这篇文章学到更多知识。更多详情敬请关注创新互联行业资讯频道。
分享名称:怎么在Java中利用Set&List实现迭代器
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