HashMap的iterator()方法理解

HashMap開放3個迭代接口，共同繼承了其內部的抽象父類HashIterator：

final class KeyIterator extends HashIterator  // 禁止被繼承
    implements Iterator<K> {
    public final K next() { return nextNode().key; }
}

final class ValueIterator extends HashIterator
    implements Iterator<V> {
    public final V next() { return nextNode().value; }
}

final class EntryIterator extends HashIterator
    implements Iterator<Map.Entry<K,V>> {
    public final Map.Entry<K,V> next() { return nextNode(); }
}

可以看出對key以及value的迭代都是Node迭代的屬性調用

abstract class HashIterator {
    Node<K,V> next;        // next entry to return
    Node<K,V> current;     // current entry
    int expectedModCount;  // for fast-fail
    int index;             // current slot

    HashIterator() {
        expectedModCount = modCount;
        Node<K,V>[] 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);  // while循環遍歷賦值方式，之前在美劇硅谷中看主角使用過，可以不需要do {}語句
        }
    }

    public final boolean hasNext() {
        return next != null;
    }

    final Node<K,V> nextNode() {  // default級別訪問權限，僅允許在本類（包）被調用
        Node<K,V>[] t;
        Node<K,V> e = next;
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
        if (e == null)
            throw new NoSuchElementException();
        if ((next = (current = e).next) == null && (t = table) != null) {  // 當前值變量current
            do {} while (index < t.length && (next = t[index++]) == null);  // 可以看出hashmap的遍歷順序：從索引0開始遍歷table數組，然后遍歷該索引位置的鏈表或紅黑樹
        }
        return e;
    }

    public final void remove() {
        Node<K,V> p = current;
        if (p == null)
            throw new IllegalStateException();
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
        current = null;
        K key = p.key;
        removeNode(hash(key), key, null, false, false);
        expectedModCount = modCount;
    }
}

最后附上正常調用iterator()方法的入口：

public Set<Map.Entry<K,V>> entrySet() {
    Set<Map.Entry<K,V>> es;
    return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
}
final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
　　public final Iterator<Map.Entry<K,V>> iterator() {
       return new EntryIterator();
　　}
}