HashMap的遍历源码

遍历可以分为上面两种方法，hm.entrySet().iterator()和 hm.keySet().iterator(). entrySet会比keySet快很多。在查资料的时候看到说 hm.entrySet()的时候会拷贝出hashtable中的数据到一个新的Set中，其实不是。

public class IterHashMap {
	public static void main(String args[]) {
		HashMap<Integer, Integer> hm = new HashMap<Integer, Integer>();
		for (int i = 0; i < 10000; i++) {
			hm.put(i, i);
		}

		Date start = new Date();
		System.out.println("****************Method 1***************");
		Iterator iter = hm.entrySet().iterator();
		while (iter.hasNext()) {
			Map.Entry<Integer, Integer> entry = (Map.Entry<Integer, Integer>) iter
					.next();
			System.out.print("Key:" + entry.getKey() + ", Value"
					+ entry.getValue() + "\n");
		}

		Date end = new Date();

		System.out.print((end.getTime() - start.getTime()) + "\n");

		start = new Date();

		System.out.println("****************Method 2***************");
		Iterator iterKey = hm.keySet().iterator();
		while (iterKey.hasNext()) {
			Integer entry = (Integer) iterKey.next();
			System.out.print("Key:" + entry + ", Value" + hm.get(entry) + "\n");
		}
		 end = new Date();
		System.out.print((end.getTime() - start.getTime()) + "\n");
	}
}

HashMap使用了内部类来实现： 下面代码中的table是hashmap中的变量，用来存储数据

private abstract class HashIterator<E> implements Iterator<E> {
  final Entry<K,V> nextEntry() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            Entry<K,V> e = next;
            if (e == null)
                throw new NoSuchElementException();

            if ((next = e.next) == null) {
                Entry[] t = table;
                while (index < t.length && (next = t[index++]) == null)
                    ;
            }
	    current = e;
            return e;
        }
}

可以看到用keyIterator和EntryIterator的唯一区别是keyIterator会调用getKey（）获得key，然后返回。

  private final class KeyIterator extends HashIterator<K> {
        public K next() {
            return nextEntry().getKey();
        }
    }

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