HashMap extends AbstractMap implements Map

AbstractMap implements Map

AbstractMap已經實現了map裡的一些公共的介面，比如
size()，isEmpty() 等。在AbstractMap裡留下了一個abstract 方法

public abstract Set<Entry<K,V>> entrySet()

HashMap 實現只是簡單的new EntrySet

public Set<Map.Entry<K,V>> entrySet() {
        Set
 
<Map.Entry<K,V>> es;
        return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
    }

HashMap的成員類-EntrySet

final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
        public final int size(){ return size; }
        public final void clear() { HashMap.this
 
.clear(); }
        //建構函式裡new EntryIterator()
        public final Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator();
        }
       ...
}

HashMap的又一個成員類-EntryIterator extends HashIterator

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

HashMap裡定義的abstract class HashIterator

  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;
            //table 是HashMap裡的陣列容器,很關鍵的一步
            //transient Node<K,V>[] table;

            Node<K,V>[] t = table;
            current = next = null;
            index = 0;
            if (t != null && size > 0) { 
            //直到table[index]不為空，將 table[index]付給next
            //也就是next是Node<K,V>
            //Node<K,V> implements Map.Entry<K,V>
            //因此下面的Iterator<Map.Entry<String,String>>裡的
            //泛型是Map.Entry<String,String>
           do {} while
            (index < t.length && (next = t[index++]) == null);
            }
        }
        ....
}

呼叫map的entrySet()來遍歷

Map<String, String> map = new HashMap<String, String>();
        map.put("1", "1");
        map.put("2", "2");
        map.put("3", "3");
        Set<Map.Entry<String, String>> mapSet = map.entrySet();
        Iterator<Map.Entry<String,String>> it = mapSet.iterator();
        while(it.hasNext()){
            Map.Entry<String,String> entry = it.next();
            System.out.println(entry.getKey() + " = " +entry.getValue());
        }