HashMap 源码分析 -- entrySet()

HashMap extends AbstractMap implements Map

AbstractMap implements Map

AbstractMap已经实现了map里的一些公共的接口，比如
size()，isEmpty() 等。在AbstractMap里留下了一个abstract 方法

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

HashMap 实现只是简单的new EntrySet

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

HashMap的成员类-EntrySet

final class EntrySet extends AbstractSet<Map.Entry<K,V>> {        public final int size(){ return size; }        public final void clear() { HashMap.this.clear(); }        //构造函数里new EntryIterator()        public final Iterator<Map.Entry<K,V>> iterator() {            return new EntryIterator();        }       ...}

HashMap的又一个成员类-EntryIterator extends HashIterator

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

HashMap里定义的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 slotHashIterator() {            expectedModCount = modCount;            //table 是HashMap里的数组容器,很关键的一步            //transient Node<K,V>[] table;            Node<K,V>[] t = table;            current = next = null;            index = 0;            if (t != null && size > 0) {             //直到table[index]不为空，将 table[index]付给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>>里的            //泛型是Map.Entry<String,String>           do {} while            (index < t.length && (next = t[index++]) == null);            }        }        ....}

调用map的entrySet()来遍历

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());        }