Map.Entry<K,V>分析

一。好处

你是否已经对每次从Map中取得关键字然后再取得相应的值感觉厌倦？

1 Set keys = map.keySet( );
2 if(keys != null) {
3     Iterator iterator = keys.iterator( );
4    while(iterator.hasNext( )) {
5            Object key = iterator.next( );
6            Object value = map.get(key);
7      }
8 }

二。用法

使用Map.Entry类，你可以得到在同一时间得到所有的信息。标准的Map访问方法如下：

1   for( Entry<Integer, Integer> e : mm.entrySet()){
2       System.out.println("key:"+e.getKey() + "  value:"+e.getValue());
3      
4    }
5    for( Iterator<Entry<Integer, Integer>> i = mm.entrySet().iterator();i.hasNext(); ){
6       Entry<Integer, Integer> e = i.next();
7       System.out.println("key:"+e.getKey() + "  value:"+e.getValue());
8 
9    }

三。分析

1.HashMap内部静态类Entry的成员变量和构造函数

 1     static class Entry<K,V> implements Map.Entry<K,V> {
 2         final K key;
 3         V value;
 4         Entry<K,V> next;
 5         final int hash;
 6 
 7         /**
 8          * Creates new entry.
 9          */
10         Entry(int h, K k, V v, Entry<K,V> n) {
11             value = v;
12             next = n;
13             key = k;
14             hash = h;
15         }
16       
17          。。。。。
18      }

2.Iterator

 1     private final class ValueIterator extends HashIterator<V> {
 2         public V next() {
 3             return nextEntry().value;
 4         }
 5     }
 6 
 7     private final class KeyIterator extends HashIterator<K> {
 8         public K next() {
 9             return nextEntry().getKey();
10         }
11     }
12 
13     private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
14         public Map.Entry<K,V> next() {
15             return nextEntry();
16         }
17     }

3.HashIterator，注意快速报错机制实现

 1     private abstract class HashIterator<E> implements Iterator<E> {
 2         Entry<K,V> next;    // next entry to return
 3         int expectedModCount;    // For fast-fail
 4         int index;        // current slot
 5         Entry<K,V> current;    // current entry
 6 
 7         HashIterator() {
 8             expectedModCount = modCount;
 9             if (size > 0) { // advance to first entry
10                 Entry[] t = table;
11                 while (index < t.length && (next = t[index++]) == null)
12                     ;
13             }
14         }
15 
16         public final boolean hasNext() {
17             return next != null;
18         }
19 
20         final Entry<K,V> nextEntry() {
21             if (modCount != expectedModCount)
22                 throw new ConcurrentModificationException();
23             Entry<K,V> e = next;
24             if (e == null)
25                 throw new NoSuchElementException();
26 
27             if ((next = e.next) == null) {
28                 Entry[] t = table;
29                 while (index < t.length && (next = t[index++]) == null)
30                     ;
31             }
32             current = e;
33             return e;
34         }
35 
36         public void remove() {
37             if (current == null)
38                 throw new IllegalStateException();
39             if (modCount != expectedModCount)
40                 throw new ConcurrentModificationException();
41             Object k = current.key;
42             current = null;
43             HashMap.this.removeEntryForKey(k);
44             expectedModCount = modCount;
45         }
46 
47     }

3.EntrySet

 1     private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
 2         public Iterator<Map.Entry<K,V>> iterator() {
 3             return newEntryIterator();                                                 //创建EntryIterator
 4         }
 5         public boolean contains(Object o) {
 6             if (!(o instanceof Map.Entry))
 7                 return false;
 8             Map.Entry<K,V> e = (Map.Entry<K,V>) o;
 9             Entry<K,V> candidate = getEntry(e.getKey());
10             return candidate != null && candidate.equals(e);
11         }
12         public boolean remove(Object o) {
13             return removeMapping(o) != null;
14         }
15         public int size() {
16             return size;
17         }
18         public void clear() {
19             HashMap.this.clear();
20         }
21     }