Map. Entry <K, V> Analysis

I. Benefits

Are you tired of getting keywords from map every time and getting the corresponding values?

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 } 

 

II. Usage

Using the map. Entry class, you can get all the information at the same time. The standard MAP access method is as follows:

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    }

 

3. Analysis

1. member variables and constructors of static entry in hashmap

 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. Pay attention to the implementation of the Fast Error Reporting mechanism.

 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 entry10                 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 (); // create 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}