HashMap 源码分析 -- entrySet()

约定不等于承诺〃 2022-06-05 02:14 111阅读 0赞

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 slot
    
    HashIterator() {
                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());
            }