Java HashMap三种循环遍历方式及其性能对比实例分析

Map<String, String> map = new HashMap<String, String>();
for (Entry<String, String> entry : map.entrySet()) {
  entry.getKey();
  entry.getValue();
}

Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
  entry.getKey();
  entry.getValue();
}

Map<String, String> map = new HashMap<String, String>();
for (String key : map.keySet()) {
  map.get(key);
}

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

private final class KeyIterator extends HashIterator<K> {
  public K next() {
    return nextEntry().getKey();
  }
}

public V get(Object key) {
  if (key == null)
    return getForNullKey();
  Entry<K,V> entry = getEntry(key);
  return null == entry ? null : entry.getValue();
}
/**
 1. Returns the entry associated with the specified key in the
 2. HashMap. Returns null if the HashMap contains no mapping
 3. for the key.
 */
final Entry<K,V> getEntry(Object key) {
  int hash = (key == null) ? 0 : hash(key);
  for (Entry<K,V> e = table[indexFor(hash, table.length)];
     e != null;
     e = e.next) {
    Object k;
    if (e.hash == hash &&
      ((k = e.key) == key || (key != null && key.equals(k))))
      return e;
  }
  return null;
}