JavaHashMap三种循环遍历方式及其性能对比实例分析

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

Iterator> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
  entry.getKey();
  entry.getValue();
}

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

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

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

public V get(Object key) {
  if (key == null)
    return getForNullKey();
  Entry entry = getEntry(key);
  return null == entry &＃63; 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 getEntry(Object key) {
  int hash = (key == null) &＃63; 0 : hash(key);
  for (Entry 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;
}