java面试题――详解HashMap和Hashtable 的区别

 public class Hashtable 
  extends Dictionary 
  implements Map, Cloneable, java.io.Serializable 

public class HashMap 
 extends AbstractMap 
 implements Map, Cloneable, Serializable 

public synchronized V put(K key, V value) { //###### 注意这里1 
 // Make sure the value is not null 
 if (value == null) { //###### 注意这里 2 
 throw new NullPointerException(); 
 } 
 // Makes sure the key is not already in the hashtable. 
 Entry tab[] = table; 
 int hash = key.hashCode(); //###### 注意这里 3 
 int index = (hash & 0x7FFFFFFF) % tab.length; 
 for (Entry e = tab[index]; e != null; e = e.next) { 
 if ((e.hash == hash) && e.key.equals(key)) { 
  V old = e.value; 
  e.value = value; 
  return old; 
 } 
 } 
 modCount++; 
 if (count >= threshold) { 
 // Rehash the table if the threshold is exceeded 
 rehash(); 
 tab = table; 
 index = (hash & 0x7FFFFFFF) % tab.length; 
 } 
 // Creates the new entry. 
 Entry e = tab[index]; 
 tab[index] = new Entry(hash, key, value, e); 
 count++; 
 return null; 
}

public V put(K key, V value) { //###### 注意这里 1 
 if (key == null) //###### 注意这里 2 
 return putForNullKey(value); 
 int hash = hash(key.hashCode()); 
 int i = indexFor(hash, table.length); 
 for (Entry e = table[i]; e != null; e = e.next) { 
 Object k; 
 if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { 
  V oldValue = e.value; 
  e.value = value; 
  e.recordAccess(this); 
  return oldValue; 
 } 
 } 
 modCount++; 
 addEntry(hash, key, value, i); //###### 注意这里 
 return null; 
}