public class Hashtable  extends Dictionary  implements Map, Cloneable, <a href="http://lib.csdn.net/base/javase" class='replace_word' title="Java SE知识库" target='_blank' style='color:#df3434; font-weight:bold;'>Java</a>.io.Serializable  

public class HashMap  extends AbstractMap  implements Map, Cloneable, Serializable  

 1 public synchronized V put(K key, V value) {  //###### 注意这里1
 2   // Make sure the value is not null
 3   if (value == null) { //###### 注意这里 2
 4     throw new NullPointerException();
 5   }
 6   // Makes sure the key is not already in the hashtable.
 7   Entry tab[] = table;
 8   int hash = key.hashCode(); //###### 注意这里 3
 9   int index = (hash & 0x7FFFFFFF) % tab.length;
10   for (Entry e = tab[index]; e != null; e = e.next) {
11     if ((e.hash == hash) && e.key.equals(key)) {
12       V old = e.value;
13       e.value = value;
14       return old;
15     }
16   }
17   modCount++;
18   if (count >= threshold) {
19     // Rehash the table if the threshold is exceeded
20     rehash();
21     tab = table;
22     index = (hash & 0x7FFFFFFF) % tab.length;
23   }
24   // Creates the new entry.
25   Entry e = tab[index];
26   tab[index] = new Entry(hash, key, value, e);
27   count++;
28   return null;
29 }  

 1 public V put(K key, V value) { //###### 注意这里 1
 2   if (key == null)  //###### 注意这里 2
 3     return putForNullKey(value);
 4   int hash = hash(key.hashCode());
 5   int i = indexFor(hash, table.length);
 6   for (Entry e = table[i]; e != null; e = e.next) {
 7     Object k;
 8     if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
 9       V oldValue = e.value;
10       e.value = value;
11       e.recordAccess(this);
12       return oldValue;
13     }
14   }
15   modCount++;
16   addEntry(hash, key, value, i);  //###### 注意这里
17   return null;
18 }