HashTable继承自Dictionary，而HashMap继承自AbstractMap。Dictionary已经被废弃。
/** * Hash table based implementation of the <tt>Map</tt> interface. This * implementation provides all of the optional map operations, and permits * <tt>null</tt> values and the <tt>null</tt> key. (The <tt>HashMap</tt> * class is roughly equivalent to <tt>Hashtable</tt>, except that it is * unsynchronized and permits nulls.) This class makes no guarantees as to * the order of the map; in particular, it does not guarantee that the order * will remain constant over time. **/ //HashMap大致相等于HashTable，除了不同步（不线程安全）和允许为null。 public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable { ... } public class Hashtable<K, V> extends Dictionary<K, V> implements Map<K, V>, Cloneable, Serializable { ... }继续看Dictionary和AbstractMap的源码，Dictionary就是一个抽象类，废弃，而AbstractMap是实现接口Map的抽象类。
/** <strong>NOTE: This class is obsolete. New implementations should * implement the Map interface, rather than extending this class.</strong> */ //这个类已经过时了。新的实现应该实现MAP接口，而不是扩展这个类。 public abstract class Dictionary<K,V> { ... } public abstract class AbstractMap<K,V> implements Map<K,V> { ... }下面是他们的继承关系： hashtable的继承图： hashMap的继承图：
从代码中看， Hashtable的put方法如果put一个null则抛出NullPointerException异常，而HashMap则是不会判断是否为null，可以插入null值。第二对于HashTable，方法前加入synchronized，是同步的。
public class Hashtable<K, V> extends Dictionary<K, V> implements Map<K, V>, Cloneable, Serializable { ... public synchronized V put(K var1, V var2) { if (var2 == null) { throw new NullPointerException(); } else { ... } } } public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable { public V put(K key, V value) { return putVal(hash(key), key, value, false, true); } /** * Implements Map.put and related methods * * @param hash hash for key * @param key the key * @param value the value to put * @param onlyIfAbsent if true, don't change existing value * @param evict if false, the table is in creation mode. * @return previous value, or null if none */ final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<K,V>[] tab; Node<K,V> p; int n, i; //判断tab是否为空，是否还有剩余空间 if ((tab = table) == null || (n = tab.length) == 0) //扩容 n = (tab = resize()).length; //判断tab是否为空 if ((p = tab[i = (n - 1) & hash]) == null) //创建一个Node，可以传null值进去 tab[i] = newNode(hash, key, value, null); else { Node<K,V> e; K k; //比较哈希值是否相同,比较key实例是否相同,比较key值是否相同 if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) e = p; //如果p是TreeNode（LinkedHashMap） else if (p instanceof TreeNode) //在红黑树中插入键值对 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); else { for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) { p.next = newNode(hash, key, value, null); if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st 如果链表的长度大于TREEIFY_THRESHOLD，转成红黑树 treeifyBin(tab, hash); break; } //找到key，跳出循环 if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) break; p = e; } } //为key赋值 if (e != null) { // existing mapping for key V oldValue = e.value; if (!onlyIfAbsent || oldValue == null) //可以替换成null值 e.value = value; afterNodeAccess(e); return oldValue; } } ++modCount; if (++size > threshold) resize(); afterNodeInsertion(evict); return null; } }转载于:https://www.cnblogs.com/sufferingStriver/p/9041166.html
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