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{"name":"Hashtable&HashMap的区别","tagline":"","body":"# 先总结：\r\nHashtable\r\n注意1 方法是同步的\r\n注意2 方法不允许value==null\r\n注意3 方法调用了key的hashCode方法，如果key==null,会抛出空指针异常\r\nHashMap  \r\n注意1 键和值都是对象\r\n注意2 方法是非同步的(但是需要的时候可以做同步，性能比较高)\r\n注意3 方法允许key==null,但是只有一个\r\n注意4 方法并没有对value进行任何调用，所以允许为null\r\nHashtable和HashMap采用的hash/rehash算法都大概一样，所以性能不会有很大的差\r\n\r\n# 我们先看2个类的定义\r\npublic class Hashtable  \r\n    extends Dictionary  \r\n    implements Map, Cloneable, java.io.Serializable  \r\n\r\npublic class HashMap  \r\n    extends AbstractMap  \r\n    implements Map, Cloneable, Serializable\r\n\r\n### 可见Hashtable 继承自 Dictiionary 而 HashMap继承自AbstractMap\r\n\r\n# Hashtable的put方法如下\r\npublic synchronized V put(K key, V value) {  //###### 注意这里1  \r\n  // Make sure the value is not null  \r\n  if (value == null) { //###### 注意这里 2  \r\n    throw new NullPointerException();  \r\n  }  \r\n  // Makes sure the key is not already in the hashtable.  \r\n  Entry tab[] = table;  \r\n  int hash = key.hashCode(); //###### 注意这里 3  \r\n  int index = (hash & 0x7FFFFFFF) % tab.length;  \r\n  for (Entry e = tab[index]; e != null; e = e.next) {  \r\n    if ((e.hash == hash) && e.key.equals(key)) {  \r\n      V old = e.value;  \r\n      e.value = value;  \r\n      return old;  \r\n    }  \r\n  }  \r\n  modCount++;  \r\n  if (count >= threshold) {  \r\n    // Rehash the table if the threshold is exceeded  \r\n    rehash();  \r\n    tab = table;  \r\n    index = (hash & 0x7FFFFFFF) % tab.length;  \r\n  }  \r\n  // Creates the new entry.  \r\n  Entry e = tab[index];  \r\n  tab[index] = new Entry(hash, key, value, e);  \r\n  count++;  \r\n  return null;  \r\n}  \r\n\r\n# HashMap的put方法如下\r\npublic V put(K key, V value) { //###### 注意这里 1  \r\n  if (key == null)  //###### 注意这里 2  \r\n    return putForNullKey(value);  \r\n  int hash = hash(key.hashCode());  \r\n  int i = indexFor(hash, table.length);  \r\n  for (Entry e = table[i]; e != null; e = e.next) {  \r\n    Object k;  \r\n    if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {  \r\n      V oldValue = e.value;  \r\n      e.value = value;  \r\n      e.recordAccess(this);  \r\n      return oldValue;  \r\n    }  \r\n  }  \r\n  modCount++;  \r\n  addEntry(hash, key, value, i);  //###### 注意这里   \r\n  return null;  \r\n}  \r\n\r\n### 附：Hashtable容易被误解的部分\r\nhashmap 有以下这两个方法：\r\nnumbers.containsKey(a)\r\nnumbers.containsValue(value)、\r\nhashtable 有一个contain()方法：\r\n他不是判断是否存在关联的键，而是判断是否存在关键的值\r\n继承于Map接口\r\ncontains（Object value）\r\n测试此映射表中是否存在与指定值关联的键（如果此 Hashtable 将一个或多个键映射到此值，则返回 true）。\r\n判断容器中是否存在值\r\n但是contains并非和containsKey的功能一致而是和containsValue功能等同. 之所以说是等同原因是需要实现MAP中的containsValue:\r\npublic boolean containsValue(Object value) {\r\n return contains(value);\r\n}\r\n\r\n","google":"","note":"Don't delete this file! It's used internally to help with page regeneration."}