Hashtable 和 HashMap的区别

分类： 面试题2008-06-05 06:303950人阅读评论(11)收藏举报

hashmapnulltabledictionaryclassobject

我们先看2个类的定义

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

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

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1. public class HashMap
2. extends AbstractMap
3. implements Map, Cloneable, Serializable

public class HashMap    extends AbstractMap    implements Map, Cloneable, Serializable

可见Hashtable 继承自 Dictiionary 而 HashMap继承自AbstractMap

Hashtable的put方法如下

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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. }

  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;  }

注意1 方法是同步的
注意2 方法不允许value==null
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 HashMap的put方法如下

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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. }

  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;  }

注意1 方法是非同步的
注意2 方法允许key==null
注意3 方法并没有对value进行任何调用，所以允许为null

补充：
Hashtable 有一个 contains方法，容易引起误会，所以在HashMap里面已经去掉了
当然，2个类都用containsKey和containsValue方法。

结论： ashMap 在大多数情况下是优先选择的。