HashTable 类
1、key- value 存放元素,key、value 可以是任意引用数据类型
2、无序(添加、取出顺序不一致)、无索引,不保证映射顺序
3、维护一个哈希表:数组 + 单向链表
4、key 不允许重复,value 允许重复,key、value 不允许为 null
5、添加 key - value,若集合有相同 key,value 会被替换
6、线程安全,实现同步互斥
7、使用方法与 HashMap 一致
8、继承 Dictionary 类,实现 Map 接口
底层
1、无参构造器
public Hashtable() {
this(11, 0.75f);
}
public Hashtable(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load: "+loadFactor);
if (initialCapacity==0)
initialCapacity = 1;
this.loadFactor = loadFactor;
table = new Entry<?,?>[initialCapacity];
threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}
(1)table 数组初始容量 11
private transient Entry<?,?>[] table;
(2)负载因子:0.75f
private float loadFactor;
(3)记录 table 有效节点数
private transient int count;
2、添加、扩容机制
(1)待添加 key - value 的 hash 值 = key.hashCode(); 索引值 = (hash & 0x7FFFFFFF) % tab.length;
(2)(entry.hash == hash) && entry.key.equals(key) 判断是否为同一个 key,若成立,替换 value
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry<?,?> tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];
for(; entry != null ; entry = entry.next) {
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}
addEntry(hash, key, value, index);
return null;
}
(3)若不成立,即为新的 key - value
(4)添加 key - value 前,判断是否到达扩容临界值 = 原容量 * 0.75(负载因子)
(5)若扩容,调用 rehash 方法,新容量 = (原容量 * 2) +1
protected void rehash() {
int oldCapacity = table.length;
Entry<?,?>[] oldMap = table;
// overflow-conscious code
int newCapacity = (oldCapacity << 1) + 1;
if (newCapacity - MAX_ARRAY_SIZE > 0) {
if (oldCapacity == MAX_ARRAY_SIZE)
// Keep running with MAX_ARRAY_SIZE buckets
return;
newCapacity = MAX_ARRAY_SIZE;
}
Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];
modCount++;
threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
table = newMap;
for (int i = oldCapacity ; i-- > 0 ;) {
for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
old = old.next;
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = (Entry<K,V>)newMap[index];
newMap[index] = e;
}
}
}
(6)添加 key - value:调用 addEntry 方法,key - value 添加到 index 位置的链表尾
private void addEntry(int hash, K key, V value, int index) {
modCount++;
Entry<?,?> tab[] = table;
if (count >= threshold) {
// Rehash the table if the threshold is exceeded
rehash();
tab = table;
hash = key.hashCode();
index = (hash & 0x7FFFFFFF) % tab.length;
}
// Creates the new entry.
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>) tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count++;
}
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