構造函數
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);
}
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
public Hashtable() {
this(11, 0.75f);
}
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
分析:
Hashtable(int initialCapacity, float loadFactor)構造函數默認容量是11,加載因子(負載因子)0.75,如果第四個構造函數,那麼就取2*size與11更大的那個作爲初始容量。我們知道HashMap的容量一定是2整數次冪,用的初始容量爲16,HashTable爲啥用11爲默認初始容量?我們先帶着這個問題繼續往下看。
Put方法
public synchronized void putAll(Map<? extends K, ? extends V> t) {
for (Map.Entry<? extends K, ? extends V> e : t.entrySet())
put(e.getKey(), e.getValue());
}
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;
}
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++;
}
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;
}
}
}
分析:
int index = (hash & 0x7FFFFFFF) % tab.length;這句話是做什麼用的呢?(hash & 0x7FFFFFFF) 的&位運算是爲了確保是一個正整數,而hash是一個有符號數。如果hash值相同,那麼再去比較key是不是相等,如果相等那麼替換old值,並返回old值。如果沒有找到相同的key,那麼說明不同的key發生了碰撞,那麼就利用鏈地址法解決這個問題,把新元素連接到該index後面的尾部。在連接到index後面之前,是否進行擴容的判斷,如果達到臨界值,那麼進行擴容,一旦擴容之後需要重新構建hash表,int newCapacity = (oldCapacity << 1) + 1;這句表示擴容後的大小(2n+1),然後把原來的hashtable值拷貝到新hashtable中,從這裏我們就可以看出,hashtable選用的就是通過利用奇數作爲容量,那麼這麼做是否能夠讓元素分佈更加均勻呢?如果有知道的同學可以跟我分析你的看法。
總結:
1.HashTable是線程安全的
2.HashTable底層實現跟HashMap一樣,使用的是鏈表數組。
3.HashTable的擴容公式是:2n+1,默認容量11或者2*size,選用奇數作爲容量。
4.HashTable中沒有hash函數了,直接取的是Object.hashCode值作爲hash值
5.HashTable中沒有再做類似HashMap中的擾動來降低碰撞概率