public class Hashtable<K,V> extends Dictionary<K,V>
						    implements Map<K,V>, Cloneable, java.io.Serializable

/**
 * NOTE: This class is obsolete. New implementations should implement the Map interface, rather than extending this class.
 *
 **/
public abstract class Dictionary<K,V> {
    public Dictionary() {
    }
    abstract public int size();
    abstract public boolean isEmpty();
    abstract public Enumeration<K> keys();
    abstract public Enumeration<V> elements();
    abstract public V get(Object key);
    abstract public V put(K key, V value);
    abstract public V remove(Object key);
}

private static class Entry<K,V> implements Map.Entry<K,V> {
    // 鍵值對(duì)節(jié)點(diǎn)中key的哈希值
    final int hash;
    // 鍵值對(duì)節(jié)點(diǎn)中的key
    final K key;
    // 鍵值對(duì)節(jié)點(diǎn)中的value
    V value;
    // 鏈表中當(dāng)前節(jié)點(diǎn)的下一個(gè)節(jié)點(diǎn)
    Entry<K,V> next;
    protected Entry(int hash, K key, V value, Entry<K,V> next) {
        this.hash = hash;
        this.key =  key;
        this.value = value;
        this.next = next;
    }
}

// 哈希表中的數(shù)組
private transient Entry<?,?>[] table;
// 哈希表中鍵值對(duì)節(jié)點(diǎn)數(shù)量
private transient int count;
// 擴(kuò)容閾值
private int threshold;
// 加載因子，默認(rèn)0.75
private float loadFactor;
// 結(jié)構(gòu)性修改次數(shù)，用于快速失敗
private transient int modCount = 0;

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);
}

public Hashtable(int initialCapacity) {
    this(initialCapacity, 0.75f);
}

public Hashtable(Map<? extends K, ? extends V> t) {
    this(Math.max(2*t.size(), 11), 0.75f);
    putAll(t);
}

protected void rehash() {
    // 擴(kuò)容前的哈希數(shù)組容量
    int oldCapacity = table.length;
    // 擴(kuò)容前的哈希數(shù)組
    Entry<?,?>[] oldMap = table;
    // 1.計(jì)算擴(kuò)容后的容量，新容量= 原容量*2 +1
    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;
    // 2. 遍歷原哈希表，將原哈希表中的鍵值對(duì)重新計(jì)算數(shù)組下標(biāo)后，通過(guò)頭插法，放至新的哈希表中。
    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;
            // 重新計(jì)算數(shù)組下標(biāo)
            int index = (e.hash & 0x7FFFFFFF) % newCapacity;
            // 頭插法
            e.next = (Entry<K,V>)newMap[index];
            newMap[index] = e;
        }
    }
}

private void addEntry(int hash, K key, V value, int index) {
    modCount++;
    Entry<?,?> tab[] = table;
    if (count >= threshold) {
        // 擴(kuò)容
        rehash();
        tab = table;
        hash = key.hashCode();
        index = (hash & 0x7FFFFFFF) % tab.length;
    }
    // Creates the new entry.
    @SuppressWarnings("unchecked")
    // 頭插法
    Entry<K,V> e = (Entry<K,V>) tab[index];
    tab[index] = new Entry<>(hash, key, value, e);
    count++;
}

public synchronized int size() {
    return count;
}

public synchronized boolean isEmpty() {
    return count == 0;
}

public synchronized boolean contains(Object value) {
    if (value == null) {
        throw new NullPointerException();
    }
    Entry<?,?> tab[] = table;
    for (int i = tab.length ; i-- > 0 ;) {
        for (Entry<?,?> e = tab[i] ; e != null ; e = e.next) {
            if (e.value.equals(value)) {
                return true;
            }
        }
    }
    return false;
}
public boolean containsValue(Object value) {
    return contains(value);
}

public synchronized boolean containsKey(Object key) {
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            return true;
        }
    }
    return false;
}

public synchronized V get(Object key) {
    Entry<?,?> tab[] = table;
    // 計(jì)算哈希值確認(rèn)哈希表中數(shù)組的下標(biāo)
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    // 遍歷鏈表，查找到相同的key的鍵值對(duì)，并返回該鍵值對(duì)的value
    for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            return (V)e.value;
        }
    }
    return null;
}

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;
    // 根據(jù)參數(shù)key計(jì)算出對(duì)應(yīng)的數(shù)組下標(biāo)
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    Entry<K,V> entry = (Entry<K,V>)tab[index];
    // 對(duì)該數(shù)組下標(biāo)上的鏈表進(jìn)行遍歷，如果存在key相等的鍵值對(duì)，則對(duì)其對(duì)應(yīng)的value值進(jìn)行覆蓋
    for(; entry != null ; entry = entry.next) {
        if ((entry.hash == hash) && entry.key.equals(key)) {
            V old = entry.value;
            // 覆蓋value
            entry.value = value;
            return old;
        }
    }
    // 如果不存在key相等的鍵值對(duì)，則調(diào)用addEntry()方法通過(guò)頭插法保存鍵值對(duì)
    addEntry(hash, key, value, index);
    return null;
}

public synchronized V remove(Object key) {
    Entry<?,?> tab[] = table;
    // 計(jì)算數(shù)組下標(biāo)
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    // 遍歷該數(shù)組下標(biāo)上的鏈表
    Entry<K,V> e = (Entry<K,V>)tab[index];
    for(Entry<K,V> prev = null ; e != null ; prev = e, e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            modCount++;
            if (prev != null) {
                prev.next = e.next;
            } else {
                tab[index] = e.next;
            }
            count--;
            V oldValue = e.value;
            e.value = null;
            return oldValue;
        }
    }
    return null;
}