类及属性

public class Hashtable<K,V> extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable {
    // Hashtable保存数据的数组
    private transient Entry<?,?>[] table;
    // hashtable的容量
    private transient int count;
    // 阈值
    private int threshold;
    // 负载因子
    private float loadFactor;
    // 结构性修改
    private transient int modCount = 0;
}

构造方法

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);
    }

put方法

public synchronized V put(K key, V value) {
        //值不允许为null
        if (value == null) {
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry<?,?> tab[] = table;
        //得到键的hash
        int hash = key.hashCode();
        //得到对应hash在数组中的桶索引
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        //得到桶中链表头节点
        Entry<K,V> entry = (Entry<K,V>)tab[index];
        //从头开始遍历
        for(; entry != null ; entry = entry.next) {
            //一旦hash值相等并且键相等，替换旧值
            if ((entry.hash == hash) && entry.key.equals(key)) {
                V old = entry.value;
                entry.value = value;
                return old;
            }
        }
        //如果没有找到相同键，那么添加新节点
        addEntry(hash, key, value, index);
        return null;
    }

addEntry方法

private void addEntry(int hash, K key, V value, int index) {
    modCount++;

    Entry<?,?> tab[] = table;
    // 如果容量大于了阈值，扩容
    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];
    // 在数组索引index位置保存
    tab[index] = new Entry<>(hash, key, value, e);
    count++;
}

rehash方法

protected void rehash() {
    int oldCapacity = table.length;
    Entry<?,?>[] oldMap = table;

    // 扩容为原来的2倍加1
    int newCapacity = (oldCapacity << 1) + 1;
    // 扩容后的数量校验
    if (newCapacity - MAX_ARRAY_SIZE > 0) {
        if (oldCapacity == MAX_ARRAY_SIZE)
            // Keep running with MAX_ARRAY_SIZE buckets
            return;
        newCapacity = MAX_ARRAY_SIZE;
    }
    // 新数组
    Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];

    modCount++;
    // 阈值计算
    threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    table = newMap;
    // 双层循环，将原数组中数据复制到新数组中
    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;
            // 重新根据hash计算index
            int index = (e.hash & 0x7FFFFFFF) % newCapacity;
            e.next = (Entry<K,V>)newMap[index];
            newMap[index] = e;
        }
    }
}

get方法

public synchronized V get(Object key) {
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    // 计算数组index
    int index = (hash & 0x7FFFFFFF) % tab.length;
    // 比较返回
    for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            return (V)e.value;
        }
    }
    return null;
}

remove方法

public synchronized V remove(Object key) {
        Entry<?,?> tab[] = table;
        //计算hash值
        int hash = key.hashCode();
        //得到桶的索引
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        Entry<K,V> e = (Entry<K,V>)tab[index];
        //遍历
        for(Entry<K,V> prev = null ; e != null ; prev = e, e = e.next) {
            //如果匹配,修改节点
            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;
    }