前言

聽說Hashtabe、HashMap、LinkedHashMap、ConcurrentHashMap面試被問較多，平時也會去注意，但畢竟還是要自己看看原始碼，這樣才能在開發過程中選擇最合適的資料結構，簡單來說，

HashMap：陣列+單鏈表，陣列（桶，buckets）用以存放單獨的元素或是每條連結串列的表頭；
Hashtable：將HashMap的資料讀寫操作加上synchronized，使集合可以達到執行緒安全的目的；
LinkedHashMap：陣列+雙向連結串列，陣列（桶，buckets）用以存放單獨的元素或是子雙向連結串列的表頭，所有的單獨元素和子雙向列表由域head作為表頭構成一個雙向連結串列，每次訪問元素時將雙向連結串列的元素移動至連結串列尾部，故而可以實現排序或是LRU；
ConcurrentHashMap

：在HashMap的基礎上對陣列的元素進行寫操作時加鎖synchronized (buckets[i])，所以比起Hashtable，它的效率要高些。

HashMap（其實說它是Hashtable更合適）桶、連結串列、Entry(Node)的關係如下：

以下通過分析Map子類的put與get操作來學習各個Map之間的特性。

HashMap

put函式

首先看put操作，通過put的分析，你將瞭解：

• HashMap怎麼建立和擴容的？
• HashMap載入因子loadFactor如何工作的？
• HashMap = 陣列 + 連結串列？

    /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
 

    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

    /**
     * Implements Map.put and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @param value the value to put
     * @param onlyIfAbsent if true, don't change existing value
     * @param evict if false, the table is in creation mode.
     * @return previous value, or null if none
     */
 

    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
        //resize()桶為空時建立new Node[newCap]作為桶，
        //或桶的容量不夠時建立新的大容量的桶，並將舊資料填入新桶中，增大幅度由factor決定
            n = (tab = resize()).length;
        //(n - 1) & hash 決定在桶的哪個部位，如果桶的該位置為空則直接放入Node
        //如果不為空，則將該Node方入連結串列尾部或者替換key相同的Node的value值
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                //找到key相同的節點p，將e指向p，用以後續中進行替換
                e = p;
            else if (p instanceof TreeNode)
            //當Map的元素數量達到一定值後，改用紅黑二叉樹來儲存桶位置相同的元素
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                    //沒有發現相同key的node，新增節點至連結串列尾部
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    //在連結串列中發現含有相同key的node
                    p = e;
                }
            }
            //替換
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        //modCount在寫資料時會+1，當使用iterator進行資料訪問時，會檢測modCount的值
        //如果值發生改變將丟擲ava.util.ConcurrentModificationException
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }
    /**
     * Initializes or doubles table size.  If null, allocates in
     * accord with initial capacity target held in field threshold.
     * Otherwise, because we are using power-of-two expansion, the
     * elements from each bin must either stay at same index, or move
     * with a power of two offset in the new table.
     *
     * @return the table
     */
    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {
			//涉及loadFactor的擴容
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        //建立新的桶
        @SuppressWarnings({"rawtypes","unchecked"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
        //將桶中的元素轉移
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                    //保留連結串列順序的情況下將連結串列轉移到新的容器中
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            next = e.next;
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

get函式

get方法比較簡單：

• 支援null查詢？

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

    /**
     * Implements Map.get and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @return the node, or null if none
     */
    final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                //在桶中直接得到了節點
                return first;
            //節點可能在桶所在[(n - 1) & hash]位置的連結串列或紅黑樹中
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)
                //從樹種查詢節點
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        //連結串列中發現節點
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }

Hashtable

put函式

Hashtable的實現和HashMap的資料結構設計，不同的是：

• 在Hashtable中不會使用紅黑樹
• 讀寫操作都是以synchronized修飾
• 不支援null的讀寫

    /**
     * Maps the specified <code>key</code> to the specified
     * <code>value</code> in this hashtable. Neither the key nor the
     * value can be <code>null</code>. <p>
     *
     * The value can be retrieved by calling the <code>get</code> method
     * with a key that is equal to the original key.
     *
     * @param      key     the hashtable key
     * @param      value   the value
     * @return     the previous value of the specified key in this hashtable,
     *             or <code>null</code> if it did not have one
     * @exception  NullPointerException  if the key or value is
     *               <code>null</code>
     * @see     Object#equals(Object)
     * @see     #get(Object)
     */
    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.
        HashtableEntry<?,?> tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        HashtableEntry<K,V> entry = (HashtableEntry<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;
    }

get函式

get沒有特別需要說明的。

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key.equals(k))},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * @param key the key whose associated value is to be returned
     * @return the value to which the specified key is mapped, or
     *         {@code null} if this map contains no mapping for the key
     * @throws NullPointerException if the specified key is null
     * @see     #put(Object, Object)
     */
    @SuppressWarnings("unchecked")
    public synchronized V get(Object key) {
        HashtableEntry<?,?> tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (HashtableEntry<?,?> e = tab[index] ; e != null ; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                return (V)e.value;
            }
        }
        return null;
    }

LinkedHashMap

接下來看神奇的LinkedHashMap，在涉及快取的時候往往會用到這個東西來實現LRU效果。

• LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>
• LinkedHashMap是如何完成雙向佇列的

put函式

LinkedHashMap中完全繼承HashMap中的put操作，只是重寫了afterNodeAccess方法，此方法在HashMap沒由任何操作，
這裡只觀察afterNodeAccess函式即可：

//將當前元素移動至雙向連結串列
    void afterNodeAccess(Node<K,V> e) { // move node to last
        LinkedHashMapEntry<K,V> last;
        if (accessOrder && (last = tail) != e) {
        //將p指向當前節點，b指向上一節點，a指向下一個節點
            LinkedHashMapEntry<K,V> p =
                (LinkedHashMapEntry<K,V>)e, b = p.before, a = p.after;
                //當前節點的->斷開
            p.after = null;
            //無前置節點則設為表頭
            if (b == null)
                head = a;
            //否則b->a，正向移除當前節點
            else
                b.after = a;
            //如果後置節點不為空，b<-a反響移除當前節點
            if (a != null)
                a.before = b;
            //無後置節點，則前置節點為表尾
            else
                last = b;
            //無後置節點也無前置節點，則只有一個節點，並設定為head(field)
            if (last == null)
                head = p;
            else {
            //移除當前節點，並將當前節點置於表尾
                p.before = last;
                last.after = p;
            }
            tail = p;
            ++modCount;
        }
    }

get函式

get中需要注意的是get訪問會afterNodeAccess函式，從而影響雙向連結串列的排序。

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     */
    public V get(Object key) {
        Node<K,V> e;
        if ((e = getNode(hash(key), key)) == null)
            return null;
        if (accessOrder)
            afterNodeAccess(e);
        return e.value;
    }

removeEldestEntry函式

除了put與get外，removeEldestEntry在建立快取時通常也會被重寫，用以控制快取的大小。當put和putAll被呼叫後此函式將被呼叫，如果函式返回true則在LRU中最近最少訪問的元素將被移除。

    /**
     * Returns <tt>true</tt> if this map should remove its eldest entry.
     * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after
     * inserting a new entry into the map.  It provides the implementor
     * with the opportunity to remove the eldest entry each time a new one
     * is added.  This is useful if the map represents a cache: it allows
     * the map to reduce memory consumption by deleting stale entries.
     *
     * <p>Sample use: this override will allow the map to grow up to 100
     * entries and then delete the eldest entry each time a new entry is
     * added, maintaining a steady state of 100 entries.
     * <pre>
     *     private static final int MAX_ENTRIES = 100;
     *
     *     protected boolean removeEldestEntry(Map.Entry eldest) {
     *        return size() &gt; MAX_ENTRIES;
     *     }
     * </pre>
     *
     * <p>This method typically does not modify the map in any way,
     * instead allowing the map to modify itself as directed by its
     * return value.  It <i>is</i> permitted for this method to modify
     * the map directly, but if it does so, it <i>must</i> return
     * <tt>false</tt> (indicating that the map should not attempt any
     * further modification).  The effects of returning <tt>true</tt>
     * after modifying the map from within this method are unspecified.
     *
     * <p>This implementation merely returns <tt>false</tt> (so that this
     * map acts like a normal map - the eldest element is never removed).
     *
     * @param    eldest The least recently inserted entry in the map, or if
     *           this is an access-ordered map, the least recently accessed
     *           entry.  This is the entry that will be removed it this
     *           method returns <tt>true</tt>.  If the map was empty prior
     *           to the <tt>put</tt> or <tt>putAll</tt> invocation resulting
     *           in this invocation, this will be the entry that was just
     *           inserted; in other words, if the map contains a single
     *           entry, the eldest entry is also the newest.
     * @return   <tt>true</tt> if the eldest entry should be removed
     *           from the map; <tt>false</tt> if it should be retained.
     */
    protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
        return false;
    }

ConcurrentHashMap

put函式

ConcurrentHashMap

    /**
     * Maps the specified key to the specified value in this table.
     * Neither the key nor the value can be null.
     *
     * <