詳解 Java 8 HashMap 實現原理

HashMap 是 Java 開發過程中常用的工具類之一，也是面試過程中常問的內容，此篇檔案通過作者自己的理解和網上眾多資料對其進行一個解析。作者本地的 JDK 版本為 64 位元的 1.8.0_171。參考資料推薦以下兩篇文章：

• www.cnblogs.com/little-fly/p/7344285.html
• monkeysayhi.github.io/2017/08/26/HashMap實現原理

資料結構

結合及源碼可以看出，HashMap 底層資料結構為 Node 類型數組，Node 類型為 HashMap 的內部類，資料結構為鏈表。

/** * The table, initialized on first use, and resized as * necessary. When allocated, length is always a power of two. * (We also tolerate length zero in some operations to allow * bootstrapping mechanics that are currently not needed.) */transient Node<K,V>[] table;/** * Basic hash bin node, used for most entries.  (See below for * TreeNode subclass, and in LinkedHashMap for its Entry subclass.) */static class Node<K,V> implements Map.Entry<K,V> {    final int hash;    final K key;    V value;    Node<K,V> next;    Node(int hash, K key, V value, Node<K,V> next) {        this.hash = hash;        this.key = key;        this.value = value;        this.next = next;    }    public final K getKey()        { return key; }    public final V getValue()      { return value; }    public final String toString() { return key + "=" + value; }    public final int hashCode() {        return Objects.hashCode(key) ^ Objects.hashCode(value);    }    public final V setValue(V newValue) {        V oldValue = value;        value = newValue;        return oldValue;    }    public final boolean equals(Object o) {        if (o == this)            return true;        if (o instanceof Map.Entry) {            Map.Entry<?,?> e = (Map.Entry<?,?>)o;            if (Objects.equals(key, e.getKey()) &&                Objects.equals(value, e.getValue()))                return true;        }        return false;    }}

初始大小

HashMap 預設的初始大小為 16，如有特殊情況下需要自訂初始化大小時可調用 HashMap(int initialCapacity) 方法進行自訂。

/** * The default initial capacity - MUST be a power of two. */static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16/** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and the default load factor (0.75). * * @param  initialCapacity the initial capacity. * @throws IllegalArgumentException if the initial capacity is negative. */public HashMap(int initialCapacity) {    this(initialCapacity, DEFAULT_LOAD_FACTOR);}

負載因子

負載因子預設為 0.75，當 HashMap 當前已使用容量大於當前大小 * 負載因子時，自動擴容一倍空間，如有特殊情況下需要自訂初始化大小時可調用 以下方法進行自訂。

/** * The load factor used when none specified in constructor. */static final float DEFAULT_LOAD_FACTOR = 0.75f;/** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and load factor. * * @param  initialCapacity the initial capacity * @param  loadFactor      the load factor * @throws IllegalArgumentException if the initial capacity is negative *         or the load factor is nonpositive */public HashMap(int initialCapacity, float loadFactor) {    if (initialCapacity < 0)        throw new IllegalArgumentException("Illegal initial capacity: " +                                            initialCapacity);    if (initialCapacity > MAXIMUM_CAPACITY)        initialCapacity = MAXIMUM_CAPACITY;    if (loadFactor <= 0 || Float.isNaN(loadFactor))        throw new IllegalArgumentException("Illegal load factor: " +                                            loadFactor);    this.loadFactor = loadFactor;    this.threshold = tableSizeFor(initialCapacity);}

樹型閥值

樹型閥值這個名字是作者根據字面意思自己翻譯的，大家看看就好了，對應參數為TREEIFY_THRESHOLD，之前提到過 HashMap 的結構為 Node 型數組，而 Node 的資料結構為鏈表，樹型閥值就是當鏈表長度超過這個值時，將 Node 的資料結構修改為紅/黑樹狀結構，以便最佳化尋找時間，預設值為8

/** * The bin count threshold for using a tree rather than list for a * bin.  Bins are converted to trees when adding an element to a * bin with at least this many nodes. The value must be greater * than 2 and should be at least 8 to mesh with assumptions in * tree removal about conversion back to plain bins upon * shrinkage. */static final int TREEIFY_THRESHOLD = 8;

初始化

HashMap 提供以下四種構造方法進行初始化，前三種主要區別在於設定以上介紹的幾個參數，第四種方法為通過其他 Map 實現建立 HashMap。

/** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and load factor. * * @param  initialCapacity the initial capacity * @param  loadFactor      the load factor * @throws IllegalArgumentException if the initial capacity is negative *         or the load factor is nonpositive */public HashMap(int initialCapacity, float loadFactor) {    if (initialCapacity < 0)        throw new IllegalArgumentException("Illegal initial capacity: " +                                            initialCapacity);    if (initialCapacity > MAXIMUM_CAPACITY)        initialCapacity = MAXIMUM_CAPACITY;    if (loadFactor <= 0 || Float.isNaN(loadFactor))        throw new IllegalArgumentException("Illegal load factor: " +                                            loadFactor);    this.loadFactor = loadFactor;    this.threshold = tableSizeFor(initialCapacity);}/** * Constructs an empty <tt>HashMap</tt> with the specified initial * capacity and the default load factor (0.75). * * @param  initialCapacity the initial capacity. * @throws IllegalArgumentException if the initial capacity is negative. */public HashMap(int initialCapacity) {    this(initialCapacity, DEFAULT_LOAD_FACTOR);}/** * Constructs an empty <tt>HashMap</tt> with the default initial capacity * (16) and the default load factor (0.75). */public HashMap() {    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted}/** * Constructs a new <tt>HashMap</tt> with the same mappings as the * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with * default load factor (0.75) and an initial capacity sufficient to * hold the mappings in the specified <tt>Map</tt>. * * @param   m the map whose mappings are to be placed in this map * @throws  NullPointerException if the specified map is null */public HashMap(Map<? extends K, ? extends V> m) {    this.loadFactor = DEFAULT_LOAD_FACTOR;    putMapEntries(m, false);}

設定 HashMap 的值

put 方法是 HashMap 中使用率非常高的 API 之一，其源碼實現如下，通過源碼我們可以發現其原理主要分為以下兩步：

• 對 key 進行 hash 運算，然後再與當前 map 最後一個下標進行與運算確定其在數組中的位置，正是因為這個演算法，我們可以得知 HashMap 中元素是無序的。
• 確定其下標以後，如果當前位置為空白則直接賦值，如果不為空白則放到下一個節點，如果當前為鏈表且添加元素後的長度達到樹型閥值，則將鏈錶轉換為紅/黑樹狀結構

/** * 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)        n = (tab = resize()).length;    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))))            e = p;        else if (p instanceof TreeNode)            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);        else {            for (int binCount = 0; ; ++binCount) {                if ((e = p.next) == null) {                    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;                p = e;            }        }        if (e != null) { // existing mapping for key            V oldValue = e.value;            if (!onlyIfAbsent || oldValue == null)                e.value = value;            afterNodeAccess(e);            return oldValue;        }    }    ++modCount;    if (++size > threshold)        resize();    afterNodeInsertion(evict);    return null;}

擷取HashMap中的值

get 方法同樣是 HashMap 中常用的 API 之一，參照其源碼，其原理與 put 方法正好相反，分為以下兩個部分：

• 根據 key 的 hash 運算值擷取數組中對應下標的內容
• 迴圈鏈表或紅/黑樹狀結構，然後匹配 value 值直至獲得對應的值或返回 null

/** * 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))))            return first;        if ((e = first.next) != null) {            if (first instanceof TreeNode)                return ((TreeNode<K,V>)first).getTreeNode(hash, key);            do {                if (e.hash == hash &&                    ((k = e.key) == key || (key != null && key.equals(k))))                    return e;            } while ((e = e.next) != null);        }    }    return null;}/** * Calls find for root node. */final TreeNode<K,V> getTreeNode(int h, Object k) {    return ((parent != null) ? root() : this).find(h, k, null);}