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,此篇文件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 位的 1.8.0_171。

數據結構

詳解 Java 8 HashMap 實現原理

結合上圖及源碼可以看出,HashMap 底層數據結構為 Node 類型數組,Node 類型為 HashMap 的內部類,數據結構為鏈表。

/**

* 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[] table;

/**

* Basic hash bin node, used for most entries. (See below for

* TreeNode subclass, and in LinkedHashMap for its Entry subclass.)

*/

static class Node implements Map.Entry {

final int hash;

final K key;

V value;

Node next;

Node(int hash, K key, V value, Node 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 默認的初始大小為 16,如有特殊情況下需要自定義初始化大小時可調用 HashMap(int initialCapacity) 方法進行自定義。

/**

* The default initial capacity - MUST be a power of two.

*/

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

/**

* Constructs an empty HashMap 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

負載因子默認為 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定義初始化大小時可調用 以下方法進行自定義。

/**

* The load factor used when none specified in constructor.

*/

static final float DEFAULT_LOAD_FACTOR = 0.75f;

/**

* Constructs an empty HashMap 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 的數據結構為鏈表,樹型閥值就是當鏈表長度超過這個值時,將 Node 的數據結構修改為紅黑樹,以便優化查找時間,默認值為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置以上介紹的幾個參數,第四種方法為通過其他 Map 實現創建 HashMap。

/**

* Constructs an empty HashMap 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 HashMap 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 HashMap 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 HashMap with the same mappings as the

* specified Map. The HashMap

is created with

* default load factor (0.75) and an initial capacity sufficient to

* hold the mappings in the specified Map.

*

* @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);

}

設置 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,如果當前位置為空則直接賦值,如果不為空則放到下一個節點,如果當前為鏈表且添加元素後的長度達到樹型閥值,則將鏈表轉換為紅黑樹

/**

* 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 key, or

* null if there was no mapping for key.

* (A null return can also indicate that the map

* previously associated null with key.)

*/

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[] tab; Node 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 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)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;

}

獲取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 運算值獲取數組中對應下標的內容
  • 循環鏈表或紅黑樹,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.

*

*

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.)

*

*

A return value of {@code null} does not necessarily

* 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 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 getNode(int hash, Object key) {

Node[] tab; Node 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)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 getTreeNode(int h, Object k) {

return ((parent != null) ? root() : this).find(h, k, null);


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