HashMap 連結串列插入方式 → 頭插為何改成尾插 ?

青石路發表於2020-11-19

HashMap

開心一刻

　　一天，樓主在路上碰到了一個很久沒見的朋友，找了個餐館，坐下聊了起來

　　樓主：在哪上班了 ?

　　朋友：火葬場啊

　　樓主：在那上班，一個月多少錢啊 ?

　　朋友：兩萬多啊

　　樓主（不可思議）：多少？

　　朋友（非常淡定）：兩萬多

　　樓主：你們那還要人嗎？

　　朋友：要啊，24小時都要

　　樓主：不是，我的意思是你們那還收人嗎

　　朋友：收，天天都收

　　樓主：我是說，我能進去不？

　　朋友：那200多斤的胖子都能進去，你進不去？

　　樓主：不是，你是非要把我給煉了是咋地？我能進去不，我能自己進去不？

　　朋友：那有點懸，都是推進去的

　　樓主：我是說，你們那還招工嗎

　　朋友：招，不分公母，都招

　　樓主：老闆，買單

　　老闆：你還沒點菜了

　　樓主：不點了，再不走就要被煉了

資料結構

　　對 HashMap 的底層資料結構，相信大家都有所瞭解，不同的版本，底層資料結構會有所不同

　　1.7 的底層資料結構

/**
 * An empty table instance to share when the table is not inflated.
 */
static final Entry<?,?>[] EMPTY_TABLE = {};

/**
 * The table, resized as necessary. Length MUST Always be a power of two.
 */
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

static class Entry<K,V> implements Map.Entry<K,V> {
    final K key;
    V value;
    Entry<K,V> next;
    int hash;
    
    ...
}

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　　1.8 的底層資料結構

/**
 * 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;

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    Node<K,V> next;

    ...
}

/**
 * Entry for Tree bins. Extends LinkedHashMap.Entry (which in turn
 * extends Node) so can be used as extension of either regular or
 * linked node.
 */
static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
    TreeNode<K,V> parent;  // red-black tree links
    TreeNode<K,V> left;
    TreeNode<K,V> right;
    TreeNode<K,V> prev;    // needed to unlink next upon deletion
    boolean red;
    
    ...
}

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　　但基礎結構還是：陣列 + 連結串列，稱作雜湊表或雜湊表

　　只是 1.8 做了優化，引進了紅黑樹，來提升連結串列中元素獲取的速度

JDK1.7 頭插

　　只有元素新增的時候，才會出現連結串列元素的插入，那麼我們先來看看 put 方法

　　put - 新增元素

　　　　原始碼如下

    /**
     * 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) {
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);
        }
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key);
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

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　　　　直接看程式碼可能不夠直觀，我們結合流程圖來看

　　　　什麼？還是不夠直觀？（樓主也這麼覺得）

　　　　那我們就結合具體案例來看下這個流程

　　　　假設 HashMap 初始狀態

　　　　然後依次往裡面新增元素：(2,b), (3,w), (5,e), (9,t), (16,p)

　　　　再利用斷點除錯，我們來看看真實情況

　　　　一切都對得上，進展的也挺順利

　　resize - 陣列擴容

　　　　上述提到了擴容，但是沒細講，我們來看看擴容的實現

　　　　關鍵程式碼如下

/**
 * Rehashes the contents of this map into a new array with a
 * larger capacity.  This method is called automatically when the
 * number of keys in this map reaches its threshold.
 *
 * If current capacity is MAXIMUM_CAPACITY, this method does not
 * resize the map, but sets threshold to Integer.MAX_VALUE.
 * This has the effect of preventing future calls.
 *
 * @param newCapacity the new capacity, MUST be a power of two;
 *        must be greater than current capacity unless current
 *        capacity is MAXIMUM_CAPACITY (in which case value
 *        is irrelevant).
 */
void resize(int newCapacity) {
    Entry[] oldTable = table;
    int oldCapacity = oldTable.length;
    if (oldCapacity == MAXIMUM_CAPACITY) {
        threshold = Integer.MAX_VALUE;
        return;
    }

    Entry[] newTable = new Entry[newCapacity];
    transfer(newTable, initHashSeedAsNeeded(newCapacity));
    table = newTable;
    threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}

/**
 * Transfers all entries from current table to newTable.
 */
void transfer(Entry[] newTable, boolean rehash) {
    int newCapacity = newTable.length;
    for (Entry<K,V> e : table) {
        while(null != e) {
            Entry<K,V> next = e.next;
            if (rehash) {
                e.hash = null == e.key ? 0 : hash(e.key);
            }
            int i = indexFor(e.hash, newCapacity);
            e.next = newTable[i];
            newTable[i] = e;
            e = next;
        }
    }
}

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　　　　主要做了兩件事：1、建立一個新的 Entry 空陣列，長度是原陣列的 2 倍，2、遍歷原陣列，對每個元素重新計算新陣列的索引值，然後放入到新陣列的對應位置