/**
     * The head (eldest) of the doubly linked list.
     */
    transient LinkedHashMap.Entry<K,V> head;
    /**
     * The tail (youngest) of the doubly linked list.
     */
    transient LinkedHashMap.Entry<K,V> tail;

    public V get(Object key) {
        Node<K,V> e;
        if ((e = getNode(key)) == null)
            return null;
        if (accessOrder)
            afterNodeAccess(e);
        return e.value;
    }

void afterNodeAccess(Node<K,V> e) { // move node to last
    LinkedHashMap.Entry<K,V> last;
    // 雙向鏈表尾部節(jié)點(diǎn) last
    if (accessOrder && (last = tail) != e) {
        // 訪問(wèn)的節(jié)點(diǎn)標(biāo)記為 p ，p 的前后節(jié)點(diǎn)保存到 b 、a 中
        LinkedHashMap.Entry<K,V> p = (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
        // 訪問(wèn)節(jié)點(diǎn)的后續(xù)節(jié)點(diǎn)設(shè)置為 null ，因?yàn)槭亲詈笠粋€(gè)節(jié)點(diǎn)，所以后續(xù)節(jié)點(diǎn)為 null
        p.after = null; 
        // 處理刪除 e 節(jié)點(diǎn)的邏輯
        if (b == null)
             // p 的前面不存在節(jié)點(diǎn)，頭節(jié)點(diǎn)設(shè)置為 a 
            head = a;
        else 
            // p 的前面存在節(jié)點(diǎn)，將 p 的后續(xù)節(jié)點(diǎn)設(shè)置為 p 前一個(gè)節(jié)點(diǎn)的下一個(gè)節(jié)點(diǎn)  b -> p -> a 更新為 b -> a （刪除 p）
            b.after = a;
        if (a != null)
            // p 存在后續(xù)節(jié)點(diǎn) a , 將 a 的 before 指針更新為 b
            a.before = b;
        else
            // p 不存在后續(xù)節(jié)點(diǎn) a , last 指針更新為 b
            last = b;
        // 處理尾部節(jié)點(diǎn)的邏輯
        if (last == null)
            // last 指針為空，更新頭節(jié)點(diǎn)
            head = p;
        else {
            // last 指針不為空，更新鏈表順序?yàn)?last 
            p.before = last;
            last.after = p;
        }
        tail = p;
        ++modCount;
    }
}

public class LruCache<K, V> {
    private final LinkedHashMap<K, V> map;
		//...
}

public class LruCache<K, V> {
    private final LinkedHashMap<K, V> map;
    private int size;
    private int maxSize;
    private int putCount;
    private int createCount;
    private int evictionCount;
    private int hitCount;
    private int missCount;
    public LruCache(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;
      	// 啟用了 訪問(wèn)排序 accessOrder = true
        this.map = new LinkedHashMap<K, V>(0, 0.75f, true); 
    }
		//...
}

    public void resize(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        synchronized (this) {
            this.maxSize = maxSize;
        }
        trimToSize(maxSize);
    }

    // 刪除最老的元素，直到剩余元素的總數(shù)等于或低于請(qǐng)求的大小。
    public void trimToSize(int maxSize) {
        while (true) {
            K key;
            V value;
            synchronized (this) {
                if (size < 0 || (map.isEmpty() && size != 0)) {
                    throw new IllegalStateException(getClass().getName() + ".sizeOf() is reporting inconsistent results!");
                }
                if (size <= maxSize) {
                    break;
                }
                Map.Entry<K, V> toEvict = map.eldest();
                if (toEvict == null) {
                    break;
                }
                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);
                size -= safeSizeOf(key, value);
                evictionCount++;
            }
            entryRemoved(true, key, value, null);
        }
    }

		// in LinkedHashMap
		public Map.Entry<K, V> eldest() {
        return head;
    }

    public final V get(K key) {
      	// key 空檢查
        if (key == null) {
            throw new NullPointerException("key == null");
        }
        V mapValue;
        synchronized (this) {
            mapValue = map.get(key); // 從 map 中讀取元素
            if (mapValue != null) { // 緩存中存在元素，直接返回
                hitCount++; 
                return mapValue; 
            }
            missCount++; 
        }
				// 緩存中不存在對(duì)應(yīng) key 的元素，創(chuàng)建一個(gè)新元素
        V createdValue = create(key); 
        if (createdValue == null) { // 未緩存且無(wú)法創(chuàng)建，返回 null
            return null;
        }
				// 創(chuàng)建成功，存入到 map ，如果 key 已存在對(duì)應(yīng)值，優(yōu)先更新為之前的值
        synchronized (this) {
            createCount++;	
            mapValue = map.put(key, createdValue); // 存入新的元素，并獲取前一個(gè) key 對(duì)應(yīng)的值 mapValue。
            if (mapValue != null) {
                map.put(key, mapValue);
            } else {
                size += safeSizeOf(key, createdValue); // 新元素導(dǎo)致當(dāng)前容量 + 1
            }
        }
        if (mapValue != null) { // key 對(duì)應(yīng)的元素已存在
          	// 當(dāng)一個(gè)值被逐出以騰出空間、通過(guò)調(diào)用 remove 移除或通過(guò)調(diào)用 put 替換時(shí)，將調(diào)用此方法。 默認(rèn)實(shí)現(xiàn)什么也不做。
            entryRemoved(false, key, createdValue, mapValue);
            return mapValue; 
        } else { // key 對(duì)應(yīng)的元素不存在
            trimToSize(maxSize); // 擴(kuò)容
            return createdValue; //返回最新插入的元素
        }
    }

    protected V create(K key) {
        return null;
    }

protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}

    public final V put(K key, V value) {
        if (key == null || value == null) {
            throw new NullPointerException("key == null || value == null");
        }
        V previous;
        synchronized (this) {
            putCount++;
            size += safeSizeOf(key, value); // 當(dāng)前容量 + 1
            previous = map.put(key, value); // 取出先前的值
            if (previous != null) {
                size -= safeSizeOf(key, previous); // map 中已存在 key 的情況下，保證 size 不變
            }
        }
				// 先前存在元素，執(zhí)行元素移除后的自定義操作
        if (previous != null) {
            entryRemoved(false, key, previous, value);
        }
				// 容量處理
        trimToSize(maxSize);
        return previous;
    }

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

    final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
            // ...
						if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e); // 【*】
                return oldValue;
            }
            // ...
		}

    // Callbacks to allow LinkedHashMap post-actions
    void afterNodeAccess(Node<K,V> p) { }
    void afterNodeInsertion(boolean evict) { }
    void afterNodeRemoval(Node<K,V> p) { }

    public V remove(Object key) {
        Node<K,V> e;
        return (e = removeNode(hash(key), key, null, false, true)) == null ?
            null : e.value;
    }

    final Node<K,V> removeNode(int hash, Object key, Object value, boolean matchValue, boolean movable) {
        Node<K,V>[] tab; Node<K,V> p; int n, index;
        if ((tab = table) != null && (n = tab.length) > 0 && (p = tab[index = (n - 1) & hash]) != null) {
            Node<K,V> node = null, e; K k; V v;
          	// ...
            if (node != null && (!matchValue || (v = node.value) == value || (value != null && value.equals(v)))) {
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount;
                --size;
                afterNodeRemoval(node); // 【*】
                return node;
            }
        }
        return null;
    }

    void afterNodeRemoval(Node<K,V> e) { // unlink
        LinkedHashMapEntry<K,V> p = (LinkedHashMapEntry<K,V>)e, b = p.before, a = p.after;
        p.before = p.after = null;
        if (b == null)
            head = a;
        else
            b.after = a;
        if (a == null)
            tail = b;
        else
            a.before = b;
    }

int cacheSize = 4 * 1024 * 1024; // 4MiB   
LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) {       
  	protected int sizeOf(String key, Bitmap value) {           
      	return value.getByteCount();       
    }   
}

class LruCache(private val capacity: Int) {
    data class DLinkedNode(
        var key: Int? = null,
        var value: Int? = null,
        var prev: DLinkedNode? = null,
        var next: DLinkedNode? = null
    )
    private val cache = HashMap<Int, DLinkedNode>()
    private var size = 0
    private var head = DLinkedNode()
    private var tail = DLinkedNode()
    fun get(key: Int): Int {
        val node = cache[key] ?: return -1
        // 如果 key 存在，先通過(guò)哈希表定位，再移到頭部
        moveToHead(node)
        return node.value ?: -1
    }
    fun put(key: Int, value: Int) {
        val node = cache[key]
        if (node == null) {
            // 如果 key 不存在，創(chuàng)建一個(gè)新的節(jié)點(diǎn)
            val newNode = DLinkedNode(key, value)
            // 添加進(jìn)哈希表
            cache[key] = newNode
            // 添加至雙向鏈表的頭部
            addToHead(newNode)
            ++size;
            if (size > capacity) {
                // 如果超出容量，刪除雙向鏈表的尾部節(jié)點(diǎn)
                val tail = removeTail()
                // 刪除哈希表中對(duì)應(yīng)的項(xiàng)
                cache.remove(tail?.key)
                --size;
            }
        }
        else {
            // 如果 key 存在，先通過(guò)哈希表定位，再修改 value，并移到頭部
            node.value = value;
            moveToHead(node);
        }
    }
    private fun addToHead(node: DLinkedNode?) {
        node?.prev = head
        node?.next = head.next
        head.next?.prev = node
        head.next = node
    }
    private fun removeNode(node: DLinkedNode?) {
        node?.prev?.next = node?.next
        node?.next?.prev = node?.prev
    }
    private fun moveToHead(node: DLinkedNode?) {
        removeNode(node)
        addToHead(node)
    }
    private fun removeTail(): DLinkedNode? {
        val res = tail.prev 
        removeNode(res)
        return res
    }
}

class LruCacheByLinkedHashMap(val capacity: Int): LinkedHashMap<Int, Int>(capacity, 0.75f, true) {
    override fun get(key: Int): Int {
        return getOrDefault(key, -1)
    }
    override fun put(key: Int, value: Int): Int? {
        return super.put(key, value)
    }
    override fun removeEldestEntry(eldest: MutableMap.MutableEntry<Int, Int>?): Boolean {
        return size > capacity
    }
}