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

static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

public class LinkedHashMap<K,V>
    extends HashMap<K,V>
    implements Map<K,V>

private static class Entry<K,V> extends HashMap.Entry<K,V> {
    Entry<K,V> before, after;
    ……
}

public LinkedHashMap(int initialCapacity, float loadFactor,boolean accessOrder) {
    super(initialCapacity, loadFactor);
    this.accessOrder = accessOrder;
}

void reinitialize() {
        super.reinitialize();
        head = tail = null;
 }

　　　　//1.插入元素的顺序
        Map<String, String> map = new LinkedHashMap<String, String>(16,0.75f,false);
        //2.访问元素的顺序
        //Map<String, String> map = new LinkedHashMap<String, String>(16,0.75f,true);

        map.put("apple", "苹果");
        map.put("watermelon", "西瓜");
        map.put("banana", "香蕉");
        map.put("peach", "桃子");

        map.get("watermelon");
        map.get("peach");

        Iterator iter = map.entrySet().iterator();
        while (iter.hasNext()) {
            Map.Entry entry = (Map.Entry) iter.next();
            System.out.println(entry.getKey() + "=" + entry.getValue());
        }

 Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
        LinkedHashMap.Entry<K,V> p =
            new LinkedHashMap.Entry<K,V>(hash, key, value, e);
        linkNodeLast(p);
        return p;
    }

    Node<K,V> replacementNode(Node<K,V> p, Node<K,V> next) {
        LinkedHashMap.Entry<K,V> q = (LinkedHashMap.Entry<K,V>)p;
        LinkedHashMap.Entry<K,V> t =
            new LinkedHashMap.Entry<K,V>(q.hash, q.key, q.value, next);
        transferLinks(q, t);
        return t;
    }

    TreeNode<K,V> newTreeNode(int hash, K key, V value, Node<K,V> next) {
        TreeNode<K,V> p = new TreeNode<K,V>(hash, key, value, next);
        linkNodeLast(p);
        return p;
    }

    TreeNode<K,V> replacementTreeNode(Node<K,V> p, Node<K,V> next) {
        LinkedHashMap.Entry<K,V> q = (LinkedHashMap.Entry<K,V>)p;
        TreeNode<K,V> t = new TreeNode<K,V>(q.hash, q.key, q.value, next);
        transferLinks(q, t);
        return t;
    }

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

    // 在节点删除之后unlink,即双端都设置为null   
    void afterNodeRemoval(Node<K,V> e) { // unlink
        LinkedHashMap.Entry<K,V> p =
            (LinkedHashMap.Entry<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;
    }

    //插入节点之后，很有可能要移除元素    
    void afterNodeInsertion(boolean evict) { // possibly remove eldest
        LinkedHashMap.Entry<K,V> first;
        if (evict && (first = head) != null && removeEldestEntry(first)) {
            K key = first.key;
            removeNode(hash(key), key, null, false, true);
        }
    }
    

   // 在元素被访问之后，用于修改链表顺序，将被访问的的元素放置到最后  
    void afterNodeAccess(Node<K,V> e) { // move node to last
        LinkedHashMap.Entry<K,V> last;
        if (accessOrder && (last = tail) != e) {
            LinkedHashMap.Entry<K,V> p =
                (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
            p.after = null;
            if (b == null)
                head = a;
            else
                b.after = a;
            if (a != null)
                a.before = b;
            else
                last = b;
            if (last == null)
                head = p;
            else {
                p.before = last;
                last.after = p;
            }
            tail = p;
            ++modCount;
        }
    }

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;
    }

    /**
     * {@inheritDoc}
     */
    public V getOrDefault(Object key, V defaultValue) {
       Node<K,V> e;
       if ((e = getNode(hash(key), key)) == null)
           return defaultValue;
       if (accessOrder)
           afterNodeAccess(e);
       return e.value;
   }

package ysz.demo;

import java.util.LinkedHashMap;
import java.util.Collection;
import java.util.Map;
import java.util.ArrayList;

/**
 * An LRU cache, based on <code>LinkedHashMap</code>.
 *
 * <p>
 * This cache has a fixed maximum number of elements (<code>cacheSize</code>).
 * If the cache is full and another entry is added, the LRU (least recently
 * used) entry is dropped.
 *
 * <p>
 * This class is thread-safe. All methods of this class are synchronized.
 *
 * <p>
 * Author: Christian d‘Heureuse, Inventec Informatik AG, Zurich, Switzerland<br>
 * Multi-licensed: EPL / LGPL / GPL / AL / BSD.
 */
public class LRUCache<K, V> {
    private static final float hashTableLoadFactor = 0.75f;
    private LinkedHashMap<K, V> map;
    private int cacheSize;

    /**
     * Creates a new LRU cache. 在该方法中，new LinkedHashMap<K,V>(hashTableCapacity,
     * hashTableLoadFactor, true)中，true代表使用访问顺序
     *
     * @param cacheSize
     *            the maximum number of entries that will be kept in this cache.
     */
    public LRUCache(int cacheSize) {
        this.cacheSize = cacheSize;
        int hashTableCapacity = (int) Math
                .ceil(cacheSize / hashTableLoadFactor) + 1;
        map = new LinkedHashMap<K, V>(hashTableCapacity, hashTableLoadFactor,
                true) {
            // (an anonymous inner class)
            private static final long serialVersionUID = 1;

            @Override
            protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
                return size() > LRUCache.this.cacheSize;
            }
        };
    }

    /**
     * Retrieves an entry from the cache.<br>
     * The retrieved entry becomes the MRU (most recently used) entry.
     *
     * @param key
     *            the key whose associated value is to be returned.
     * @return the value associated to this key, or null if no value with this
     *         key exists in the cache.
     */
    public synchronized V get(K key) {
        return map.get(key);
    }

    /**
     * Adds an entry to this cache. The new entry becomes the MRU (most recently
     * used) entry. If an entry with the specified key already exists in the
     * cache, it is replaced by the new entry. If the cache is full, the LRU
     * (least recently used) entry is removed from the cache.
     *
     * @param key
     *            the key with which the specified value is to be associated.
     * @param value
     *            a value to be associated with the specified key.
     */
    public synchronized void put(K key, V value) {
        map.put(key, value);
    }

    /**
     * Clears the cache.
     */
    public synchronized void clear() {
        map.clear();
    }

    /**
     * Returns the number of used entries in the cache.
     *
     * @return the number of entries currently in the cache.
     */
    public synchronized int usedEntries() {
        return map.size();
    }

    /**
     * Returns a <code>Collection</code> that contains a copy of all cache
     * entries.
     *
     * @return a <code>Collection</code> with a copy of the cache content.
     */
    public synchronized Collection<Map.Entry<K, V>> getAll() {
        return new ArrayList<Map.Entry<K, V>>(map.entrySet());
    }

    // Test routine for the LRUCache class.
    public static void main(String[] args) {
        LRUCache<String, String> c = new LRUCache<String, String>(3);
        c.put("1", "one"); // 1
        c.put("2", "two"); // 2 1
        c.put("3", "three"); // 3 2 1
        c.put("4", "four"); // 4 3 2
        if (c.get("2") == null)
            throw new Error(); // 2 4 3
        c.put("5", "five"); // 5 2 4
        c.put("4", "second four"); // 4 5 2
        // Verify cache content.
        if (c.usedEntries() != 3)
            throw new Error();
        if (!c.get("4").equals("second four"))
            throw new Error();
        if (!c.get("5").equals("five"))
            throw new Error();
        if (!c.get("2").equals("two"))
            throw new Error();
        // List cache content.
        for (Map.Entry<String, String> e : c.getAll())
            System.out.println(e.getKey() + " : " + e.getValue());
    }
}