LinkedHashMap源碼分析

為什麼要有LinkedHashMap？

在分析HashMap的時候提到了HashMap是無序的，即添加節點的順序和遍歷的順序不一致

@Test
	public void test1() {
		HashMap<String,String> hashMap=new HashMap<String, String>();
		hashMap.put("tom", "american");
		hashMap.put("jack", "chainese");
		hashMap.put("mary", "japanese");
		Set<Entry<String, String>> entrySet = hashMap.entrySet();
		Iterator<Entry<String, String>> iterator = entrySet.iterator();
		while(iterator.hasNext()) {
			Entry<String, String> entry = iterator.next();
			String key = entry.getKey();
			String value = entry.getValue();
			System.out.println(key+":"+value);
		}
	}
輸出：
tom:american
mary:japanese
jack:chainese

LinkedHashMap保證節點的順序，這也是LinkedHashMap和HashMap的主要區別

@Test
	public void test2() {
		LinkedHashMap<String,String> linkedHashMap=new LinkedHashMap();
		linkedHashMap.put("tom", "american");
		linkedHashMap.put("jack", "chainese");
		linkedHashMap.put("mary", "japanese");
		Set<Entry<String, String>> entrySet = linkedHashMap.entrySet();
		Iterator<Entry<String, String>> iterator = entrySet.iterator();
		while(iterator.hasNext()) {
			Entry<String, String> entry = iterator.next();
			String key = entry.getKey();
			String value = entry.getValue();
			System.out.println(key+":"+value);
		}
	}
輸出：
tom:american
jack:chainese
mary:japanese

存儲示意圖

類結構

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

LinkedHashMap是HashMap的子類，它對HashMap做了一些增強

節點

static class Entry<K,V> extends HashMap.Node<K,V> {//LinkedHashMap的Entry節點是HashMap的Node節點的子類
        Entry<K,V> before, after;//新增了before、after分別指向前驅和後繼
        Entry(int hash, K key, V value, Node<K,V> next) {
          //直接使用HashMap的Node節點構造方法
            super(hash, key, value, next);
        }
    }

屬性

//頭指針指向第一個添加節點
transient LinkedHashMap.Entry<K,V> head;

//尾指針指向最後一個添加節點
transient LinkedHashMap.Entry<K,V> tail;

//排序規則，true的話按照訪問順序排序，最近訪問的放到最後，false也是預設按照插入順序排序
final boolean accessOrder;

構造方法

//指定初始化容量和載入因數
public LinkedHashMap(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor);
        accessOrder = false;
    }

//指定初始化容量
public LinkedHashMap(int initialCapacity) {
        super(initialCapacity);
        accessOrder = false;
    }

//無參構造方法
public LinkedHashMap() {
        super();
        accessOrder = false;
    }

//使用Map初始化
public LinkedHashMap(Map<? extends K, ? extends V> m) {
        super();
        accessOrder = false;
        putMapEntries(m, false);
    }

//指定初始化容量、載入因數、排序規則
public LinkedHashMap(int initialCapacity,
                         float loadFactor,
                         boolean accessOrder) {
        super(initialCapacity, loadFactor);
        this.accessOrder = accessOrder;
    }

方法

LinkedHashMap中並沒有put方法，所以使用的是父類HashMap的put方法

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) {
        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)
          //當根據hash計算的下標位置沒放節點，調用LinkedHashMap的newNode方法
            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;
              //如果是指定訪問順序排序，那麼替換後，把節點移動到最後
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

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;
    }

private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {
  //保存LinkedHashMap的為指針
        LinkedHashMap.Entry<K,V> last = tail;
        tail = p;
        if (last == null)//尾指針為null說明LinkedHashMap中沒元素
            head = p;
        else {//有元素
          //新節點的前驅指向添加之前的尾指針
            p.before = last;
          //添加之前的尾指針節點的後繼指向新節點
            last.after = p;
        }
    }

void afterNodeAccess(Node<K,V> e) { // move node to last
        LinkedHashMap.Entry<K,V> last;
        if (accessOrder && (last = tail) != e) {//如果指定了按訪問順序排序且替換的節點不是最末尾的節點
            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;
        }
  
  void afterNodeInsertion(boolean evict) { // possibly remove eldest
        LinkedHashMap.Entry<K,V> first;
        if (evict && (first = head) != null && removeEldestEntry(first)) {//removeEldestEntry方法返回false所以不會進入if
            K key = first.key;
            removeNode(hash(key), key, null, false, true);
        }
    }

get(Object）根據key獲取值

public V get(Object key) {
        Node<K,V> e;
        if ((e = getNode(hash(key), key)) == null)//調用HashMap的方法拿到值
            return null;
        if (accessOrder)//如果是按照訪問順序排序的話
          //訪問過後要修改順序
            afterNodeAccess(e);
        return e.value;
    }

//把訪問的節點移動到鏈表的最末端
void afterNodeAccess(Node<K,V> e) { 
        LinkedHashMap.Entry<K,V> last;
        if (accessOrder && (last = tail) != e) {//按訪問順序排序並且訪問節點不是最後一個節點
            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;
        }
    }

remove(Object)方法是調用父類HashMap的方法

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 (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            else if ((e = p.next) != null) {
                if (p instanceof TreeNode)
                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
                else {
                    do {
                        if (e.hash == hash &&
                            ((k = e.key) == key ||
                             (key != null && key.equals(k)))) {
                            node = e;
                            break;
                        }
                        p = e;
                    } while ((e = e.next) != null);
                }
            }
            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;
              //調用LinkedHashMap·的方法來實現雙鏈的刪除
                afterNodeRemoval(node);
                return node;
            }
        }
        return null;
    }

void afterNodeRemoval(Node<K,V> e) { // unlink
        LinkedHashMap.Entry<K,V> p =
            (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
  //把要移除節點的前驅後繼置為null
        p.before = p.after = null;
        if (b == null)//b為null即移除的就是第一個元素
          //頭指針指向移除元素的後繼
            head = a;
        else
            b.after = a;
        if (a == null)//a為null即移除的元素是最後一個元素
          //尾指針指向移除元素的前驅
            tail = b;
        else
            a.before = b;
    }