引言
LinkedHashMap的成员变量
区别于1.7中的成员变量标志位置只有一个header节点;
JDK1.8中有一个head和一个tail节点;
JDK1.7
/**
* The head of the doubly linked list.
*/
private transient Entry<K,V> header;
/**
* The iteration ordering method for this linked hash map: <tt>true</tt>
* for access-order, <tt>false</tt> for insertion-order.
*
* @serial
*/
private final boolean accessOrder;
JDK1.8
private static final long serialVersionUID = 3801124242820219131L;
// 用于指向双向链表的头部, 双向链表头节点(最老)
transient LinkedHashMap.Entry<K,V> head;
//用于指向双向链表的尾部,双向列表尾节点(最新)
transient LinkedHashMap.Entry<K,V> tail;
/**
* 用来指定LinkedHashMap的迭代顺序,
* true则表示按照基于访问的顺序来排列,意思就是最近使用的entry,放在链表的最末尾
* false则表示按照插入顺序来
*/
final boolean accessOrder;
构造函数
1.8
public LinkedHashMap(int initialCapacity, float loadFactor) {
super(initialCapacity, loadFactor);
accessOrder = false;
}
public LinkedHashMap(int initialCapacity) {
super(initialCapacity);
accessOrder = false;
}
/**
* Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
* with the default initial capacity (16) and load factor (0.75).
*/
public LinkedHashMap() {
super();
accessOrder = false;
}
/**
* Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with
* the same mappings as the specified map. The <tt>LinkedHashMap</tt>
* instance is created with a 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 LinkedHashMap(Map<? extends K, ? extends V> m) {
super();
accessOrder = false;
putMapEntries(m, false);
}
/**
* Constructs an empty <tt>LinkedHashMap</tt> instance with the
* specified initial capacity, load factor and ordering mode.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @param accessOrder the ordering mode - <tt>true</tt> for
* access-order, <tt>false</tt> for insertion-order
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder) {
super(initialCapacity, loadFactor);
this.accessOrder = accessOrder;
}
1.8的构造函数和1.7的构造函数有差异的在接收map的构造函数;
1.7
public LinkedHashMap(Map<? extends K, ? extends V> m) {
super(m);
accessOrder = false;
}
super(m)调用hashMap的构造函数进行初始化:
public HashMap(Map<? extends K, ? extends V> m) {
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
inflateTable(threshold);
putAllForCreate(m);
}
putAllForCreate(m)
内部循环调用putForCreate(K key, V value)
private void putForCreate(K key, V value) {
int hash = null == key ? 0 : hash(key);
int i = indexFor(hash, table.length);
/**
* Look for preexisting entry for key. This will never happen for
* clone or deserialize. It will only happen for construction if the
* input Map is a sorted map whose ordering is inconsistent w/ equals.
*/
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) {
e.value = value;
return;
}
}
//如果没有存在的key对应的Entry,那么新建Entry
createEntry(hash, key, value, i);
}
void createEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<>(hash, key, value, e);
size++;
}
1.8
public LinkedHashMap(Map<? extends K, ? extends V> m) {
super();
accessOrder = false;
putMapEntries(m, false);
}
/**
* Implements Map.putAll and Map constructor
*
* @param m the map
* @param evict false when initially constructing this map, else
* true (relayed to method afterNodeInsertion).
*/
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > 0) {
if (table == null) { // pre-size
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t);
}
else if (s > threshold)
resize();
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
K key = e.getKey();
V value = e.getValue();
//evict false when initially constructing this map, else true (relayed to method afterNodeInsertion).
//调用putVal函数
putVal(hash(key), key, value, false, evict);
}
}
}
Entry
LinkedHashMap 1.8中的Entry和1.7中的区别不大,都是Map.Entry<K,V>
的实现
同样的:
Entry里面的属性:
1、K key
2、V value
3、Entry<K, V> next
4、int hash
5、Entry<K, V> before
6、Entry<K, V> after
其中前面四个,也就是红色部分是从HashMap.Entry中继承过来的;后面两个,也就是蓝色部分是LinkedHashMap独有的。不要搞错了next和before、After,next是用于维护HashMap指定table位置上连接的Entry的顺序的,before、After是用于维护Entry插入的先后顺序的。
put
get 查
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;
}
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;
}
}