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Map

Map

Map是一个接口,下面介绍一下Map接口的一些常用的实现类

Hashtable

Hashtable是在java1.0中实现的最早的Map,继承自Dictionary类,底层使用的哈希表,是线程安全的,因为该类中的方法都是用了synchronized修饰,但是也因此存在了效率问题

如果想要使用具有用线程安全能力的map可以使用Collections.synchronizedMap()方法或者使用ConcurrentHashMap

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public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable {
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@SuppressWarnings("unchecked")
public synchronized V get(Object key) {
Entry<?,?> tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return (V)e.value;
}
}
return null;
}

内部所使用的是一个Entry数组进行存储的,Entry实际上是一个链表,key和value都保存在Entry中

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/**
* The hash table data.
*/
private transient Entry<?,?>[] table;

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

protected Entry(int hash, K key, V value, Entry<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}

@SuppressWarnings("unchecked")
protected Object clone() {
return new Entry<>(hash, key, value,
(next==null ? null : (Entry<K,V>) next.clone()));
}

// Map.Entry Ops

public K getKey() {
return key;
}

public V getValue() {
return value;
}

public V setValue(V value) {
if (value == null)
throw new NullPointerException();

V oldValue = this.value;
this.value = value;
return oldValue;
}

public boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<?,?> e = (Map.Entry<?,?>)o;

return (key==null ? e.getKey()==null : key.equals(e.getKey())) &&
(value==null ? e.getValue()==null : value.equals(e.getValue()));
}

public int hashCode() {
return hash ^ Objects.hashCode(value);
}

public String toString() {
return key.toString()+"="+value.toString();
}
}

HashTable的key和value都不允许为null,可以看到在put操作的时候,会校验value值是否为null,而且会获取key的hashCode,所以key和value都不可以为null

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public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}

// Makes sure the key is not already in the hashtable.
Entry<?,?> tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];
for(; entry != null ; entry = entry.next) {
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}

addEntry(hash, key, value, index);
return null;
}

HashMap

当前用的最多的还是HashMap

HashMap是线程不安全的,底层也是哈希表,与HashTable不同的是,key和value可以为null

可以看到在put方法取key的hash值时对key进行了判断,key为null的话,hash值为0

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public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

HashMap的底层使用Node数组来存储的

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transient Node<K,V>[] table;

HashMap如果出现hash冲突的话,会在放到链表中,但是如果hash冲突过多的话,会导致链表太长,查询时性能会下降,在链表长度超过一定值时,会进行结构改造,将链表转换为树状结构,这里TREEIFY_THRESHOLD是8

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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) // table为null,进行初始化
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
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 {
// 遍历map
for (int binCount = 0; ; ++binCount) {
// 遍历完依旧没有该key,需要添加一个新的节点
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
// 链表转为树状结构
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;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}

TreeMap

TreeMap底层是红黑树,实现了SortedMap接口,顺序由key控制(默认是按key进行,key必须实现Comparable接口或者在构造器传入自定义的Comparator),通过Comparator或者Comparable决定,

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public class TreeMap<K,V>
extends AbstractMap<K,V>
implements NavigableMap<K,V>, Cloneable, java.io.Serializable

public interface NavigableMap<K,V> extends SortedMap<K,V>
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// key进行比较
final int compare(Object k1, Object k2) {
return comparator==null ? ((Comparable<? super K>)k1).compareTo((K)k2)
: comparator.compare((K)k1, (K)k2);
}

LinkedHashMap

LinkedHashMap属于一个双向链表,通过键值来维护,遍历顺序为插入顺序,相当于一个有顺序的HashMap

增加了两个属性来保证迭代顺序,分别是双向链表的头结点header和标志位accessOrder(值为true表示按照访问顺序迭代,值为false表示按照插入顺序迭代)

默认构造器accessOrder为false是按照插入顺序迭代的

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public LinkedHashMap() {
super();
accessOrder = false;
}

ConcurrentHashMap

ConcurrentHashMap是并发包下的类,属于线程安全的HashMap。

java8中ConcurrentHashMap放弃了Segment分段加锁的机制,采用了Node+CAS+Syncronized保证并发安全。

大量地采用了自旋+CAS操作

key和value都不可以为null,否则会抛出空指针异常

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final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode());
int binCount = 0;
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) {
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) {
Node<K,V> p;
binCount = 2;
if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}

HashMap和HashTable的区别

  • HashMap线程不安全,HashTable线程安全
  • HashMap性能比HashTable好
  • HashMap键值允许为null,HashTable键值不允许为null
  • HashMap继承AbstractMap,HashTable继承Dictionary类
  • HashMap初始容量为16,HashTable初始容量为11
  • HashMap扩容是翻一倍,HashTable是翻一倍+1
  • HashMap的Iterator迭代器是fail-fast的,而HashTable的Enumerator是fail-safe的