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Map（接口）
Map是一个独立的接口，内部包含一个Entry的接口，用于管理每个节点
public interface Map<K,V>{
interface Entry<K,V> {
}
}
AbstractMap 抽象类实现Map接口，常用的HashMap和TreeMap都继承此类
public abstract class AbstractMap<K,V> implements Map<K,V>
这个类下面提供了一个将Map的values转换为Collection实现的子类的方法，十分的方便
transient volatile Collection<V> values = null; //这里定义一个Collection的集合
/**
* {@inheritDoc}
*
* <p>This implementation returns a collection that subclasses {@link
* AbstractCollection}. The subclass‘s iterator method returns a
* "wrapper object" over this map‘s <tt>entrySet()</tt> iterator.
* The <tt>size</tt> method delegates to this map‘s <tt>size</tt>
* method and the <tt>contains</tt> method delegates to this map‘s
* <tt>containsValue</tt> method.
*
* <p>The collection is created the first time this method is called, and
* returned in response to all subsequent calls. No synchronization is
* performed, so there is a slight chance that multiple calls to this
* method will not all return the same collection.
*/
public Collection<V> values() {
if (values == null) {
values = new AbstractCollection<V>() {
public Iterator<V> iterator() {
return new Iterator<V>() {
private Iterator<Entry<K,V>> i = entrySet().iterator();
public boolean hasNext() {
return i.hasNext();
}
public V next() {
return i.next().getValue();
}
public void remove() {
i.remove();
}
};
}
public int size() {
return AbstractMap.this.size();
}
public boolean isEmpty() {
return AbstractMap.this.isEmpty();
}
public void clear() {
AbstractMap.this.clear();
}
public boolean contains(Object v) {
return AbstractMap.this.containsValue(v);
}
};
}
return values;
}
1、HashMap
先看下头部的定义
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 //初始化的大小
/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30; //最大2的30次方大小
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f; //默认的负载因子
/**
* An empty table instance to share when the table is not inflated.
*/
static final Entry<?,?>[] EMPTY_TABLE = {};
/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE; //这里是具体存entry的哈系表
从上面的代码可以看出 所有的k v 的值都是存在Entry里，那么什么是Entry呢
Entry 先是定义在Map接口中的一个子接口
interface Map<K,V> {
interface Entry<K,V> {
K getKey();
V getValue();
V setValue(V value);
boolean equals(Object o);
int hashCode();
//需要实现的方法
}
}
然后在HashMap中实现了他的方法
static class Entry<K,V> implements Map.Entry<K,V>{
final K key;
V value;
Entry<K,V> next;
int hash;
/**
* Creates new entry.
*/
Entry(int h, K k, V v, Entry<K,V> n) {
value = v;
next = n;
key = k;
hash = h;
}
......
}
首先看下源码中的hash算法
final int hash(Object k) {
int h = hashSeed;
if (0 != h && k instanceof String) {
return sun.misc.Hashing.stringHash32((String) k);
}
h ^= k.hashCode();
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
这个方法会在每次put操作的时候计算下hash的值
public V put(K key, V value) {
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
if (key == null)
return putForNullKey(value);
int hash = hash(key); //计算hash
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i); //添加entry
return null;
}
/**
* Adds a new entry with the specified key, value and hash code to
* the specified bucket. It is the responsibility of this
* method to resize the table if appropriate.
*
* Subclass overrides this to alter the behavior of put method.
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
if ((size >= threshold) && (null != table[bucketIndex])) {
resize(2 * table.length);
hash = (null != key) ? hash(key) : 0;
bucketIndex = indexFor(hash, table.length);
}
createEntry(hash, key, value, bucketIndex);
}
/**
* Like addEntry except that this version is used when creating entries
* as part of Map construction or "pseudo-construction" (cloning,
* deserialization). This version needn‘t worry about resizing the table.
*
* Subclass overrides this to alter the behavior of HashMap(Map),
* clone, and readObject.
*/
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++;
}
在看下删除entry，通过key进行remove方法最终调用的也是这个方法，通过value remove的方法与这个类似
/**
* Removes and returns the entry associated with the specified key
* in the HashMap. Returns null if the HashMap contains no mapping
* for this key.
*/
final Entry<K,V> removeEntryForKey(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
int i = indexFor(hash, table.length); //根据hash值计算下 在hash表中的位置，通过位置拿到entry
Entry<K,V> prev = table[i];
Entry<K,V> e = prev;
while (e != null) {
Entry<K,V> next = e.next;
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) {
modCount++;
size--;
if (prev == e)
table[i] = next;
else
prev.next = next; //以上执行删除，相当于移动了下游标
e.recordRemoval(this); //这里是类似hook的方式，这个方法是个空方法，留给子类去做，为一些定制化的扩展留个口
return e;
}
prev = e;
e = next;
}
return e;
}
get方法也是通过计算hash 然后在找hash表中的位置来取值
ps：以上的jdk源码是 openjdk7u75-b13版本
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原文地址：http://my.oschina.net/u/268957/blog/500205
