HashSet和HashMap (一)

Java集合实际是多个引用变量组成的集合，修改引用对象的属性后表现在集合内对象的属性也会相应被修改。但是可验证，如果集合内元素的类型是包装类类型，如Integer，则不满足上述称述。

对Map而言，每个元素都是key-value的Set集合。HashSet的实现基于HashMap，HashMap的实现用到了Entry数组。于是，可以将Set集合扩展成Map集合。

本文主要阅读HashSet和HashMap的JDK源码，了解其实现。

HashSet源码如下：

[java]
public class HashSet
extends AbstractSet
implements Set, Cloneable, java.io.Serializable
{
static final long serialVersionUID = -5024744406713321676L;

private transient HashMap map; //基于HashMap

// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object(); //private static final Object


/**
* Constructs a new, empty set; the backing HashMap instance has
* default initial capacity (16) and load factor (0.75).
*/
public HashSet() {
map = new HashMap();
}//后面代码略........

public class HashSet
extends AbstractSet
implements Set, Cloneable, java.io.Serializable
{
static final long serialVersionUID = -5024744406713321676L;

private transient HashMap map; //基于HashMap

// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object(); //private static final Object

/**
* Constructs a new, empty set; the backing HashMap instance has
* default initial capacity (16) and load factor (0.75).
*/
public HashSet() {
map = new HashMap();
}//后面代码略........可以看到HashSet基于HashMap来实现的，下面来看HashMap的实现。

下面通过HashMap的两个重要的方法get 和put 方法来认识HashMap。

[java]
public V get(Object key) { //HashMap的方法
if (key == null)
return getForNullKey();
int hash = hash(key.hashCode());
for (Entry e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) //此处可见hashCode提升性能！！！！！
return e.value;
}
return null;
}

public V get(Object key) { //HashMap的方法
if (key == null)
return getForNullKey();
int hash = hash(key.hashCode());
for (Entry e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) //此处可见hashCode提升性能！！！！！
return e.value;
}
return null;
}

[java]
public V put(K key, V value) { //HashMap的方法
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
int i = indexFor(hash, table.length);
for (Entry e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { //示例
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}

modCount++;
addEntry(hash, key, value, i);//优雅地将新Entry加至链头。Entry是HashMap的内部类。具体请阅读jdk源码。
return null;
}

public V put(K key, V value) { //HashMap的方法
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
int i = indexFor(hash, table.length);
for (Entry e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { //示例