Map作为键值对储存的集合，其实现类包括HashMap,HashTable等，这里展示HashMap的底层原理。
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next;
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
首先是在HashMap类中定义了一个Node 类，用于储存键值对,该类实现了Map.Entry<K,V>作为储存键值对的结构。
transient Node<K,V>[] table;
/**
* Holds cached entrySet(). Note that AbstractMap fields are used
* for keySet() and values().
*/
transient Set<Map.Entry<K,V>> entrySet;
/**
* The number of key-value mappings contained in this map.
*/
transient int size;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
*/
transient int modCount;
定义了包含Node类的数组和一个Set类，数组是作为哈希表储存数组，那么Set类的作用是什么呢？
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();
putVal(hash(key), key, value, false, evict);
}
}
}
在上述数组中出现了m.entrySet()方法，接着追溯
public Set<Map.Entry<K,V>> entrySet() {
Set<Map.Entry<K,V>> es;
return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
}
可以看到m.entrySet()的作用就是创建entrySet = new EntrySet()实例。
那么EntrySet又是什么类呢？
final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
public final int size() { return size; }
public final void clear() { HashMap.this.clear(); }
public final Iterator<Map.Entry<K,V>> iterator() {
return new EntryIterator();
}
public final boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<?,?> e = (Map.Entry<?,?>) o;
Object key = e.getKey();
Node<K,V> candidate = getNode(hash(key), key);
return candidate != null && candidate.equals(e);
}
public final boolean remove(Object o) {
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>) o;
Object key = e.getKey();
Object value = e.getValue();
return removeNode(hash(key), key, value, true, true) != null;
}
return false;
}
EntrySet类的作用就是实现抽象set集合，其中Map.Entry<K,V>就是返回键对应的这个
interface Entry<K,V> {
/**
* Returns the key corresponding to this entry.
*
* @return the key corresponding to this entry
* @throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
K getKey();
在源码中可以看到，EntrySet是内部类并且没有值域，也就是说调用entrySet()方法不会返回真正意义上的所有Map.Entey实例的Set对象,那么为什么要使用这个EnteySet类呢？
final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
public final int size() { return size; }
public final void clear() { HashMap.this.clear(); }
public final Iterator<Map.Entry<K,V>> iterator() {
return new EntryIterator();
}
可以看到里面存在new EntryIterator()返回迭代器实例，这才是使用EntrySet类的原因，从抽象上可以理解为迭代的过程和Set迭代一致，但是深入内部的实现原理，可以看到EntryIterator基本就是调用HashIterator
final class EntryIterator extends HashIterator
implements Iterator<Map.Entry<K,V>> {
public final Map.Entry<K,V> next() { return nextNode(); }
}
abstract class HashIterator {
Node<K,V> next; // next entry to return
Node<K,V> current; // current entry
int expectedModCount; // for fast-fail
int index; // current slot
HashIterator() {
expectedModCount = modCount;
Node<K,V>[] t = table;
current = next = null;
index = 0;
if (t != null && size > 0) { // advance to first entry
do {} while (index < t.length && (next = t[index++]) == null);
}
}
public final boolean hasNext() {
return next != null;
}
final Node<K,V> nextNode() {
Node<K,V>[] t;
Node<K,V> e = next;
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (e == null)
throw new NoSuchElementException();
if ((next = (current = e).next) == null && (t = table) != null) {
do {} while (index < t.length && (next = t[index++]) == null);
}
return e;
}
HashIterator的抽象类已经实现了针对哈希链表模式的遍历，这是针对HashMap实现类的遍历实现，与AbstracSet的遍历并没有太多的关系，那么为什么EntrySet要实现AbstracSet类呢，追溯回这个类
public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {
/**
* Sole constructor. (For invocation by subclass constructors, typically
* implicit.)
*/
protected AbstractSet() {
}
// Comparison and hashing
/**
* Compares the specified object with this set for equality. Returns
* <tt>true</tt> if the given object is also a set, the two sets have
* the same size, and every member of the given set is contained in
* this set. This ensures that the <tt>equals</tt> method works
* properly across different implementations of the <tt>Set</tt>
* interface.<p>
*
* This implementation first checks if the specified object is this
* set; if so it returns <tt>true</tt>. Then, it checks if the
* specified object is a set whose size is identical to the size of
* this set; if not, it returns false. If so, it returns
* <tt>containsAll((Collection) o)</tt>.
*
* @param o object to be compared for equality with this set
* @return <tt>true</tt> if the specified object is equal to this set
*/
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Set))
return false;
Collection<?> c = (Collection<?>) o;
if (c.size() != size())
return false;
try {
return containsAll(c);
} catch (ClassCastException unused) {
return false;
} catch (NullPointerException unused) {
return false;
}
}
/**
* Returns the hash code value for this set. The hash code of a set is
* defined to be the sum of the hash codes of the elements in the set,
* where the hash code of a <tt>null</tt> element is defined to be zero.
* This ensures that <tt>s1.equals(s2)</tt> implies that
* <tt>s1.hashCode()==s2.hashCode()</tt> for any two sets <tt>s1</tt>
* and <tt>s2</tt>, as required by the general contract of
* {@link Object#hashCode}.
*
* <p>This implementation iterates over the set, calling the
* <tt>hashCode</tt> method on each element in the set, and adding up
* the results.
*
* @return the hash code value for this set
* @see Object#equals(Object)
* @see Set#equals(Object)
*/
public int hashCode() {
int h = 0;
Iterator<E> i = iterator();
while (i.hasNext()) {
E obj = i.next();
if (obj != null)
h += obj.hashCode();
}
return h;
}
AbstracSet类仅仅实现了两个方法，hashCode()和equals而正好这也是Map.EntrySet需要的,因为这个类内部实现了contains()和remove()方法，这两个方法都需要使用hash值定位以及key值查找同一数组下标的链表中符合要求的key的节点。
另外，从源码可以看出来，Map插入null值key的实现方式和网上的putForNullKey()不同，这个方法已经没有了，取而代之解释null的hash值为0，也就是在下标为0处保存key=null的键值对，而map同样不允许key重复，所有当多个null的key插入时会不断覆盖之前的value，结果就是只能保存最近一个key为null的键值对的值
