文章目录
Java 基础 ——HashMap构造、PUT、GET
- HashMap的数据结构包括了初始数组、链表、红黑树；
- 插入数据的时候使用key%size来进行插入数据；
- 当两个或者两个以上的key的key相同，且key值不同的时候（即%【取余】发生冲突，就会挂在数组初始化位置的链表后）
- 当某个节点后出现过多的链表节点的时候，就会换成红黑树以提高效率；
HashMap结构
Key是通过Set组织的，即不允许重复
/**
* Returns a {@link Set} view of the keys contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation), the results of
* the iteration are undefined. The set supports element removal,
* which removes the corresponding mapping from the map, via the
* <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
* operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
* operations.
*
* @return a set view of the keys contained in this map
*/
public Set<K> keySet() {
Set<K> ks = keySet;
if (ks == null) {
ks = new KeySet();
keySet = ks;
}
return ks;
}
Value是通过Collection组织的，即体现允许重复数据(ArrayList、LinkedList)
/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa. If the map is
* modified while an iteration over the collection is in progress
* (except through the iterator's own <tt>remove</tt> operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Collection.remove</tt>, <tt>removeAll</tt>,
* <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
* support the <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a view of the values contained in this map
*/
public Collection<V> values() {
Collection<V> vs = values;
if (vs == null) {
vs = new Values();
values = vs;
}
return vs;
}
HashMap(JDK8之前)
HashMap是数组+链表存储结构
- 数组特点是查询速度快、增删较慢
- 链表特点是查询速度慢、增删较快
HashMap结合两者特点组成
如图所示（HashMap存储结构 JDK8之前）：
HashMap数组长度默认是16,16个数组（桶）中每个元素存储的就是链表的头结点
/**
* 默认初始容量，必须是2的幂次方，即桶默认是16个
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
HashMap(JDK8之后)
HashMap是数组+链表+红黑树结构
- 数组特点是查询速度快、增删较慢
- 链表特点是查询速度慢、增删较快
- 通过常量TREEIFY_THRESHOLD来控制是否将链表转换成红黑树来存储
以下对HashMap基于JDK8讲解
HashMap内部结构
通过数组Node<K,V>与链表Set<Map.Entry<K,V>>组成的复合结构
- 基础结构
// 数组结构
transient Node<K,V>[] table;
// 链表结构
transient Set<Map.Entry<K,V>> entrySet;
- Node<K,V>
/**
* Node是通过hash值、键值对、以及指向下一个节点来组成的
*/
static class Node<K,V> implements Map.Entry<K,V> {
// hash值
final int hash;
// 键
final K key;
// 值
V value;
// 指向下一个节点
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;
}
public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
数组被分为一个个的backet（桶）,通过hash值决定了键值对在这个数组的寻址，相同的键值对以链表形式存储!!!
链表的大小超过TREEIFY_THRESHOLD阈值（默认是8），会改造成红黑树
/**
* The bin count threshold for using a tree rather than list for a
* bin. Bins are converted to trees when adding an element to a
* bin with at least this many nodes. The value must be greater
* than 2 and should be at least 8 to mesh with assumptions in
* tree removal about conversion back to plain bins upon
* shrinkage.
*/
// TREEIFY_THRESHOLD（树化）
static final int TREEIFY_THRESHOLD = 8;
而某个桶，上面的链表由于删除了某些值，低于了阈值（UNTREEIFY_THRESHOLD），红黑树又被转变为链表，来确保性能
/**
* The bin count threshold for untreeifying a (split) bin during a
* resize operation. Should be less than TREEIFY_THRESHOLD, and at
* most 6 to mesh with shrinkage detection under removal.
*/
static final int UNTREEIFY_THRESHOLD = 6;
HashMap构造函数
构造函数不是直接指定大小，而是给一些成员变量赋值，所以HashMap是在首次使用的时候才被初始化。
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
总结HashMap结构
HashMap是数组+链表+红黑树的结构默认数组为16个桶构造函数不做大小指定，在首次使用（构造函数指定大小或者put）时对大小进行指定
具体使用数组、链表、红黑树如何转变的通过put函数说明。
HashMap添加元素（put）
- 源码分析
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
// table 为空时，调用resize()方法来初始化table
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// 进行hash运算，算出键值对，在table中的具体位置
if ((p = tab[i = (n - 1) & hash]) == null)
// 当没有元素时，直接new该键值对的node放到数组当中
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
// 如果存在键值对，且传入进来的键值对是一致的，则直接替换数组中的元素
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
// 否则接着判断，判断当前数组位置存储的是否是已经树化之后的节点
else if (p instanceof TreeNode)
// 如果是树化的，按照树的方式尝试存储键值对
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
// 如果不是树化的，按照链表的插入方式往链表中添加元素，同时判断链表元素的总数，一旦超过TREEIFY_THRESHOLD，则将链表进行树化
else {
for (int binCount = 0; ; ++binCount) {
// 如果不是树化的，按照链表的插入方式往链表中添加元素
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// 同时判断链表元素的总数(binCount)，一旦超过TREEIFY_THRESHOLD，则将链表进行树化(treeifyBin)
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
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;
// 如果插入的键位存在于HashMap中，则对对应的键位进行更新操作
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
// 当HashMap中的size，当元素大于阈值（threshold）时，对hashMap进行扩容（resize方法）
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
resize即具备初始化，也具备扩容的功能
总结put逻辑
1.若HashMap未被初始化，则进行初始化操作；2.对Key求Hash值，依据Hash值计算下标；3.若未发生碰撞，则直接放入桶中；
"碰撞"：就是计算得到相同的hash值4.若发生碰撞，则以链表的方式链接到后面；5.若链表长度超过阈值，且HashMap元素超过最低树化容量，则将链表转成红黑树；
默认阈值：TREEIFY_THRESHOLD=8
默认最低树化容量：MIN_TREEIFY_CAPACITY=64
即：当前桶的容量超过8，并且整个HashMap的元素超过64就会将链表转换为红黑树。如果桶容量超过8，但是整个HashMap元素没有超过64，只会发生resize（扩容），而不会发生树化（红黑树树化）6.若节点已存在，则用新值替换旧值；7.若桶满了（默认容量16*扩容因子0.75），就需要resize(扩容2倍后重排）；
HashMap获取元素（get）
主要使用键值对的hashcode，通过hash算法找到backet（桶）的位置，找到backet位置调用key.equlas(k)方法，去找到链表中正确的节点，最终找到要找的值并返回
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
// 通过hash算法找到backet（桶）的位置 (first = tab[(n - 1) & hash]) != null)
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // always check first node 找到backet位置调用key.equlas(k)方法，去找到链表中正确的节点
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
// 最终找到要找的值并返回
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
总结
- HashMap由数组+链表+红黑树组成
- 数组也被称为桶（backed），默认为16个桶
- 在PUT操作时，会先进行hash运算，计算数组下标是否出现“hash碰撞”
- 如果没有出现hash碰撞，则直接插入桶中
- 如果出现hash碰撞，则以链表方式链接到这个桶的后边（默认每个桶大小为8个）
- 当桶中链表长度超过阈值（8）时，且hashMap桶超过（64）个时，则发生树化（红黑树树化）
- 若节点已经存在，则用新值替换旧值
- 若桶满了（默认容量16*扩容因子0.75）,就需要resize（扩容2倍后重排）