2018-08-10 07:19:35 +08:00
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### <font face="楷体"> **目录:**</font>
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<a href="#1" target="_self">0-1. 简介</a>
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<a href="#2" target="_self">0-2. 内部结构分析</a>
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<a href="#2.1" target="_self">0-2-1. JDK18之前</a>
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<a href="#2.2" target="_self">0-2-2. JDK18之后</a>
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<a href="#3" target="_self">0-3. LinkedList源码分析</a>
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<a href="#3.1" target="_self">0-3-1. 构造方法</a>
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<a href="#3.2" target="_self">0-3-2. put方法</a>
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<a href="#3.3" target="_self">0-3-3. get方法</a>
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<a href="#3.4" target="_self">0-3-4. resize方法</a>
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<a href="#4" target="_self">0-4. HashMap常用方法测试</a>
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## <font face="楷体" id="1">简介</font>
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<font color="red">HashMap</font>主要用来存放<font color="red">键值对</font>,它<font color="red">基于哈希表的Map接口实现</font>,是常用的Java集合之一。与HashTable主要区别为<font color="red">不支持同步和允许null作为key和value</font>,所以如果你想要保证线程安全,可以使用<font color="red">ConcurrentHashMap</font>代替而不是线程安全的HashTable,因为HashTable基本已经被淘汰。
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## <font face="楷体" id="2">内部结构分析
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### <font face="楷体" id="2.1">JDK1.8之前:</font>
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JDK1.8之前HashMap底层是<font color="red">数组和链表</font>结合在一起使用也就是<font color="red">链表散列</font>。HashMap通过key的hashCode来计算hash值,当hashCode相同时,通过<font color="red">“拉链法”</font>解决冲突。
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所谓<font color="red">“拉链法”</font>就是:将链表和数组相结合。也就是说创建一个链表数组,数组中每一格就是一个链表。若遇到哈希冲突,则将冲突的值加到链表中即可。
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![jdk1.8之前的内部结构](https://user-gold-cdn.xitu.io/2018/3/20/16240dbcc303d872?w=348&h=427&f=png&s=10991)
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简单来说,JDK1.8之前HashMap由<font color="red">数组+链表组成的,数组是HashMap的主体,链表则是主要为了解决哈希冲突而存在的,如果定位到的数组位置不含链表(当前entry的next指向null),那么对于查找,添加等操作很快,仅需一次寻址即可;如果定位到的数组包含链表,对于添加操作,其时间复杂度依然为O(1),因为最新的Entry会插入链表头部,急需要简单改变引用链即可,而对于查找操作来讲,此时就需要遍历链表,然后通过key对象的equals方法逐一比对查找.</font>
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### <font face="楷体" id="2.2">JDK1.8之后:</font>
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相比于之前的版本,jdk1.8在解决哈希冲突时有了较大的变化,当链表长度大于阈值(默认为8)时,将链表转化为红黑树,以减少搜索时间。
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![JDK1.8之后的内部结构](https://user-gold-cdn.xitu.io/2018/3/20/16240e0e30123cfc?w=552&h=519&f=png&s=15827)
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**类的属性:**
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```java
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public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {
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// 序列号
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private static final long serialVersionUID = 362498820763181265L;
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// 默认的初始容量是16
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static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
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// 最大容量
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static final int MAXIMUM_CAPACITY = 1 << 30;
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// 默认的填充因子
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static final float DEFAULT_LOAD_FACTOR = 0.75f;
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// 当桶(bucket)上的结点数大于这个值时会转成红黑树
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static final int TREEIFY_THRESHOLD = 8;
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// 当桶(bucket)上的结点数小于这个值时树转链表
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static final int UNTREEIFY_THRESHOLD = 6;
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// 桶中结构转化为红黑树对应的table的最小大小
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static final int MIN_TREEIFY_CAPACITY = 64;
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// 存储元素的数组,总是2的幂次倍
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transient Node<k,v>[] table;
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// 存放具体元素的集
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transient Set<map.entry<k,v>> entrySet;
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// 存放元素的个数,注意这个不等于数组的长度。
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transient int size;
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// 每次扩容和更改map结构的计数器
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transient int modCount;
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// 临界值 当实际大小(容量*填充因子)超过临界值时,会进行扩容
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int threshold;
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// 填充因子
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final float loadFactor;
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}
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```
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<font color="red">(1)loadFactor加载因子</font>
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loadFactor加载因子是控制数组存放数据的疏密程度,loadFactor越趋近于1,那么 数组中存放的数据(entry)也就越多,也就越密,也就是会让链表的长度增加,load Factor越小,也就是趋近于0,
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**loadFactor太大导致查找元素效率低,太小导致数组的利用率低,存放的数据会很分散。loadFactor的默认值为0.75f是官方给出的一个比较好的临界值**。
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<font color="red">(2)threshold</font>
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**threshold = capacity * loadFactor**,**当Size>=threshold**的时候,那么就要考虑对数组的扩增了,也就是说,这个的意思就是 **衡量数组是否需要扩增的一个标准**。
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**Node节点类源码:**
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```java
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// 继承自 Map.Entry<K,V>
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static class Node<K,V> implements Map.Entry<K,V> {
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final int hash;// 哈希值,存放元素到hashmap中时用来与其他元素hash值比较
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final K key;//键
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V value;//值
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// 指向下一个节点
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Node<K,V> next;
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Node(int hash, K key, V value, Node<K,V> next) {
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this.hash = hash;
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this.key = key;
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this.value = value;
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this.next = next;
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}
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public final K getKey() { return key; }
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public final V getValue() { return value; }
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public final String toString() { return key + "=" + value; }
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// 重写hashCode()方法
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public final int hashCode() {
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return Objects.hashCode(key) ^ Objects.hashCode(value);
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}
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public final V setValue(V newValue) {
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V oldValue = value;
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value = newValue;
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return oldValue;
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}
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// 重写 equals() 方法
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public final boolean equals(Object o) {
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if (o == this)
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return true;
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if (o instanceof Map.Entry) {
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Map.Entry<?,?> e = (Map.Entry<?,?>)o;
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if (Objects.equals(key, e.getKey()) &&
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Objects.equals(value, e.getValue()))
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return true;
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}
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return false;
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}
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}
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```
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**树节点类源码:**
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```java
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static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
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TreeNode<K,V> parent; // 父
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TreeNode<K,V> left; // 左
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TreeNode<K,V> right; // 右
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TreeNode<K,V> prev; // needed to unlink next upon deletion
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boolean red; // 判断颜色
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TreeNode(int hash, K key, V val, Node<K,V> next) {
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super(hash, key, val, next);
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}
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// 返回根节点
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final TreeNode<K,V> root() {
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for (TreeNode<K,V> r = this, p;;) {
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if ((p = r.parent) == null)
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return r;
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r = p;
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}
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```
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## <font face="楷体" id="3">HashMap源码分析</font>
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### <font face="楷体" id="3.1">构造方法</font>
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![四个构造方法](https://user-gold-cdn.xitu.io/2018/3/20/162410d912a2e0e1?w=336&h=90&f=jpeg&s=26744)
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```java
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// 默认构造函数。
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public More ...HashMap() {
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this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
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}
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// 包含另一个“Map”的构造函数
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public More ...HashMap(Map<? extends K, ? extends V> m) {
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this.loadFactor = DEFAULT_LOAD_FACTOR;
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putMapEntries(m, false);//下面会分析到这个方法
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}
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// 指定“容量大小”的构造函数
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public More ...HashMap(int initialCapacity) {
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this(initialCapacity, DEFAULT_LOAD_FACTOR);
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}
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// 指定“容量大小”和“加载因子”的构造函数
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public More ...HashMap(int initialCapacity, float loadFactor) {
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if (initialCapacity < 0)
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throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
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if (initialCapacity > MAXIMUM_CAPACITY)
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initialCapacity = MAXIMUM_CAPACITY;
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if (loadFactor <= 0 || Float.isNaN(loadFactor))
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throw new IllegalArgumentException("Illegal load factor: " + loadFactor);
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this.loadFactor = loadFactor;
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this.threshold = tableSizeFor(initialCapacity);
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}
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```
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putMapEntries方法:
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```java
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final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
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int s = m.size();
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if (s > 0) {
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// 判断table是否已经初始化
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if (table == null) { // pre-size
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// 未初始化,s为m的实际元素个数
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float ft = ((float)s / loadFactor) + 1.0F;
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int t = ((ft < (float)MAXIMUM_CAPACITY) ?
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(int)ft : MAXIMUM_CAPACITY);
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// 计算得到的t大于阈值,则初始化阈值
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if (t > threshold)
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threshold = tableSizeFor(t);
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}
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// 已初始化,并且m元素个数大于阈值,进行扩容处理
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else if (s > threshold)
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resize();
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// 将m中的所有元素添加至HashMap中
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for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
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K key = e.getKey();
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V value = e.getValue();
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putVal(hash(key), key, value, false, evict);
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}
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}
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}
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```
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### <font face="楷体" id="3.2">put方法</font>
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HashMap只提供了put用于添加元素,putVal方法只是给put方法调用的一个方法,并没有提供给用户使用。
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```java
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public V put(K key, V value) {
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return putVal(hash(key), key, value, false, true);
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}
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final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
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boolean evict) {
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Node<K,V>[] tab; Node<K,V> p; int n, i;
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// table未初始化或者长度为0,进行扩容
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if ((tab = table) == null || (n = tab.length) == 0)
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n = (tab = resize()).length;
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// (n - 1) & hash 确定元素存放在哪个桶中,桶为空,新生成结点放入桶中(此时,这个结点是放在数组中)
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if ((p = tab[i = (n - 1) & hash]) == null)
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tab[i] = newNode(hash, key, value, null);
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// 桶中已经存在元素
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else {
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Node<K,V> e; K k;
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// 比较桶中第一个元素(数组中的结点)的hash值相等,key相等
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if (p.hash == hash &&
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((k = p.key) == key || (key != null && key.equals(k))))
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// 将第一个元素赋值给e,用e来记录
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e = p;
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// hash值不相等,即key不相等;为红黑树结点
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else if (p instanceof TreeNode)
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// 放入树中
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e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
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// 为链表结点
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else {
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// 在链表最末插入结点
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for (int binCount = 0; ; ++binCount) {
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// 到达链表的尾部
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if ((e = p.next) == null) {
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// 在尾部插入新结点
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p.next = newNode(hash, key, value, null);
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// 结点数量达到阈值,转化为红黑树
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if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
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treeifyBin(tab, hash);
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// 跳出循环
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break;
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}
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// 判断链表中结点的key值与插入的元素的key值是否相等
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if (e.hash == hash &&
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((k = e.key) == key || (key != null && key.equals(k))))
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// 相等,跳出循环
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break;
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// 用于遍历桶中的链表,与前面的e = p.next组合,可以遍历链表
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p = e;
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}
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}
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// 表示在桶中找到key值、hash值与插入元素相等的结点
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if (e != null) {
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// 记录e的value
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V oldValue = e.value;
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// onlyIfAbsent为false或者旧值为null
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if (!onlyIfAbsent || oldValue == null)
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//用新值替换旧值
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e.value = value;
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|
|
|
// 访问后回调
|
|
|
|
|
afterNodeAccess(e);
|
|
|
|
|
// 返回旧值
|
|
|
|
|
return oldValue;
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
// 结构性修改
|
|
|
|
|
++modCount;
|
|
|
|
|
// 实际大小大于阈值则扩容
|
|
|
|
|
if (++size > threshold)
|
|
|
|
|
resize();
|
|
|
|
|
// 插入后回调
|
|
|
|
|
afterNodeInsertion(evict);
|
|
|
|
|
return null;
|
|
|
|
|
}
|
|
|
|
|
```
|
|
|
|
|
### <font face="楷体" id="3.3">get方法</font>
|
|
|
|
|
```java
|
|
|
|
|
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;
|
|
|
|
|
if ((tab = table) != null && (n = tab.length) > 0 &&
|
|
|
|
|
(first = tab[(n - 1) & hash]) != null) {
|
|
|
|
|
// 数组元素相等
|
|
|
|
|
if (first.hash == hash && // always check first node
|
|
|
|
|
((k = first.key) == key || (key != null && key.equals(k))))
|
|
|
|
|
return first;
|
|
|
|
|
// 桶中不止一个节点
|
|
|
|
|
if ((e = first.next) != null) {
|
|
|
|
|
// 在树中get
|
|
|
|
|
if (first instanceof TreeNode)
|
|
|
|
|
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
|
|
|
|
|
// 在链表中get
|
|
|
|
|
do {
|
|
|
|
|
if (e.hash == hash &&
|
|
|
|
|
((k = e.key) == key || (key != null && key.equals(k))))
|
|
|
|
|
return e;
|
|
|
|
|
} while ((e = e.next) != null);
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
return null;
|
|
|
|
|
}
|
|
|
|
|
```
|
|
|
|
|
### <font face="楷体" id="3.4">resize方法</font>
|
|
|
|
|
进行扩容,会伴随着一次重新hash分配,并且会遍历hash表中所有的元素,是非常耗时的。在编写程序中,要尽量避免resize。
|
|
|
|
|
```java
|
|
|
|
|
final Node<K,V>[] resize() {
|
|
|
|
|
Node<K,V>[] oldTab = table;
|
|
|
|
|
int oldCap = (oldTab == null) ? 0 : oldTab.length;
|
|
|
|
|
int oldThr = threshold;
|
|
|
|
|
int newCap, newThr = 0;
|
|
|
|
|
if (oldCap > 0) {
|
|
|
|
|
// 超过最大值就不再扩充了,就只好随你碰撞去吧
|
|
|
|
|
if (oldCap >= MAXIMUM_CAPACITY) {
|
|
|
|
|
threshold = Integer.MAX_VALUE;
|
|
|
|
|
return oldTab;
|
|
|
|
|
}
|
|
|
|
|
// 没超过最大值,就扩充为原来的2倍
|
|
|
|
|
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY)
|
|
|
|
|
newThr = oldThr << 1; // double threshold
|
|
|
|
|
}
|
|
|
|
|
else if (oldThr > 0) // initial capacity was placed in threshold
|
|
|
|
|
newCap = oldThr;
|
|
|
|
|
else {
|
|
|
|
|
signifies using defaults
|
|
|
|
|
newCap = DEFAULT_INITIAL_CAPACITY;
|
|
|
|
|
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
|
|
|
|
|
}
|
|
|
|
|
// 计算新的resize上限
|
|
|
|
|
if (newThr == 0) {
|
|
|
|
|
float ft = (float)newCap * loadFactor;
|
|
|
|
|
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE);
|
|
|
|
|
}
|
|
|
|
|
threshold = newThr;
|
|
|
|
|
@SuppressWarnings({"rawtypes","unchecked"})
|
|
|
|
|
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
|
|
|
|
|
table = newTab;
|
|
|
|
|
if (oldTab != null) {
|
|
|
|
|
// 把每个bucket都移动到新的buckets中
|
|
|
|
|
for (int j = 0; j < oldCap; ++j) {
|
|
|
|
|
Node<K,V> e;
|
|
|
|
|
if ((e = oldTab[j]) != null) {
|
|
|
|
|
oldTab[j] = null;
|
|
|
|
|
if (e.next == null)
|
|
|
|
|
newTab[e.hash & (newCap - 1)] = e;
|
|
|
|
|
else if (e instanceof TreeNode)
|
|
|
|
|
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
|
|
|
|
|
else {
|
|
|
|
|
Node<K,V> loHead = null, loTail = null;
|
|
|
|
|
Node<K,V> hiHead = null, hiTail = null;
|
|
|
|
|
Node<K,V> next;
|
|
|
|
|
do {
|
|
|
|
|
next = e.next;
|
|
|
|
|
// 原索引
|
|
|
|
|
if ((e.hash & oldCap) == 0) {
|
|
|
|
|
if (loTail == null)
|
|
|
|
|
loHead = e;
|
|
|
|
|
else
|
|
|
|
|
loTail.next = e;
|
|
|
|
|
loTail = e;
|
|
|
|
|
}
|
|
|
|
|
// 原索引+oldCap
|
|
|
|
|
else {
|
|
|
|
|
if (hiTail == null)
|
|
|
|
|
hiHead = e;
|
|
|
|
|
else
|
|
|
|
|
hiTail.next = e;
|
|
|
|
|
hiTail = e;
|
|
|
|
|
}
|
|
|
|
|
} while ((e = next) != null);
|
|
|
|
|
// 原索引放到bucket里
|
|
|
|
|
if (loTail != null) {
|
|
|
|
|
loTail.next = null;
|
|
|
|
|
newTab[j] = loHead;
|
|
|
|
|
}
|
|
|
|
|
// 原索引+oldCap放到bucket里
|
|
|
|
|
if (hiTail != null) {
|
|
|
|
|
hiTail.next = null;
|
|
|
|
|
newTab[j + oldCap] = hiHead;
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
return newTab;
|
|
|
|
|
}
|
|
|
|
|
```
|
|
|
|
|
## <font face="楷体" id="4">HashMap常用方法测试</font>
|
|
|
|
|
```java
|
|
|
|
|
package map;
|
|
|
|
|
|
|
|
|
|
import java.util.Collection;
|
|
|
|
|
import java.util.HashMap;
|
|
|
|
|
import java.util.Set;
|
|
|
|
|
|
|
|
|
|
public class HashMapDemo {
|
|
|
|
|
|
|
|
|
|
public static void main(String[] args) {
|
|
|
|
|
HashMap<String, String> map = new HashMap<String, String>();
|
|
|
|
|
// 键不能重复,值可以重复
|
|
|
|
|
map.put("san", "张三");
|
|
|
|
|
map.put("si", "李四");
|
|
|
|
|
map.put("wu", "王五");
|
|
|
|
|
map.put("wang", "老王");
|
|
|
|
|
map.put("wang", "老王2");// 老王被覆盖
|
|
|
|
|
map.put("lao", "老王");
|
|
|
|
|
System.out.println("-------直接输出hashmap:-------");
|
|
|
|
|
System.out.println(map);
|
|
|
|
|
/**
|
|
|
|
|
* 遍历HashMap
|
|
|
|
|
*/
|
|
|
|
|
// 1.获取Map中的所有键
|
|
|
|
|
System.out.println("-------foreach获取Map中所有的键:------");
|
|
|
|
|
Set<String> keys = map.keySet();
|
|
|
|
|
for (String key : keys) {
|
|
|
|
|
System.out.print(key+" ");
|
|
|
|
|
}
|
|
|
|
|
System.out.println();//换行
|
|
|
|
|
// 2.获取Map中所有值
|
|
|
|
|
System.out.println("-------foreach获取Map中所有的值:------");
|
|
|
|
|
Collection<String> values = map.values();
|
|
|
|
|
for (String value : values) {
|
|
|
|
|
System.out.print(value+" ");
|
|
|
|
|
}
|
|
|
|
|
System.out.println();//换行
|
|
|
|
|
// 3.得到key的值的同时得到key所对应的值
|
|
|
|
|
System.out.println("-------得到key的值的同时得到key所对应的值:-------");
|
|
|
|
|
Set<String> keys2 = map.keySet();
|
|
|
|
|
for (String key : keys2) {
|
|
|
|
|
System.out.print(key + ":" + map.get(key)+" ");
|
|
|
|
|
|
|
|
|
|
}
|
|
|
|
|
/**
|
|
|
|
|
* 另外一种不常用的遍历方式
|
|
|
|
|
*/
|
|
|
|
|
// 当我调用put(key,value)方法的时候,首先会把key和value封装到
|
|
|
|
|
// Entry这个静态内部类对象中,把Entry对象再添加到数组中,所以我们想获取
|
|
|
|
|
// map中的所有键值对,我们只要获取数组中的所有Entry对象,接下来
|
|
|
|
|
// 调用Entry对象中的getKey()和getValue()方法就能获取键值对了
|
|
|
|
|
Set<java.util.Map.Entry<String, String>> entrys = map.entrySet();
|
|
|
|
|
for (java.util.Map.Entry<String, String> entry : entrys) {
|
|
|
|
|
System.out.println(entry.getKey() + "--" + entry.getValue());
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
/**
|
|
|
|
|
* HashMap其他常用方法
|
|
|
|
|
*/
|
|
|
|
|
System.out.println("after map.size():"+map.size());
|
|
|
|
|
System.out.println("after map.isEmpty():"+map.isEmpty());
|
|
|
|
|
System.out.println(map.remove("san"));
|
|
|
|
|
System.out.println("after map.remove():"+map);
|
|
|
|
|
System.out.println("after map.get(si):"+map.get("si"));
|
|
|
|
|
System.out.println("after map.containsKey(si):"+map.containsKey("si"));
|
|
|
|
|
System.out.println("after containsValue(李四):"+map.containsValue("李四"));
|
|
|
|
|
System.out.println(map.replace("si", "李四2"));
|
|
|
|
|
System.out.println("after map.replace(si, 李四2):"+map);
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
```
|