HashMap 是 Java 开发过程中常用的工具类之一，也是面试过程中常问的内容，此篇文件通过作者自己的理解和网上众多资料对其进行一个解析。作者本地的 JDK 版本为 64 位的 1.8.0_171。参考资料推荐以下两篇文章：
https://www.cnblogs.com/little-fly/p/7344285.html
数据结构
结合上图及源码可以看出，HashMap 底层数据结构为 Node 类型数组，Node 类型为 HashMap 的内部类，数据结构为链表。
/**
* The table, initialized on first use, and resized as
* necessary. When allocated, length is always a power of two.
* (We also tolerate length zero in some operations to allow
* bootstrapping mechanics that are currently not needed.)
*/transient Node<K,V>[] table;/**
* Basic hash bin node, used for most entries. (See below for
* TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
*/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;
} 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;
}
}初始大小
HashMap 默认的初始大小为 16，如有特殊情况下需要自定义初始化大小时可调用 HashMap(int initialCapacity) 方法进行自定义。
/**
* The default initial capacity - MUST be a power of two.
*/static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16/**
* 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);
}负载因子
负载因子默认为 0.75，当 HashMap 当前已使用容量大于当前大小 * 负载因子时，自动扩容一倍空间，如有特殊情况下需要自定义初始化大小时可调用 以下方法进行自定义。
/**
* The load factor used when none specified in constructor.
*/static final float DEFAULT_LOAD_FACTOR = 0.75f;/**
* 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);
}树型阀值
树型阀值这个名字是作者根据字面意思自己翻译的，大家看看就好了，对应参数为TREEIFY_THRESHOLD，之前提到过 HashMap 的结构为 Node 型数组，而 Node 的数据结构为链表，树型阀值就是当链表长度超过这个值时，将 Node 的数据结构修改为红黑树，以便优化查找时间，默认值为8
/** * 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. */static final int TREEIFY_THRESHOLD = 8;
初始化
HashMap 提供以下四种构造方法进行初始化，前三种主要区别在于设置以上介绍的几个参数，第四种方法为通过其他 Map 实现创建 HashMap。
/**
* 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);
}设置 HashMap 的值
put 方法是 HashMap 中使用率非常高的 API 之一，其源码实现如下，通过源码我们可以发现其原理主要分为以下两步：
对 key 进行 hash 运算，然后再与当前 map 最后一个下标进行与运算确定其在数组中的位置，正是因为这个算法，我们可以得知 HashMap 中元素是无序的。
确定其下标以后，如果当前位置为空则直接赋值，如果不为空则放到下一个节点，如果当前为链表且添加元素后的长度达到树型阀值，则将链表转换为红黑树
/**
* 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; if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length; if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null); else {
Node<K,V> e; K k; if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p; else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); else { for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null); 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; if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e); return oldValue;
}
}
++modCount; if (++size > threshold)
resize();
afterNodeInsertion(evict); return null;
}获取HashMap中的值
get 方法同样是 HashMap 中常用的 API 之一，参照其源码，其原理与 put 方法正好相反，分为以下两个部分：
根据 key 的 hash 运算值获取数组中对应下标的内容
循环链表或红黑树，然后匹配 value 值直至获得对应的值或返回 null
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
* <p>A return value of {@code null} does not <i>necessarily</i>
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link #containsKey containsKey} operation may be used to
* distinguish these two cases.
*
* @see #put(Object, Object)
*/public V get(Object key) {
Node<K,V> e; return (e = getNode(hash(key), key)) == null ? null : e.value;
}/**
* Implements Map.get and related methods
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/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) { if (first.hash == hash && // always check first node
((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 { if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) return e;
} while ((e = e.next) != null);
}
} return null;
}/**
* Calls find for root node.
*/final TreeNode<K,V> getTreeNode(int h, Object k) { return ((parent != null) ? root() : this).find(h, k, null);
}原文出处：https://www.cnblogs.com/aotian/p/9491346.html
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