1. 基本结构
HashMap 的核心是一个数组，每个数组元素是一个链表或红黑树（JDK 1.8 及以后）。当哈希冲突发生时，链表或红黑树用于存储多个键值对。
java
// HashMap的基本结构
public class HashMap<K, V> extends AbstractMap<K, V>
implements Map<K, V>, Cloneable, Serializable {
// 默认初始容量为16
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
// 最大容量为2^30
static final int MAXIMUM_CAPACITY = 1 << 30;
// 默认加载因子为0.75
static final float DEFAULT_LOAD_FACTOR = 0.75f;
// 链表转化为红黑树的阈值
static final int TREEIFY_THRESHOLD = 8;
// 红黑树转化为链表的阈值
static final int UNTREEIFY_THRESHOLD = 6;
// 最小树化容量
static final int MIN_TREEIFY_CAPACITY = 64;
// 存储元素的数组
transient Node<K, V>[] table;
// 存储具体键值对的集合
transient Set<Map.Entry<K, V>> entrySet;
// 存储的键值对的数量
transient int size;
// 扩容和其他结构修改次数
transient int modCount;
// 阈值
int threshold;
// 加载因子
final float loadFactor;
// Node 是 HashMap 的基本结构
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;
}
}
// 其他重要的方法和类
// ...
}2. 哈希算法
HashMap 使用的哈希算法通过扰动函数减少哈希冲突，提高查找效率。
java
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}这个方法对键的哈希码进行高低16位混合，以增加随机性，减少冲突。
3. 初始化和扩容
HashMap 在插入元素时，会检查当前容量是否需要扩容，如果需要，就进行扩容。扩容时，新容量是旧容量的两倍。
java
// 初始化
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);
}
// 扩容
final Node<K, V>[] resize() {
Node<K, V>[] oldTable = table;
int oldCap = (oldTable == null) ? 0 : oldTable.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTable;
}
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 { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
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>[] newTable = (Node<K, V>[])new Node[newCap];
table = newTable;
if (oldTable != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K, V> e;
if ((e = oldTable[j]) != null) {
oldTable[j] = null;
if (e.next == null)
newTable[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((TreeNode<K, V>)e).split(this, newTable, j, oldCap);
else { // preserve order
Node<K, V> loHead = null, loTail = null;
Node<K, V> hiHead = null, hiTail = null;
Node<K, V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTable[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTable[j + oldCap] = hiHead;
}
}
}
}
}
return newTable;
}4. 插入元素
插入元素时，首先计算元素的哈希值，然后确定存储位置。如果发生哈希冲突，将元素链入当前链表或红黑树中。
java
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
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;