transient Node<k,v>[] table; 这个类属性就是哈希桶数组
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable { // 序列号 private static final long serialVersionUID = 362498820763181265L; // 默认的初始容量是16 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 最大容量 static final int MAXIMUM_CAPACITY = 1 << 30; // 默认的负载因子 static final float DEFAULT_LOAD_FACTOR = 0.75f; // 当桶(bucket)上的结点数大于这个值时会转成红黑树 static final int TREEIFY_THRESHOLD = 8; // 当桶(bucket)上的结点数小于这个值时树转链表 static final int UNTREEIFY_THRESHOLD = 6; // 桶中结构转化为红黑树对应的table的最小大小 static final int MIN_TREEIFY_CAPACITY = 64; // 存储元素的数组，总是2的幂次倍（哈希桶数组） transient Node<k,v>[] table; // 存放具体元素的集 transient Set<map.entry<k,v>> entrySet; // 存放元素的个数，注意这个不等于数组的长度。 transient int size; // 每次扩容和更改map结构的计数器 transient int modCount; // 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 int threshold; // 负载因子 final float loadFactor; }内部类Node
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) { ...... } 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) { .... } public final boolean equals(Object o) { ...... } } 构造函数无参构造函数默认长度16，负载因子0.75
/** * 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 }指定容量，负载因子0.75
public HashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); }指定容量和指定负载因子
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); } 重要函数 内部hash方法(获得的hash值用于putVal方法中确定哈希桶数组索引位置) static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); }第一步调用object的hashCode：h = key.hashCode() 取hashCode值
h ^ (h >>> 16) 首先进行无符号右移（>>>）运算，再通过异或运算(^)得到hash值。
put方法，put内部调用的是putVal final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) { Node<K,V>[] tab; Node<K,V> p; int n, i; //首先确定table是不是为空，如果为空进行扩容 if ((tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; //取模运算,确定哈希桶数组索引位置 if ((p = tab[i = (n - 1) & hash]) == null) tab[i] = newNode(hash, key, value, null); else { Node<K,V> e; K k; //节点key存在，直接覆盖value if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k)))) e = p; //判断是否是红黑树 else if (p instanceof TreeNode) //如果是红黑树，则直接在树中插入键值对 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //为链表 else { for (int binCount = 0; ; ++binCount) { if ((e = p.next) == null) { p.next = newNode(hash, key, value, null); //判断链表长度是否大于8，大于8把链表转换为红黑树 if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st treeifyBin(tab, hash); break; } //key已经存在直接覆盖value 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; //判断实际存在的键值对数量size是否超多了最大容量threshold，如果超过，进行扩容。 if (++size > threshold) resize(); afterNodeInsertion(evict); return null; }