一、简介
HashMap是java中常用的key/value集合，是非安全的。这里从源码上进行解读。
二、HashMap基本数据结构
HashMap基本数据结构为“数组+链表“，
数组是基本存储结构，数组中的每个元素又是一个链表，当key冲突时，就在冲突元素后以链接的形式添加新元素。
示例如下：
table[0] -> Entry-> Entry table[1] -> Entry table[2] table[3] -> Entry-> Entry- > Entry . . . .
三、源码解读
1、HashMap实现了map接口，同时继承AbstractMap抽象类，源码如下：
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {2、数据的存储单元Entry是一个类，该类实现了Map.Entry接口，也是一个链表节点。
包含的基本字段为：hash值、元素的key、元素的value、以及可能的下一个元素（链表），源码如下：
static class Entry<K,V> implements Map.Entry<K,V> { final K key; V value; Entry<K,V> next; int hash; /** * Creates new entry. */ Entry(int h, K k, V v, Entry<K,V> n) { value = v; next = n; key = k; hash = h; } public final K getKey() { return key; } public final V getValue() { return value; } public final V setValue(V newValue) { V oldValue = value; value = newValue; return oldValue; } public final boolean equals(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry e = (Map.Entry)o; Object k1 = getKey(); Object k2 = e.getKey(); if (k1 == k2 || (k1 != null && k1.equals(k2))) { Object v1 = getValue(); Object v2 = e.getValue(); if (v1 == v2 || (v1 != null && v1.equals(v2))) return true; } return false; } public final int hashCode() { return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue()); } public final String toString() { return getKey() + "=" + getValue(); } /** * This method is invoked whenever the value in an entry is * overwritten by an invocation of put(k,v) for a key k that's already * in the HashMap. */ void recordAccess(HashMap<K,V> m) { } /** * This method is invoked whenever the entry is * removed from the table. */ void recordRemoval(HashMap<K,V> m) { } }3、hash值是根据key的hashCode计算出来的，null的hash值为0，具体计算方式源码如下：
/** * Retrieve object hash code and applies a supplemental hash function to the * result hash, which defends against poor quality hash functions. This is * critical because HashMap uses power-of-two length hash tables, that * otherwise encounter collisions for hashCodes that do not differ * in lower bits. Note: Null keys always map to hash 0, thus index 0. */ final int hash(Object k) { int h = hashSeed; if (0 != h && k instanceof String) { return sun.misc.Hashing.stringHash32((String) k); } h ^= k.hashCode(); // This function ensures that hashCodes that differ only by // constant multiples at each bit position have a bounded // number of collisions (approximately 8 at default load factor). h ^= (h >>> 20) ^ (h >>> 12); return h ^ (h >>> 7) ^ (h >>> 4); }4、HashMap的初始化，在初始化时，会新建名为table的空的Entry数组，同时指定新建或扩容时的参数，
初始数组容量为16 ,最大容量为2的30次方，扩容因子为0.75，初始化参数源码如下：
/** * The default initial capacity - MUST be a power of two. */ static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 /** * The maximum capacity, used if a higher value is implicitly specified * by either of the constructors with arguments. * MUST be a power of two <= 1<<30. */ static final int MAXIMUM_CAPACITY = 1 << 30; /** * The load factor used when none specified in constructor. */ static final float DEFAULT_LOAD_FACTOR = 0.75f; /** * An empty table instance to share when the table is not inflated. */ static final Entry<?,?>[] EMPTY_TABLE = {}; /** * The table, resized as necessary. Length MUST Always be a power of two. */ transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;新建HashMap时，可指定初始化参数，或添加map形式的元素，构造函数源码如下：
/** * 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; threshold = initialCapacity; init(); } /** * 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(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR); } /** * 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(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR); inflateTable(threshold); putAllForCreate(m); }
/** * Inflates the table. */ private void inflateTable(int toSize) { // Find a power of 2 >= toSize int capacity = roundUpToPowerOf2(toSize); threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1); table = new Entry[capacity]; initHashSeedAsNeeded(capacity); } // internal utilities /** * Initialization hook for subclasses. This method is called * in all constructors and pseudo-constructors (clone, readObject) * after HashMap has been initialized but before any entries have * been inserted. (In the absence of this method, readObject would * require explicit knowledge of subclasses.) */ void init() { }5、get方法（注意key是可为null的），
当key为null时，则从table数组的第一个位置取（当key为null,默认是存在table数组的第一个位置），
当key不为null时，则根据key的hash值定位到其在table数组的存储位置，该位置元素又是一个链接，从头开始比对，最终获取元素。
源码如下：
/** * 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) { if (key == null) return getForNullKey(); Entry<K,V> entry = getEntry(key); return null == entry ? null : entry.getValue(); } /** * Offloaded version of get() to look up null keys. Null keys map * to index 0. This null case is split out into separate methods * for the sake of performance in the two most commonly used * operations (get and put), but incorporated with conditionals in * others. */ private V getForNullKey() { if (size == 0) { return null; } for (Entry<K,V> e = table[0]; e != null; e = e.next) { if (e.key == null) return e.value; } return null; } /** * Returns <tt>true</tt> if this map contains a mapping for the * specified key. * * @param key The key whose presence in this map is to be tested * @return <tt>true</tt> if this map contains a mapping for the specified * key. */ public boolean containsKey(Object key) { return getEntry(key) != null; } /** * Returns the entry associated with the specified key in the * HashMap. Returns null if the HashMap contains no mapping * for the key. */ final Entry<K,V> getEntry(Object key) { if (size == 0) { return null; } int hash = (key == null) ? 0 : hash(key); for (Entry<K,V> e = table[indexFor(hash, table.length)]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } return null; }6、put方法（注意key是可为null的），首先会进行容量检测，看是否需要扩容，接下来便是插入新元素，
a)当key为null时，则默认存在table数组的第一个位置;
b)当key不为null时，
则根据key的hash值定位到其在table数组的存储位置，若该位置元素没有元素，则保存，
若已经存在元素（其是一个链表）且key一样，则用新值覆盖旧值，同时返回旧值，否则当key不一样时，便在该链表元素的末尾新添加元素。
源码如下：
/** * 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) { if (table == EMPTY_TABLE) { inflateTable(threshold); } if (key == null) return putForNullKey(value); int hash = hash(key); int i = indexFor(hash, table.length); for (Entry<K,V> e = table[i]; e != null; e = e.next) { Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(hash, key, value, i); return null; } /** * Offloaded version of put for null keys */ private V putForNullKey(V value) { for (Entry<K,V> e = table[0]; e != null; e = e.next) { if (e.key == null) { V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(0, null, value, 0); return null; }
/** * Retrieve object hash code and applies a supplemental hash function to the * result hash, which defends against poor quality hash functions. This is * critical because HashMap uses power-of-two length hash tables, that * otherwise encounter collisions for hashCodes that do not differ * in lower bits. Note: Null keys always map to hash 0, thus index 0. */ final int hash(Object k) { int h = hashSeed; if (0 != h && k instanceof String) { return sun.misc.Hashing.stringHash32((String) k); } h ^= k.hashCode(); // This function ensures that hashCodes that differ only by // constant multiples at each bit position have a bounded // number of collisions (approximately 8 at default load factor). h ^= (h >>> 20) ^ (h >>> 12); return h ^ (h >>> 7) ^ (h >>> 4); } /** * Returns index for hash code h. */ static int indexFor(int h, int length) { // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2"; return h & (length-1); }
/** * Adds a new entry with the specified key, value and hash code to * the specified bucket. It is the responsibility of this * method to resize the table if appropriate. * * Subclass overrides this to alter the behavior of put method. */ void addEntry(int hash, K key, V value, int bucketIndex) { if ((size >= threshold) && (null != table[bucketIndex])) { resize(2 * table.length); hash = (null != key) ? hash(key) : 0; bucketIndex = indexFor(hash, table.length); } createEntry(hash, key, value, bucketIndex); } /** * Like addEntry except that this version is used when creating entries * as part of Map construction or "pseudo-construction" (cloning, * deserialization). This version needn't worry about resizing the table. * * Subclass overrides this to alter the behavior of HashMap(Map), * clone, and readObject. */ void createEntry(int hash, K key, V value, int bucketIndex) { Entry<K,V> e = table[bucketIndex]; table[bucketIndex] = new Entry<>(hash, key, value, e); size++; }