HashMap源码阅读

1.HashMap空间申请

Hash默认的初始化空间大小为16字节，最大的空间大小为1073741824字节。HashMap在构造方法中对空间的容量进行初始化。代码如下：

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();    }

从代码中，我们可以看出，当我们自定义空间容量超过1073741824字节时候，它默认只给1073741824字节的容量。

2.计算Hash码

在hashMap中，存储都是以键值对的形式来存储的，在我们传入键值对的时候，不是直接存储的，会先对键进行计算，换算成Hash码存储。代码如下：

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);    }

3.放入键值对

代码如下：

 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;    }

当我们放入键值对的时候，先判断空间是不是为空，如果为空，则申请一段空间，否则判断键是不是为空，如果为空，则存储空键的值。从代码中，我们可以发现HasMap运行存储空键的值。存储方法如下：

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;    }

3.删除操作

final Entry<K,V> removeEntryForKey(Object key) {        if (size == 0) {            return null;        }        int hash = (key == null) ? 0 : hash(key);        int i = indexFor(hash, table.length);        Entry<K,V> prev = table[i];        Entry<K,V> e = prev;        while (e != null) {            Entry<K,V> next = e.next;            Object k;            if (e.hash == hash &&                ((k = e.key) == key || (key != null && key.equals(k)))) {                modCount++;                size--;                if (prev == e)                    table[i] = next;                else                    prev.next = next;                e.recordRemoval(this);                return e;            }            prev = e;            e = next;        }        return e;    }

4.HashMap值存储的数据结构

 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) {        }    }