深入理解HashMap各个方法的源码

public V put(K key, V value) {
if (key == null)
return putForNullKey(value);  //这里判断key是否为空,若为空则调用putForNullKey处理null值
int hash = hash(key); //根据key的hashCode计算hash值
int i = indexFor(hash, table.length);//搜索该key的hash值在table中的索引,其中table是当HashMap用于存放entry的一个数组
//这里循环遍历table中对应该索引的entry，若发现存在key与put进来的key相同则覆盖其value值
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++;
//将key value 添加到 索引i处
addEntry(hash, key, value, i);
return null;
}

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);
// Find a power of 2 >= initialCapacity
int capacity = 1;
while (capacity < initialCapacity)
capacity <<= 1;
this.loadFactor = loadFactor;
threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
table = new Entry[capacity];
useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
init();
}

public V get(Object key) {
if (key == null)
return getForNullKey();
Entry<K,V> entry = getEntry(key);
return null == entry ? null : entry.getValue();
}
final Entry<K,V> getEntry(Object key) {
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;
}

Entry(int h, K k, V v, Entry<K,V> n) {
value = v;
next = n;
key = k;
hash = h;
}