HashMap源码分析

public class HashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable
{
//...

	public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
		
		//迭代这个位置下有没有key的值相同的,如果相同则覆盖这个Entry里的value
        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);//留给开发人员覆盖来记录log
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value,i);//没有覆盖value的情况就增加Entry.
        return null;
    }
	

    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;
    }
	
	 void addEntry(int hash, K key, V value, int bucketIndex) {
	Entry<K,V> e = table[bucketIndex];//其中一种情况就是赋值e为null
        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
        if (size++ >= threshold)
            resize(2 * table.length);
    }
	
	final Entry<K,V> removeEntryForKey(Object key) {
        int hash = (key == null) ? 0 : hash(key.hashCode());
        int i = indexFor(hash, table.length);
        Entry<K,V> prev = table[i];
        Entry<K,V> e = prev;

        while (e != null) {//寻找位置i上的链表上key的值相等的Entry
            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;
    }
//...	
}