util HashMap

1.HashMap涉及的数据结构

Entry[] ;

//Entry数组:存储HashMap元素的地方.

//Entry

//1.封装了key;value;

//2.本身是一个单向链表;包含hash值;next;指针;

static class Entry<K,V> implements Map.Entry<K,V> { final K key; V value; Entry<K,V> next; final int hash; /** * Creates new entry. */ Entry(int h, K k, V v, Entry<K,V> n) { value = v; next = n; key = k; hash = h; }

2.存储的过程:

通过hashcode得到index后.存储的不仅仅是该元素的key,value,hash.还有指向下一个Entry的引用.如果出现了hashcode冲突问题.则新建一个Entry;将该Entry的nex指针指向已存在的Entry.

public V put(K key, V value) { if (key == null) return putForNullKey(value); int hash = hash(key.hashCode()); 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))) { //已经存在 V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } //不存在，则插入 modCount++; addEntry(hash, key, value, i); return null; }
void addEntry(int hash, K key, V value, int bucketIndex) { Entry<K,V> e = table[bucketIndex]; table[bucketIndex] = new Entry<K,V>(hash, key, value, e); if (size++ >= threshold) resize(2 * table.length); }

3.取值的过程

还是hash-index-Entry的过程.不是直接return该index上Entry;而要检查Entry链上真正对应的那个

public V get(Object key) { if (key == null) return getForNullKey(); int hash = hash(key.hashCode()); 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.equals(k))) return e.value; } return null; }