JDK容器学习之HashMap (一) ： 底层存储结构分析

transient Node<K,V>[] table;

transient Set<Map.Entry<K,V>> entrySet;

transient int size;

transient int modCount;

int threshold;

final float loadFactor;

interface Entry<K,V> {
  K getKey();

  V getValue();

  V setValue(V value);

  boolean equals(Object o);

  int hashCode();
}

(e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
(e.getValue()==null ? 0 : e.getValue().hashCode())

 (e1.getKey()==null ? e2.getKey()==null : e1.getKey().equals(e2.getKey()))   &&
(e1.getValue()==null ? e2.getValue() ==null : e1.getValue().equals(e2.getValue()))

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    Node<K,V> next;

    Node(int hash, K key, V value, Node<K,V> next) {
        this.hash = hash;
        this.key = key;
        this.value = value;
        this.next = next;
    }

    public final K getKey()        { return key; }
    public final V getValue()      { return value; }
    public final String toString() { return key + "=" + value; }

    public final int hashCode() {
        return Objects.hashCode(key) ^ Objects.hashCode(value);
    }

    public final V setValue(V newValue) {
        V oldValue = value;
        value = newValue;
        return oldValue;
    }

    public final boolean equals(Object o) {
        if (o == this)
            return true;
        if (o instanceof Map.Entry) {
            Map.Entry<?,?> e = (Map.Entry<?,?>)o;
            if (Objects.equals(key, e.getKey()) &&
                Objects.equals(value, e.getValue()))
                return true;
        }
        return false;
    }
}

public HashMap(int initialCapacity, float loadFactor) {
    // 参数合法性校验省略
    
    this.loadFactor = loadFactor;
    this.threshold = tableSizeFor(initialCapacity);
}

/**
 * 找到大于等于cap的最小的2的幂.
 */
static final int tableSizeFor(int cap) {
  int n = cap - 1;
  n |= n >>> 1;
  n |= n >>> 2;
  n |= n >>> 4;
  n |= n >>> 8;
  n |= n >>> 16;
  return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

/**
 * Implements Map.put and related methods
 *
 * @param hash (key的hash值，通过hash方法计算)
 * @param key the key
 * @param value the value to put
 * @param onlyIfAbsent true表示在不存在kv时，才塞入数据
 * @param evict if false, the table is in creation mode.
 * @return 返回原来的value（如果之前不存在，返回null）
 */
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    
    // 首先是将tab局部变量指向 table数组
    if ((tab = table) == null || (n = tab.length) == 0) {
        // 当table数组没有初始化时，进行初始化，并返回数组长度
        n = (tab = resize()).length;
    } 
    
    
    if ((p = tab[i = (n - 1) & hash]) == null) {
        // 根据数组长度和key的hash值，计算出key放入数组的位置，若该位置没有值，则直接创建一个新的Entry（即Node），放在该位置即可
        tab[i] = newNode(hash, key, value, null);
    } else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k)))) {
        // 若根据key的hash值，从数组中获取的Entry对象，其key正好是我们指定的key，则直接修改这个Entry的value值即可
            e = p;
        }
        // 下面则表示出现hash碰撞，虽然key的hash值相同，但是这个Entry的key并不是我们指定的key
        else if (p instanceof TreeNode) {
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        } else {
        // 迭代Entry的next节点，知道找到Entry.Key 正好是我们指定的Key为止
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) { // 若一直都不存在，则创建一个新的Entry对象，并塞入table
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

Map map = new HashMap<>();
map.put(xx, xx);

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table; // null
    int oldCap = 0;
    int oldThr = 0;
    int newCap, newThr = 0;

    // zero initial threshold signifies using defaults
    newCap = 16; // DEFAULT_INITIAL_CAPACITY;
    newThr = 12; // (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);

 
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    return newTab;
}

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}