HashMap中的一些常量

默认的初始容量 16
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
hashmap 最大容量,当构造函数中传入的数值大于此值,将使用最大值
static final int MAXIMUM_CAPACITY = 1 << 30;
hashmap的加载因子。
1,加载因子设置较大,随着数组中元素装的越多,发生的冲突的概率越大,即对应的链表越长,影响查找效率。
2.加载因子设置较小,元素很容易达到设定的阈值,发生扩容操作,数组空间还有很大部分没有利用上,造成空间浪费。因此在时间与空间上的权衡考虑
DEFAULT_LOAD_FACTOR = 0.75f
树化阀值,当链表元素超过8时,hashmap存储结构由链表转化为红黑数结果,提高查找效率
static final int TREEIFY_THRESHOLD = 8;
取消树阀值,在红黑树中节点数量小于6个,将其转化为链表结构
UNTREEIFY_THRESHOLD = 6
最小的树化容量,链表树化红黑树条件:
1,链表中元素数量超过(>)8个;
2, 满足map中元素个数(size)大于等于64否则会先扩容,扩容对解决hash冲突更有效。
static final int MIN_TREEIFY_CAPACITY = 64;
threshold表示当HashMap的size大于threshold时会执行resize操作。threshold = capacity * loadFactor
threshold
hashmap修改次数,当修改次数不匹配时,会抛出ConcurrentModificationException异常
modCount

HashMap的构造函数

HashMap提供了4种构造函数。
无参构造函数

//加载因子,和初始化容量均使用默认值
public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

提供初始容量和加载因子的构造函数

//使用传入的容量和加载因子初始化hashmap,并校验参数范围
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;
        this.threshold = tableSizeFor(initialCapacity);
    }

提供初始容量的构造函数

public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

参数为map子集的构造函数

public HashMap(Map<? extends K, ? extends V> m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }
HashMap的节点结构
  • 数组和链表节点对象为HashMap内部类 Node.
  • 红黑树节点 TreeNode,继承LinkedHashMap.Entry , 而 Entry继承Node,因此 TreeNode 实际是 Node孙子.
  • Node类,重写了hashCode和 equals方法,记录了当前key, value, key的hash值,以及指向后一个元素指针.

Node类

    /**
     * Basic hash bin node, used for most entries.  (See below for
     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
     */
    static class Node<K,V> implements Map.Entry<K,V> {
        //记录当前节点的hash值
        final int hash;
        //键
        final K key;
        //值
        V value;
        //指向下一个节点元素
        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;
        }
    }
HashMap的put实现
put方法:
public V put(K key, V value) { 
    return putVal(hash(key), key, value, false, true);
}
key的hash方法
static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }
put方法的核心实现:putVal()方法
    /**
     * Implements Map.put and related methods.
     *
     * @param hash hash for key
     * @param key the key
     * @param value the value to put
     * @param onlyIfAbsent if true, don't change existing value
     * @param evict if false, the table is in creation mode.
     * @return previous value, or null if none
     */
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //初始化数组
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //确定元素在数组中的位置。
        //hashmap通过 数组长度 n - 1 与上key的hash值,来确定元素的位置,那么是怎么保证永远结果在数组长度范围内呢?
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            //比较第一个元素(数组中的结点)的hash值相等,key相等`
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                //将第一个元素赋值给e,用e来记录`
                e = p;
                //hash值不相等,即key不相等;为红黑树结点`
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
                //不是红黑树,即为链表结构,遍历链表
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                    //尾插,next 为null
                        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;
                }
            }
            // existing mapping for key,key值已经存在,新的value替换旧的value,并返回旧值
            if (e != null) { 
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        //修改次数增加
        ++modCount;
        if (++size > threshold)
        //到达阀值,hashmap重新进行resize()
            resize();
        afterNodeInsertion(evict);
        return null;
    }

Benjamin
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