Java HashMap、LinkedHashMap

如果需要使用的Map中的key无序，选择HashMap；如果要求key有序，则选择TreeMap。

但是选择TreeMap就会有性能问题，因为TreeMap的get操作的时间复杂度是O(log(n))的，

相比于HashMap的O(1)还是差不少的，LinkedHashMap的出现就是为了平衡这些因素，使得

能够以O(1)时间复杂度增加查找元素，又能够保证key的有序性

此外，LinkedHashMap提供了两种key的顺序：

• 访问顺序（access order）。非常实用，可以使用这种顺序实现LRU（Least Recently Used）缓存
• 插入顺序（insertion orde）。同一key的多次插入，并不会影响其顺序

一。HashMap

1.HashMap构造函数

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;
    threshold = initialCapacity;
    init();   //注意这个模板函数，在LinkHashMap中有使用
}

2.默认参数

//容量必须为2的指数（默认为16），想想原因？
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; 

static final int MAXIMUM_CAPACITY = 1 << 30;

//默认的平衡因子为0.75，这是权衡了时间复杂度与空间复杂度之后的取值
//过高的因子可以增加存储空间利用率但是查找的时间就会增加。
static final float DEFAULT_LOAD_FACTOR = 0.75f;

3.重设索引

    void resize(int newCapacity) {
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
        transfer(newTable, initHashSeedAsNeeded(newCapacity));
        table = newTable;
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }

    /**
     * Transfers all entries from current table to newTable.
     */
    void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        for (Entry<K,V> e : table) {
            while(null != e) {
                Entry<K,V> next = e.next;
                if (rehash) {
                    e.hash = null == e.key ? 0 : hash(e.key);
                }
                int i = indexFor(e.hash, newCapacity);    //占用哪个槽位
                e.next = newTable[i];
                newTable[i] = e;
                e = next;
            }
        }
    }

4.

    static int indexFor(int h, int length) {
        // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
        return h & (length-1);
    }

二。LinkedHashMap

1.Entry

    private static class Entry<K,V> extends HashMap.Entry<K,V> {
        // These fields comprise the doubly linked list used for iteration.
        Entry<K,V> before, after;

        Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
            super(hash, key, value, next);
        }
}

2.删除

   //删除一个节点时，需要把
    //1. 前继节点的后继指针 指向 要删除节点的后继节点
    //2. 后继节点的前继指针 指向 要删除节点的前继节点
    private void remove() {
        before.after = after;
        after.before = before;
    }

3.增加

        private void addBefore(Entry<K,V> existingEntry) {
            after  = existingEntry;            //当前节点的后继节点 指向 新节点
            before = existingEntry.before;     //
            before.after = this;               //
            after.before = this;               //
        }

4.重写的init

    @Override
    void init() {
        header = new Entry<>(-1, null, null, null);
        header.before = header.after = header;
    }

5.重写transfer

    /**
     * Transfers all entries to new table array.  This method is called
     * by superclass resize.  It is overridden for performance, as it is
     * faster to iterate using our linked list.
     */
    @Override
    void transfer(HashMap.Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        for (Entry<K,V> e = header.after; e != header; e = e.after) {    //把链表里的元素重排序
            if (rehash)
                e.hash = (e.key == null) ? 0 : hash(e.key);
            int index = indexFor(e.hash, newCapacity);
            e.next = newTable[index];
            newTable[index] = e;
        }
    }