跟我一起阅读Java源代码之HashMap（三）

package sourcecoderead.collection.map;


public class EntryHashMap<K, V> {

    /** 初始容量 */
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /** 下次扩容的临界值 */
    int threshold;

    transient int size;

    final float loadFactor;

    transient Entry[] table;

    public EntryHashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        threshold = (int) (DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
        table = new Entry[DEFAULT_INITIAL_CAPACITY];
    }

    public V put(K key, V value) {
        // 计算出新的hash
        int hash = hash(key.hashCode());
        // 计算出数组小标i
        int i = indexFor(hash, table.length);
        // 遍历table[i]，如果table[i]没有与新加入的key相等的，则新加入
        // 一个value到table[i]中的entry，否则将新的value覆盖旧的value并返回旧的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;
                return oldValue;
            }
        }
        addEntry(hash, key, value, i);
        return null;
    }

    public V get(K key) {
        // 计算出新的hash
        int hash = hash(key.hashCode());
        // 计算出数组小标i
        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))) {
                return e.value;
            }
        }
        return null;
    }

    private void addEntry(int hash, K key, V value, int bucketIndex) {
        Entry<K, V> e = table[bucketIndex];
        // 将新的元素插入链表前端
        table[bucketIndex] = new Entry<>(hash, key, value, e);
        if (size++ >= threshold)
            resize(2 * table.length);
    }

    void resize(int newCapacity) {
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        Entry[] newTable = new Entry[newCapacity];
        transfer(newTable);
        table = newTable;
        threshold = (int) (newCapacity * loadFactor);
    }

    void transfer(Entry[] newTable) {
        Entry[] src = table;
        int newCapacity = newTable.length;
        for (int j = 0; j < src.length; j++) {
            Entry<K, V> e = src[j];
            if (e != null) {
                src[j] = null;
                do {
                    Entry<K, V> next = e.next;
                    int i = indexFor(e.hash, newCapacity);
                    e.next = newTable[i];
                    newTable[i] = e;
                    e = next;
                } while (e != null);
            }
        }
    }

    /**
     * 通过hash code 和table的length得到对应的数组下标
     * 
     * @param h
     * @param length
     * @return
     */
    static int indexFor(int h, int length) {
        return h & (length - 1);
    }

    /**
     * 通过一定算法计算出新的hash值
     * 
     * @param h
     * @return
     */
    static int hash(int h) {
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

    public static void main(String[] args) {
        EntryHashMap<String, String> hashMap = new EntryHashMap<String, String>();
        hashMap.put("key", "value");
        System.out.println(hashMap.get("key"));
    }
}