HashMap源码讲解一 hash算法

1 什么是hash算法

  来自百度百科

  Hash算法可以将一个数据转换为一个标志，这个标志和源数据的每一个字节都有十分紧密的关系。Hash算法还具有一个特点，就是很难找到逆向规律

   个人批语

   通过hash算法可以把一个数据转换为一个值，这个值暂用很小的空间，但是这个转换是不可逆的

2 Java中hash算法的应用
      

        java 的 根对象 Object 存在 hashCode方法，是native修饰

    /**
     * Returns a hash code value for the object. This method is
     * supported for the benefit of hash tables such as those provided by
     * {@link java.util.HashMap}.
     * <p>
     * The general contract of {@code hashCode} is:
     * <ul>
     * <li>Whenever it is invoked on the same object more than once during
     *     an execution of a Java application, the {@code hashCode} method
     *     must consistently return the same integer, provided no information
     *     used in {@code equals} comparisons on the object is modified.
     *     This integer need not remain consistent from one execution of an
     *     application to another execution of the same application.
     * <li>If two objects are equal according to the {@code equals(Object)}
     *     method, then calling the {@code hashCode} method on each of
     *     the two objects must produce the same integer result.
     * <li>It is <em>not</em> required that if two objects are unequal
     *     according to the {@link java.lang.Object#equals(java.lang.Object)}
     *     method, then calling the {@code hashCode} method on each of the
     *     two objects must produce distinct integer results.  However, the
     *     programmer should be aware that producing distinct integer results
     *     for unequal objects may improve the performance of hash tables.
     * </ul>
     * <p>
     * As much as is reasonably practical, the hashCode method defined by
     * class {@code Object} does return distinct integers for distinct
     * objects. (This is typically implemented by converting the internal
     * address of the object into an integer, but this implementation
     * technique is not required by the
     * Java&trade; programming language.)
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.lang.System#identityHashCode
     */
    public native int hashCode();

        个人批语

             自己新建的对象需要重新这个方法，不然得到的hascode值相同

             得到的hash值都是int类型

  3 hash算法在 HashMap中的应用

     HashMap 是Key Value的存储形式，从大方向的讲解 Key的存储结构是可以独立来说的

   Key的数据结构

        数组

 /* ---------------- Fields -------------- */

    /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;

   问题反思

        数组的存储是根据索引进行的，那么如何确定一个key应该在数组何处索引位置

        数组是定长的,那么扩容的时候应该如何处理呢  

  Key如何确定索引位置   

            首先把Key取hash,得到一个int类型的hashcode

 /**
     * Computes key.hashCode() and spreads (XORs) higher bits of hash
     * to lower.  Because the table uses power-of-two masking, sets of
     * hashes that vary only in bits above the current mask will
     * always collide. (Among known examples are sets of Float keys
     * holding consecutive whole numbers in small tables.)  So we
     * apply a transform that spreads the impact of higher bits
     * downward. There is a tradeoff between speed, utility, and
     * quality of bit-spreading. Because many common sets of hashes
     * are already reasonably distributed (so don't benefit from
     * spreading), and because we use trees to handle large sets of
     * collisions in bins, we just XOR some shifted bits in the
     * cheapest possible way to reduce systematic lossage, as well as
     * to incorporate impact of the highest bits that would otherwise
     * never be used in index calculations because of table bounds.
     */
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

     计算key对应的下标值    

i = (n - 1) & hash

从源码中可以借鉴的思想

    hash算法的应用可以解决kafka消费有序性的问题

    &比直接使用取模效率高的场景