FastList和Arraylist的区别

最新推荐文章于 2024-08-29 08:27:10 发布

原创最新推荐文章于 2024-08-29 08:27:10 发布 · 4.8k 阅读

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#FastList #ArrayList #java

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1.引言

FastList是一个List接口的精简实现，只实现了接口中必要的几个方法。
JDK ArrayList每次调用get()方法时都会进行rangeCheck检查索引是否越界，FastList的实现中去除了这一检查，只要保证索引合法那么rangeCheck就成为了不必要的计算开销(当然开销极小)。此外，HikariCP使用List来保存打开的Statement，当Statement关闭或Connection关闭时需要将对应的Statement从List中移除。通常情况下，同一个Connection创建了多个Statement时，后打开的Statement会先关闭。ArrayList的remove(Object)方法是从头开始遍历数组，而FastList是从数组的尾部开始遍历，因此更为高效。

简而言之就是自定义数组类型（FastList）代替ArrayList：避免每次get()调用都要进行range check，避免调用remove()时的从头到尾的扫描

2.源码展示

ArrayList 的get() 方法
每次都要执行rangeCheck() 方法

  /**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }


  /**
     * Checks if the given index is in range.  If not, throws an appropriate
     * runtime exception.  This method does *not* check if the index is
     * negative: It is always used immediately prior to an array access,
     * which throws an ArrayIndexOutOfBoundsException if index is negative.
     */
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

然后是FastList的get()方法
是不是少了一个步骤

  /**
    * Get the element at the specified index.
    *
    * @param index the index of the element to get
    * @return the element, or ArrayIndexOutOfBounds is thrown if the index is invalid
    */
   @Override
   public T get(int index)
   {
      return elementData[index];
   }

还有就是它们的删除操作
ArrayList:

 /**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

FastList:

/**
    * This remove method is most efficient when the element being removed
    * is the last element.  Equality is identity based, not equals() based.
    * Only the first matching element is removed.
    *
    * @param element the element to remove
    */
   @Override
   public boolean remove(Object element)
   {
      for (int index = size - 1; index >= 0; index--) {
         if (element == elementData[index]) {
            final int numMoved = size - index - 1;
            if (numMoved > 0) {
               System.arraycopy(elementData, index + 1, elementData, index, numMoved);
            }
            elementData[--size] = null;
            return true;
         }
      }

      return false;
   }

3. 代码演示

FastList 位于com.zaxxer.hikari.util.FastList;包下

创建方式跟ArrayList 差不多
user 是我创建的实体类型
同时也支持lambda表达式

List<User> list=new FastList<>(User.class);
list= list.stream().sorted(Comparator.comparing(User::getId).reversed()).collect(Collectors.toList());

4.我的参考

http://cmsblogs.com/?p=2518
十分感谢这位大哥，本来我是在查看 spring boot2.0 自带数据源时，无意间看到了，觉得不错分享给大家。
此list不支持lambda表达式
本人还是java小白，如果有什么不对的请在下方留言。