Java 集合系列（二）—— ArrayList

ArrayList

　　ArrayList 是通过一个数组来实现的，因此它是在连续的存储位置存放对象的引用，只不过它比 Array 更智能，能够根据集合长度进行自动扩容。

　　假设让我们来实现一个简单的能够自动扩容的数组，我们最容易想到的点就是：

1. add()的时候需要判断当前数组size+1是否等于此时定义的数组大小；
2. 若小于直接添加即可；否则，需要先扩容再进行添加。

实际上，ArrayList的内部实现原理也是这样子，我们可以来研究分析一下ArrayList的源码

add(E e) 源码分析

   /**
      * Appends the specified element to the end of this list.
      *
      * @param e element to be appended to this list
      * @return <tt>true</tt> (as specified by {@link Collection#add})
      */
     public boolean add(E e) {
         ensureCapacityInternal(size + 1);   // 进行扩容校验
         elementData[size++] = e;            // 将值添加到数组后面，并将 size+1
         return true;
     }

     /**
      * The array buffer into which the elements of the ArrayList are stored.
      * The capacity of the ArrayList is the length of this array buffer. Any
      * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
      * will be expanded to DEFAULT_CAPACITY when the first element is added.
      */
     transient Object[] elementData; // non-private to simplify nested class access

     private void ensureCapacityInternal(int minCapacity) {
         ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));    // elementData 数组
     }

     /**
      * Default initial capacity.
      */
     private static final int DEFAULT_CAPACITY = 10;

     /**
      * Shared empty array instance used for default sized empty instances. We
      * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
      * first element is added.
      */
     private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

     // 返回最大的 index
     private static int calculateCapacity(Object[] elementData, int minCapacity) {
         if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {   //  与空数组实例对比
             return Math.max(DEFAULT_CAPACITY, minCapacity);
         }
         return minCapacity;
     }

     private void ensureExplicitCapacity(int minCapacity) {
         modCount++;

         // overflow-conscious code
         if (minCapacity - elementData.length > 0)
             grow(minCapacity);
     }

扩容调用方法，实际也就是数组复制的过程

     /**
      * Increases the capacity to ensure that it can hold at least the
      * number of elements specified by the minimum capacity argument.
      *
      * @param minCapacity the desired minimum capacity
      */
     private void grow(int minCapacity) {
         // overflow-conscious code
         int oldCapacity = elementData.length;
         int newCapacity = oldCapacity + (oldCapacity >> 1);
         if (newCapacity - minCapacity < 0)
             newCapacity = minCapacity;
         if (newCapacity - MAX_ARRAY_SIZE > 0)
             newCapacity = hugeCapacity(minCapacity);
         // minCapacity is usually close to size, so this is a win:
         elementData = Arrays.copyOf(elementData, newCapacity);
     }

add(int index, E element) 源码分析

   /**
      * Inserts the specified element at the specified position in this
      * list. Shifts the element currently at that position (if any) and
      * any subsequent elements to the right (adds one to their indices).
      *
      * @param index index at which the specified element is to be inserted
      * @param element element to be inserted
      * @throws IndexOutOfBoundsException {@inheritDoc}
      */
     public void add(int index, E element) {
         rangeCheckForAdd(index);    // 校验index是否超过当前定义的数组大小范围，超过则抛出 IndexOutOfBoundsException

         ensureCapacityInternal(size + 1);  // Increments modCount!!
         System.arraycopy(elementData, index, elementData, index + 1,
                          size - index);     // 复制，向后移动
         elementData[index] = element;
         size++;
     }

     /**
      * A version of rangeCheck used by add and addAll.
      */
     private void rangeCheckForAdd(int index) {
         if (index > size || index < 0)
             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
     }

从上面的源码分析可知，扩容和随机插入元素的消耗比较大，因此在实际开发中，应尽量指定ArrayList大小，减少在随机插入操作。

优缺点

优点

• 封装了一个动态再分配的对象数组
• 使用索引进行随机访问效率高

缺陷

• 在数组中增删一个元素，所有元素都要往后往前移动，效率低下

知识脑图

在 github 上建了一个 repository ，Java Core Knowledge Tree，各位看官若是喜欢请给个star，以示鼓励，谢谢。
https://github.com/suifeng412/JCKTree

（以上是自己的一些见解，若有不足或者错误的地方请各位指出）

作者：那一叶随风   http://www.cnblogs.com/phpstudy2015-6/

原文地址： https://www.cnblogs.com/phpstudy2015-6/p/10618707.html

声明：本博客文章为原创，只代表本人在工作学习中某一时间内总结的观点或结论。转载时请在文章页面明显位置给出原文链接