#####起因

​ 这是详解集合系列文章中的第一篇，至于为什么要写该系列的文章，想必大家都很清楚。java中提供的这些集合，不论是在工作还是日常刷算法题，都会大量的使用。因此作为一个java程序员我们必须熟悉它们的底层实现原理。

ArrayList底层源码实现
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/**
    * 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

   /**
    * The size of the ArrayList (the number of elements it contains).
    *
    * @serial
    */
   private int size;
/**
    * Constructs an empty list with the specified initial capacity.
    *
    * @param  initialCapacity  the initial capacity of the list
    * @throws IllegalArgumentException if the specified initial capacity
    *         is negative
    */
   public ArrayList(int initialCapacity) {
       if (initialCapacity > 0) {
           this.elementData = new Object[initialCapacity];
       } else if (initialCapacity == 0) {
           this.elementData = EMPTY_ELEMENTDATA;
       } else {
           throw new IllegalArgumentException("Illegal Capacity: "+
                                              initialCapacity);
       }
   }

   /**
    * Constructs an empty list with an initial capacity of ten.
    */
   public ArrayList() {
       this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
   }

   /**
    * Constructs a list containing the elements of the specified
    * collection, in the order they are returned by the collection's
    * iterator.
    *
    * @param c the collection whose elements are to be placed into this list
    * @throws NullPointerException if the specified collection is null
    */
   public ArrayList(Collection<? extends E> c) {
       elementData = c.toArray();
       if ((size = elementData.length) != 0) {
           // c.toArray might (incorrectly) not return Object[] (see 6260652)
           if (elementData.getClass() != Object[].class)
               elementData = Arrays.copyOf(elementData, size, Object[].class);
       } else {
           // replace with empty array.
           this.elementData = EMPTY_ELEMENTDATA;
       }
   }

通过上述源码可以知道的是ArrayList底层是一个Object数组，在初始化时可以指定ArrayList的大小，如果不指定大小的话，默认大小为10.不过这里有意思的是它并不是直接就创建一个长度为10的数组，看过源码你会发现，它分配的其实是一个空数组

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/**
 * 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 = {};