ArrayList：

* Resizable-array implementation of the <tt>List</tt> interface.  Implements
* all optional list operations, and permits all elements, including
* <tt>null</tt>.  In addition to implementing the <tt>List</tt> interface,
* this class provides methods to manipulate the size of the array that is
* used internally to store the list.  (This class is roughly大致的 equivalent 相等於to
* <tt>Vector</tt>, except that it is unsynchronized.)
 

原始碼第一句就給出 了ArrayList是一個實現list集合全部操作的可變陣列，可以有null，陣列大小是可以調整的，執行緒不安全。

每一個都有初始容量，當向集合中新增元素時候會進行擴容。（就不copy了）

它提供了三個構造器：

　　　　　　　　　　一個無參構造器ArrayList（）；

　　　　　　　　　　兩個帶參構造器ArrayList（int）和ArrayList（Collection< ? extends E>）

/**
 * Default initial capacity.預設初始化大小為10
 */
private static final int DEFAULT_CAPACITY = 10;
/**
 * Shared empty array instance used for empty instances.以空object陣列作為類內共享例項容器。
 */
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
 * 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 = {};
/**
 * 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).
 */
private int size;

/**
 * Constructs an empty list with the specified initial capacity.
 *帶參構造，手動設定容器初始大小，若例項化時能確定大小，最好別使用預設擴充套件容量，將提高執行效率
 */
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. 無參構造器 ，初始化容量為10
 */
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.帶參構造器,將引數集合用陣列工具類轉成陣列,並存入緩衝陣列,對緩衝陣列進行校驗.
   若轉換後的陣列並非object陣列（內巢狀無法直接引用情況下），則使用陣列工具類將該陣列使用深度拷貝
 */
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;
    }
}

/**
 * Trims the capacity of this <tt>ArrayList</tt> instance to be the
 * list's current size.  An application can use this operation to minimize
 * the storage of an <tt>ArrayList</tt> instance.去除多餘的陣列申請空間，在記憶體緊張時會用到
 */
public void trimToSize() {
    modCount++;
    if (size < elementData.length) {
        elementData = (size == 0)
          ? EMPTY_ELEMENTDATA
          : Arrays.copyOf(elementData, size);
    }
}