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    ArrayList 实现了Iterable,RandomAccess,Collection,List,Cloneable接口,其中RandomAccess,Cloneable 和Serializable是标记接口,表明实现随机访问,可克隆以及序列化
    ArrayList里主要的几个变量

    1. //默认初始化大小
    2. private static final int DEFAULT_CAPACITY = 10;
    3. //用于空实例的共享数组实例
    4. private static final Object[] EMPTY_ELEMENTDATA = {};
    5. //用于默认大小的空实例的共享空数组实例
    6. private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    7. //储存ArrayList元素的缓冲区
    8. transient Object[] elementData;
    9. //ArrayList包含的元素个数
    10. private int size;
    11. //最大容量
    12. private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    14. protected transient int modCount = 0;
    16. //最常用构造函数,构造一个初始化容量为10的数组
    17. public ArrayList() {
    18.     this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    19. }
    21. //调用add方法来向list中添加元素
    22. public boolean add(E e) {
    23.     //判断当前容量是否放的下新增元素,容量够就不变,容量不够扩容
    24.     ensureCapacityInternal(size + 1);  // Increments modCount!!
    25.     //
    26.     elementData[size++] = e;
    27.     return true;
    28. }
    30. private void ensureCapacityInternal(int minCapacity) {
    31.     ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    32. }
    34. private static int calculateCapacity(Object[] elementData, int minCapacity) {
    35.     //首次添加元素时,elementData中没东西,if表达式为true,  minCapacity=1
    36.     if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
    37.         //此处直接返回DEFAULT_CAPACITY = 10
    38.         return Math.max(DEFAULT_CAPACITY, minCapacity);
    39.     }
    40.     return minCapacity;
    41. }
    43. private void ensureExplicitCapacity(int minCapacity) {
    44.     //modCount 继承自AbstractList 用于记录list结构修改的次数,即容量改变的次数,容量改变一次此处加一
    45.     modCount++;
    47.     //首次进入minCapacity为10,if 表达式为true
    48.     // overflow-conscious code
    49.     if (minCapacity - elementData.length > 0)
    50.         grow(minCapacity);
    51. }
    53. private void grow(int minCapacity) {
    54.     // overflow-conscious code
    55.     //首次oldCapacity = 0,minCapacity
    56.     int oldCapacity = elementData.length;
    57.     //oldCapacity >> 1,右位移运算符 num >> 1,相当于num除以2,所以此处首次仍然为0,newCapacity = minCapacity = 10;复制elementData中1-10个元素到elementData中
    58.     int newCapacity = oldCapacity + (oldCapacity >> 1);
    59.     if (newCapacity - minCapacity < 0)
    60.         newCapacity = minCapacity;
    61.     if (newCapacity - MAX_ARRAY_SIZE > 0)
    62.         newCapacity = hugeCapacity(minCapacity);
    63.     // minCapacity is usually close to size, so this is a win:
    64.     elementData = Arrays.copyOf(elementData, newCapacity);
    65. }