/**
     * Default initial capacity.
     * 默认容量
     */
    private static final  int DEFAULT_CAPACITY = 10 ;
    
    /**
     * Shared empty array instance used for empty instances.
     * 用于空实例的共享空数组实例。在初始化时，你传进去的初始化大小为0，那么就用这个来做的处理。
     */
    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.
     * 共享的空数组实例，用于默认大小的空实例，默认构造器使用的。
     */ 
    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.
     * 用来存储元素的数组
     */
    transient Object[] elementData; // non-private to simplify nested class access
    
    /**
     * The size of the ArrayList (the number of elements it contains).
     * 数组的大小
     * @serial
     */
    private  int size;
    

 “第一个”
     /**
     * Constructs an empty list with an initial capacity of ten.
     * 默认无参的构造函数，会给用来存储元素的数组赋值为一个空数组实例
     */
     //默认无参的构造函数， new ArrayList() 使用默认的构造函数来创建它，
     //那么它会直接使用一个空的 Object []，初始大小是0，
     //它有一个默认的初始化大小参数 DEFAULT_CAPACITY 是 10，
     //当添加第一个元素时，这个空数据的大小才是10，我们也可以认为它给我们创建的默认的数据大小就是10 
    public  ArrayList() { 
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    
 “第二个”
    /**
     * 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
     */
     //这个构造函数可以设置数组的容量，
     //一般来说是可以预估出来数据的大小，比如几十个，一百个，
     //那么我们在初始化的时候就把这个数字带进去，避免数据容量太小在后续插入数据的时候进行频繁的扩容，创建      //新的数组。
    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);
        }
    }
   
   
“第三个”
    /**
     * 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
     */
     // 构造一个包含指定元素的列表
    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;
        }
    }

    /**
     * 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);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

/**
 * ArrayList
 */
public class ArrayListDemo {
    public static void main(String[] args) {
        List<String> stringList = new ArrayList<String>();

        stringList.add("张三");
        stringList.add("李四");
        stringList.add("王五");
    }
}

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

stringList.add(1,"赵六");

 System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);

System.arraycopy(elementData, 1, elementData, 1 + 1,
                         3 - 1);
                         
=>

System.arraycopy(elementData, 1, elementData, 2,
                         2);

    /**
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

String strTemp =  stringList.set(2,"马七")

    /**
     * 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);
    }

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

String strTemp1 = stringList.get(0);

    /**
     * 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;
    }

 String strRemoveTemp = stringList.remove(1);

拷贝前：elementData["张三","赵六","马七,"王五"]
拷贝后：elementData["张三","马七,"王五","王五"]

ensureCapacityInternal(size + 1);  // Increments modCount!!

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

private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

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

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