public class ArrayList<E> extends AbstractList<E>implements List<E>, RandomAccess, Cloneable, java.io.Serializable

private static final long serialVersionUID = 8683452581122892189L;

    /*** Default initial capacity.*/private static final int DEFAULT_CAPACITY = 10;/*** Shared empty array instance used for empty instances.*/private static final Object[] EMPTY_ELEMENTDATA = {};private transient Object[] elementData;

　　/*** 初始化一个给定大小容量的数组List，主要用于如果本事知道这个list的大小时。* 能优化如果大小超过默认容量时导致arralist每次都要去增加并创建一个新的list* @param  initialCapacity  the initial capacity of the list* @throws IllegalArgumentException if the specified initial capacity*         is negative*/public ArrayList(int initialCapacity) {super();if (initialCapacity < 0)throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);this.elementData = new Object[initialCapacity];}/*** 使用默认的大小10*/public ArrayList() {super();this.elementData = EMPTY_ELEMENTDATA;}/*** 根据一个collection来定义一个arralist** @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();size = elementData.length;// c.toArray might (incorrectly) not return Object[] (see 6260652)if (elementData.getClass() != Object[].class)elementData = Arrays.copyOf(elementData, size, Object[].class);}

    public void trimToSize() {modCount++;if (size < elementData.length) {elementData = Arrays.copyOf(elementData, size);}}

　　public boolean add(E e) {ensureCapacityInternal(size + 1);  // 判断容量是否超标elementData[size++] = e;return true;}private void ensureCapacityInternal(int minCapacity) {if (elementData == EMPTY_ELEMENTDATA) {minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);// 容量超标情况
        }ensureExplicitCapacity(minCapacity);}private void ensureExplicitCapacity(int minCapacity) {modCount++;// overflow-conscious codeif (minCapacity - elementData.length > 0)// 如果超标，则处理
            grow(minCapacity);}private void grow(int minCapacity) {// overflow-conscious codeint oldCapacity = elementData.length; // 容量大小int newCapacity = oldCapacity + (oldCapacity >> 1); //超标增加量计算法，x+x/2=50%左右if (newCapacity - minCapacity < 0)newCapacity = minCapacity;if (newCapacity - MAX_ARRAY_SIZE > 0)newCapacity = hugeCapacity(minCapacity);// 赋值原来的数组，并增加newCapacity个容量elementData = Arrays.copyOf(elementData, newCapacity);} private static int hugeCapacity(int minCapacity) {if (minCapacity < 0) // overflowthrow new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :MAX_ARRAY_SIZE;}