public class ArrayList<E> extends AbstractList<E>implements List<E>, RandomAccess, Cloneable, java.io.SerializableRandomAccess: 实现该接口可以通过下标序号快速访问
Cloneable: 能够被克隆
Serializable：这一个则是支持序列化的

public void trimToSize() {modCount++;//也就是说数组中实际的个数小于数组的容量if (size < elementData.length) {elementData = (size == 0)? EMPTY_ELEMENTDATA: Arrays.copyOf(elementData, size);}}//将集合的容量增加为 minCapacitypublic void ensureCapacity(int minCapacity) {int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)// elementData 里面有数据? 0// larger than default for default empty table. It's already// supposed to be at default size.: DEFAULT_CAPACITY;  //值为10if (minCapacity > minExpand) {ensureExplicitCapacity(minCapacity);}}private void ensureCapacityInternal(int minCapacity) {if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);}ensureExplicitCapacity(minCapacity);}private void ensureExplicitCapacity(int minCapacity) {modCount++;// overflow-conscious codeif (minCapacity - elementData.length > 0)grow(minCapacity);}private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;// *增加容量以确保它至少可以容纳由minimum capacity参数指定的元素数量。* @param minCapacity the desired minimum capacity*/private void grow(int minCapacity) {// overflow-conscious codeint 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);}private static int hugeCapacity(int minCapacity) {if (minCapacity < 0) // overflowthrow new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :MAX_ARRAY_SIZE;}

//添加元素到末尾
public boolean add(E e) {ensureCapacityInternal(size + 1);  // Increments modCount!!elementData[size++] = e;return true;}//添加元素到指定的位置public void add(int index, E element) {//顾名思义，检查检查索引是否越界
        rangeCheckForAdd(index);ensureCapacityInternal(size + 1);  // Increments modCount!!//这个是重点System.arraycopy(elementData, index, elementData, index + 1,size - index);elementData[index] = element;size++;}    private void rangeCheckForAdd(int index) {if (index > size || index < 0)throw new IndexOutOfBoundsException(outOfBoundsMsg(index));}//将集合添加到数组里面public boolean addAll(Collection<? extends E> c) {//集合转数组Object[] a = c.toArray();int numNew = a.length;ensureCapacityInternal(size + numNew);  // Increments modCountSystem.arraycopy(a, 0, elementData, size, numNew);size += numNew;return numNew != 0;}//这个是指定要将集合插入那个位置public boolean addAll(int index, Collection<? extends E> c) {rangeCheckForAdd(index);Object[] a = c.toArray();int numNew = a.length;ensureCapacityInternal(size + numNew);  // Increments modCountint numMoved = size - index;if (numMoved > 0)System.arraycopy(elementData, index, elementData, index + numNew,numMoved);System.arraycopy(a, 0, elementData, index, numNew);size += numNew;return numNew != 0;}

//删除指定位置的元素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 workreturn oldValue;}public boolean remove(Object o) {//传过来的值为nullif (o == null) {for (int index = 0; index < size; index++)if (elementData[index] == null) {fastRemove(index);return true;}} else {for (int index = 0; index < size; index++)if (o.equals(elementData[index])) {fastRemove(index);return true;}}return false;}//这个是把集合清空，也就是将里面的值都值为nullpublic void clear() {modCount++;// clear to let GC do its workfor (int i = 0; i < size; i++)elementData[i] = null;size = 0;}//删除fromIndex到toIndex之间的全部元素protected void removeRange(int fromIndex, int toIndex) {modCount++;int numMoved = size - toIndex;System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved);// clear to let GC do its workint newSize = size - (toIndex-fromIndex);for (int i = newSize; i < size; i++) {elementData[i] = null;}size = newSize;}