public ArrayList(int initialCapacity)
{if (initialCapacity > 0) {this.elementData = new Object[initialCapacity];} else if (initialCapacity == 0) {//注意初始容量为0时给的对象,EMPTY_ELEMENTDATAthis.elementData = EMPTY_ELEMENTDATA;}else {throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);}}public ArrayList(){//默认构造函数this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;}public ArrayList(Collection<? extends E> c)
{//转入的集合转换为数组elementData = c.toArray();//改变自己的sizeif ((size = elementData.length) != 0) {//这里有个坑,可能不能正确的返回Object[] Class对象,不能的话复制// 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;}}//截断,在原数组上移动
public void trimToSize()
{modCount++;if (size < elementData.length) {elementData = (size == 0)? EMPTY_ELEMENTDATA: Arrays.copyOf(elementData, size);}}//比较关心的几个方法,存放,取出,查找
//查找,运行时间上界O(n),从这里也可用看出是允许放null
public int indexOf(Object o)
{if (o == null) {for (int i = 0; i < size; i++)if (elementData[i]==null)return i;} else {for (int i = 0; i < size; i++)if (o.equals(elementData[i]))return i;}return -1;}//反向查找public int lastIndexOf(Object o)
{if (o == null) {for (int i = size-1; i >= 0; i--)if (elementData[i]==null)return i;} else {for (int i = size-1; i >= 0; i--)if (o.equals(elementData[i]))return i;}return -1;}
//返回index位置的元素
E elementData(int index)
{return (E) elementData[index];}//获取对应位置元素,O(1)public E get(int index)
{//检查size,并没有判断小于0rangeCheck(index);return elementData(index);
}//将元素放在对应位置,index<size，list当成数组使用,注意,这样的代码会报错
//List<String> list = new ArrayList<>(100);
//      list.set(10, "123");
//根据构造函数可以知道,已经开辟了100长度的数组,但是就是不让你用public E set(int index, E element)
{rangeCheck(index);E oldValue = elementData(index);elementData[index] = element;return oldValue;
}private void grow(int minCapacity)
{// overflow-conscious codeint oldCapacity = elementData.length;//原来的1.5备长度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://注意是新开辟一个数组给elementDataelementData = Arrays.copyOf(elementData, newCapacity);}private void ensureExplicitCapacity(int minCapacity)
{modCount++;// overflow-conscious code//1-0,进入if,进行内存复制,开辟新的数组if (minCapacity - elementData.length > 0)grow(minCapacity);
}private void ensureCapacityInternal(int minCapacity)
{//构造函数给0,这里是falseif (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);}//ensureExplicitCapacity(1);ensureExplicitCapacity(minCapacity);}//增加一个元素public boolean add(E e) {//为什么构造函数给0会被打死,看下面这个函数,一来就要新数组,新的数组长度也只有1ensureCapacityInternal(size + 1);  // Increments modCount!!elementData[size++] = e;return true;}//元素放到index位置,0<=index<size
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++;
}
//移除public E remove(int index)
{rangeCheck(index);modCount++;E oldValue = elementData(index);int numMoved = size - index - 1;//内存复制if (numMoved > 0)System.arraycopy(elementData, index+1, elementData, index,numMoved);elementData[--size] = null; // clear to let GC do its workreturn oldValue;
}
//O(n)的移除,注意在fastRemove方法里面也会出现内存复制
public boolean remove(Object o)
{if (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;}//为什么elementData不用置null,方便重用
public void clear()
{modCount++;// clear to let GC do its workfor (int i = 0; i < size; i++)elementData[i] = null;size = 0;}//浅克隆
public Object clone()
{try {ArrayList<?> v = (ArrayList<?>) super.clone();v.elementData = Arrays.copyOf(elementData, size);v.modCount = 0;return v;} catch (CloneNotSupportedException e) {// this shouldn't happen, since we are Cloneablethrow new InternalError(e);}}//返回副本public Object[] toArray()
{return Arrays.copyOf(elementData, size);
}结尾附上一个内存分布的代码,有指针真好
int main(int argc, char* argv[])
{//int ba[4];int a[] = { 1, 2, 3, 4 };int c = 5;int d = 6;int dd = 7;cout << a << endl;cout << a[-1] << endl;cout << a-1 << endl;cout<<&dd<<endl;cout << &d << endl;cout << &c << endl;//memcpy(ba, a, sizeof(int) * 4);//cout << ba[0] << endl;//cout << ba[1] << endl;return 0;
}