我是看一篇博客，并且在它的基础上面增加内容的。
地址为https://www.cnblogs.com/xujian2014/p/4625346.html
看源码之前要先了解一些知识
@SuppressWarnings("unchecked") 告诉编译器忽略 unchecked 警告信息
System.arrayCopy的源代码声明 :
public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)
代码解释:
Object src : 原数组
int srcPos : 从元数据的起始位置开始
Object dest : 目标数组
int destPos : 目标数组的开始起始位置
int length : 要copy的数组的长度public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
//用于序列化
private static final long serialVersionUID = 8683452581122892189L;
//默认的初始容量为10
private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
//不能被序列化 那怎么把数组传递呢？writeObject和readObject 来实现
transient Object[] elementData;
// ArrayList中实际数据的数量
private int size;
public ArrayList(int initialCapacity) //带初始容量大小的构造函数
{
if (initialCapacity > 0) //初始容量大于0,实例化数组
{
this.elementData = new Object[initialCapacity];
}
else if (initialCapacity == 0) //初始化等于0，将空数组赋给elementData
{
this.elementData = EMPTY_ELEMENTDATA;
}
else //初始容量小于，抛异常
{
throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
}
}
public ArrayList() //无参构造函数,默认容量为10
{
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
public ArrayList(Collection<? extends E> c) //创建一个包含collection的ArrayList
{
elementData = c.toArray(); //返回包含c所有元素的数组
if ((size = elementData.length) != 0)
{
//如果类型不相同 那就有可能是父子类的关系
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);//复制指定数组，使elementData具有指定长度
}
else
{
//c中没有元素
this.elementData = EMPTY_ELEMENTDATA;
}
}
//将当前容量值设为当前实际元素大小
public void trimToSize()
{
modCount++;
if (size < elementData.length)
{
//如果数组为0那就给他 一个空的数组 否则复制数组，让当前容量值设为当前实际元素大小
elementData = (size == 0)? EMPTY_ELEMENTDATA:Arrays.copyOf(elementData, size);
}
}
//将集合的capacit增加minCapacity
public void ensureCapacity(int minCapacity)
{
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)?0:DEFAULT_CAPACITY;
//如果想使用的最小容量小于最小扩容量，就使用最小容量
if (minCapacity > minExpand)
{
ensureExplicitCapacity(minCapacity);
}
}
//下面都是扩容的
private void ensureCapacityInternal(int minCapacity)
{
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
{
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
/**
* 分派给arrays的最大容量
* 为什么要减去8呢？
* 因为某些VM会在数组中保留一些头字，尝试分配这个最大存储容量，可能会导致array容量大于VM的limit，最终导致OutOfMemoryError。
*/
private void ensureExplicitCapacity(int minCapacity)
{
modCount++;
if (minCapacity - elementData.length > 0)
//扩充
grow(minCapacity);
}
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
private void grow(int minCapacity)
{
int oldCapacity = elementData.length;
//注意此处扩充capacity的方式是将其向右一位再加上原来的数，实际上是扩充了1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
//如果扩容后还不够，那就用minCapacity
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//如果扩容后大于临界值 那就进行大容量分配
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity)
{
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
//返回ArrayList的大小
public int size()
{
return size;
}
//判断ArrayList是否为空
public boolean isEmpty() {
return size == 0;
}
//判断ArrayList中是否包含Object(o)
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
//正向查找，返回ArrayList中元素Object(o)的索引位置
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;
}
//逆向查找，返回返回ArrayList中元素Object(o)的索引位置
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;
}
//返回此 ArrayList实例的浅拷贝。
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 Cloneable
throw new InternalError(e);
}
}
//返回一个包含ArrayList中所有元素的数组
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
//返回至指定索引的值
public E get(int index)
{
rangeCheck(index); //检查给定的索引值是否越界
return elementData(index);
}
//将指定索引上的值替换为新值，并返回旧值
public E set(int index, E element)
{
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
//将指定的元素添加到此列表的尾部
public boolean add(E e)
{
ensureCapacityInternal(size + 1);
elementData[size++] = e;
return true;
}
// 将element添加到ArrayList的指定位置
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1);
//从指定源数组中复制一个数组，复制从指定的位置开始，到目标数组的指定位置结束。
//arraycopy(被复制的数组, 从第几个元素开始复制, 要复制到的数组, 从第几个元素开始粘贴, 一共需要复制的元素个数)
//即在数组elementData从index位置开始，复制到index+1位置,共复制size-index个元素
System.arraycopy(elementData, index, elementData, index + 1,size - index);
elementData[index] = element;
size++;
}
//删除ArrayList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; //将原数组最后一个位置置为null
return oldValue;
}
//移除ArrayList中首次出现的指定元素(如果存在)。
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;
}
//快速删除指定位置的元素
private void fastRemove(int index)
{
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index, numMoved);
elementData[--size] = null;
}
//清空ArrayList，将全部的元素设为null
public void clear()
{
modCount++;
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
//按照c的迭代器所返回的元素顺序，将c中的所有元素添加到此列表的尾部
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
//从指定位置index开始，将指定c中的所有元素插入到此列表中
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)
//先将ArrayList中从index开始的numMoved个元素移动到起始位置为index+numNew的后面去
System.arraycopy(elementData, index, elementData, index + numNew, numMoved);
//再将c中的numNew个元素复制到起始位置为index的存储空间中去
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
//删除fromIndex到toIndex之间的全部元素
protected void removeRange(int fromIndex, int toIndex)
{
modCount++;
//numMoved为删除索引后面的元素个数
int numMoved = size - toIndex;
//将删除索引后面的元素复制到以fromIndex为起始位置的存储空间中去
System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved);
int newSize = size - (toIndex-fromIndex);
//将ArrayList后面(toIndex-fromIndex)个元素置为null
for (int i = newSize; i < size; i++)
{
elementData[i] = null;
}
size = newSize;
}
//检查索引是否越界
private void rangeCheck(int index)
{
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private void rangeCheckForAdd(int index)
{
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
//删除ArrayList中包含在c中的元素
public boolean removeAll(Collection<?> c)
{
Objects.requireNonNull(c);
return batchRemove(c, false);
}
//删除ArrayList中除包含在c中的元素，和removeAll相反
public boolean retainAll(Collection<?> c)
{
Objects.requireNonNull(c); //检查指定对象是否为空
return batchRemove(c, true);
}
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try
{
for (; r < size; r++)
if (c.contains(elementData[r]) == complement) //判断c中是否有elementData[r]元素
elementData[w++] = elementData[r];
}
finally
{
if (r != size)
{
System.arraycopy(elementData, r, elementData, w, size - r);
w += size - r;
}
if (w != size)
{
// clear to let GC do its work
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}
//将ArrayList的“容量，所有的元素值”都写入到输出流中
private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException
{
int expectedModCount = modCount;
s.defaultWriteObject();
//写入数组大小
s.writeInt(size);
//写入所有数组的元素
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
//先将ArrayList的“大小”读出，然后将“所有的元素值”读出
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
elementData = EMPTY_ELEMENTDATA;
s.defaultReadObject();
s.readInt(); // ignored
if (size > 0) {
// be like clone(), allocate array based upon size not capacity
ensureCapacityInternal(size);
Object[] a = elementData;
// Read in all elements in the proper order.
for (int i=0; i<size; i++) {
a[i] = s.readObject();
}
}
}