学习源码，应该是一件认真与钻研的功课，点滴积累。
package java.util;
public class ArrayList extends AbstractList
implements List, RandomAccess, Cloneable, java.io.Serializable
{
//
private static final long serialVersionUID = 8683452581122892189L;
//
private static final int DEFAULT_CAPACITY = 10;
//
private static final Object[] EMPTY_ELEMENTDATA = {};
//
private transient Object[] elementData;
//
private int size;
/**************** Constructor ***********************/
//
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}
//
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
//
public ArrayList(Collection c) {
elementData = c.toArray();
size = elementData.length;
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
/******************* Array size *************/
//
public void trimToSize() {
modCount++;
//
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}
//
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != EMPTY_ELEMENTDATA)
// any size if real element table
? 0
// larger than default for empty table. It's already supposed to be
// at default size.
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
private void ensureCapacityInternal(int minCapacity) {
//
if (elementData == EMPTY_ELEMENTDATA) {
//
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
//
ensureExplicitCapacity(minCapacity);
}
//
private void ensureExplicitCapacity(int minCapacity) {
modCount++; //
//
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
//
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//
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);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
//
public int size() {
return size;
}
//
public boolean isEmpty() {
return size == 0;
}
/****************************** Search Operations *************************/
//
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
//
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;
}
//
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;
}
/******************************* Clone *********************************/
//克隆函数
public Object clone() {
try {
@SuppressWarnings("unchecked")
ArrayList v = (ArrayList) super.clone();
//将当前ArrayList的全部元素拷贝到v中
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();
}
}
/********************************* toArray *****************************/
/**
* 返回一个Object数组，包含ArrayList中所有的元素
* toArray()方法扮演着array-based和collection-based API之间的桥梁
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
//返回ArrayList的模板数组
@SuppressWarnings("unchecked")
public T[] toArray(T[] a) {
//如果数组a的大小 < ArrayList的元素个数，
//则新建一个T[]数组，大小为ArrayList元素个数，并将“ArrayList”全部拷贝到新数组中。
if (a.length < size)
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
//如果数组a的大小 >= ArrayList的元素个数，
//则将ArrayList全部拷贝到新数组a中。
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
/******************** Positional Access Operations ********************/
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
//获取index位置的元素值
public E get(int index) {
rangeCheck(index); //首先判断index的范围是否合法
return elementData(index);
}
//将index位置的值设为element，并返回原来的值
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
//将e添加到ArrayList中
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
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); // Increments modCount!!
//将index以及index之后的数据复制到index+1的位置往后，即从index开始向后挪了一位
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element; //然后在index处插入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)
//向左挪一位，index位置原来的数据已经被覆盖了
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//多出来的最后一位删掉
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
//删除ArrayList中指定的元素
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;
}
//private的快速删除与上面的public普通删除区别在于，没有进行边界判断以及不返回删除值
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; // clear to let GC do its work
}
//清空ArrayList，将全部元素置为null
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
//将集合C中所有的元素添加到ArrayList中
public boolean addAll(Collection c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
//在原来数组的后面添加c中所有的元素
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
//从index位置开始，将集合C中所欲的元素添加到ArrayList中
public boolean addAll(int index, Collection c) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)
//将index开始向后的所有数据，向后移动numNew个位置，给新插入的数据腾出空间
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
//将集合C中的数据插到刚刚腾出的位置
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
//删除从fromIndex到toIndex之间的数据，不包括toIndex位置的数据
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 work
int newSize = size - (toIndex-fromIndex);
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}
//范围检测
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//add和addAll方法中的范围检测
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中所有集合C中包含的数据
public boolean removeAll(Collection c) {
return batchRemove(c, false);
}
//删除ArrayList中除了集合C中包含的数据外的其他所有数据
public boolean retainAll(Collection c) {
return batchRemove(c, true);
}
//批量删除
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)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
//官方的注释是为了保持和AbstractCollection的兼容性
//我的理解是上面c.contains抛出了异常，导致for循环终止，那么必然会导致r != size
//所以0-w之间是需要保留的数据，同时从w索引开始将剩下没有循环的数据(也就是从r开始的)拷贝回来，也保留
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
//for循环完毕，检测了所有的元素
//0-w之间保存了需要留下的数据，w开始以及后面的数据全部清空
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;
}
/***************************** Writer and Read Object *************************/
//java.io.Serializable的写入函数
//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{
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();
// Write out size as capacity for behavioural compatibility with clone()
//写入“数组的容量”，保持与clone()的兼容性
s.writeInt(size);
//写入“数组的每一个元素”
for (int i=0; i 0) {
// be like clone(), allocate array based upon size not capacity
ensureCapacityInternal(size);
Object[] a = elementData;
//从输入流中将“所有元素值”读出
for (int i=0; i 关于ArrayList有以下几点总结：####
动态扩充容量
ArrayList在每次增加元素（可能是1个，也可能是一组）时，都要调用该方法来确保足够的容量。当容量不足以容纳当前的元素个数时，就设置新的容量为旧的容量的1.5倍加1，如果设置后的新容量还不够，则直接新容量设置为传入的参数（也就是所需的容量），而后用Arrays.copyof()方法将元素拷贝到新的数组。从中可以看出，当容量不够时，每次增加元素，都要将原来的元素拷贝到一个新的数组中，非常之耗时，也因此建议在事先能确定元素数量的情况下，才使用ArrayList，否则建议使用LinkedList。Arrays.copyOf()与System.copyOf()方法
// Arrays.copyOf源码
public static T[] copyOf(T[] original, int newLength) {
return (T[]) copyOf(original, newLength, original.getClass());
}
public static T[] copyOf(U[] original, int newLength, Class newType) {
@SuppressWarnings("unchecked")
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
// 返回数组组件的类型
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
// 使用系统native函数，将数组复制到新数组中
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
- ArrayList遍历访问的效率
// 基于迭代器遍历
Integer value = null;
Iterator it = list.iterator();
while (it.hasNext()) {
value = (Integer)it.next();
}
// 基于随机访问遍历
Integer value = null;
int size = list.size();
for (int i = 0; i < size; i++) {
value = (Integer)list.get(i);
}
// 基于增强for循环遍历
Integer value = null;
for (Integer integ : list) {
value = integ;
}
三种方式遍历，经过测试，基于随机访问（索引号的方式），遍历速度最快；ArrayList查找速度快，而插入和删除速度较慢；
- ArrayList支持null类型
在查找对象的索引号，移除指定的对象时，均进行null类型进行检查，可见ArrayList支持null类型；