1. 成员变量和静态变量
private static final long serialVersionUID = 8683452581122892189L;
/**
* 默认驻足大小为10.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 空数组
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 默认空数组
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* 基础数据存储的数组，类型transient 的作用？？？？
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* 当前数组的大小
*/
private int size;
2. 构造方法
/**
* 传入初始化大小的构造
*/
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
/**
* 构造空数组，默认大小为10
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 传入其他集合的构造方法，其中使用了Arrays.copyOf方法进行集合的复制
*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// 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;
}
}
3. 增删改查方法
- 添加
/**
* 向list的末尾添加数据
*/
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!! 以及确认是否需要扩容
elementData[size++] = e;
return true;
}
/**
* 项指定位置添加数据
*/
public void add(int index, E element) {
rangeCheckForAdd(index); //查询是否下表越界
ensureCapacityInternal(size + 1); // Increments modCount!! 以及确认是否需要扩容
System.arraycopy(elementData, index, elementData, index + 1,
size - index); //将所有index之后的数据都向后挪一位
elementData[index] = element;
size++;
}
/**
* 查询下表是否越界
*/
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++; //添加操作次数
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
/**
* 扩容
*/
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1); //扩容成运来的1.5倍
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;
}
- 删除
/**
* 删除第n个数值
*/
public E remove(int index) {
rangeCheck(index); //检查下标
modCount++; //添加修改次数
E oldValue = elementData(index);
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved); //向前移动index之后的数据
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
/**
*删除某个数值
*/
public boolean remove(Object o) {
if (o == null) { //从这里可以看得出来Arraylist可以存储null
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index); //遍历，快速删除
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
/*
*这个私有的方法和remove方法不同有两点：1.没有检查下标 2.没有返回老的数值
*/
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
}
- 修改和获取
/**
* 获取第index个字段
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
/**
* 更新字段，返回老子段
*/
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
4. fast-fail机制
public E next() {
checkForComodification(); //检查是否有修改次数的增加
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification(); //检查是否有修改次数的增加
try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
5.序列化
基于数组实现，保存元素的数组使用 transient 修饰，该关键字声明数组默认不会被序列化。ArrayList 具有动态扩容特性，因此保存元素的数组不一定都会被使用，那么就没必要全部进行序列化。ArrayList 重写了 writeObject() 和 readObject() 来控制只序列化数组中有元素填充那部分内容。
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()
s.writeInt(size);
// Write out all elements in the proper order.
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
elementData = EMPTY_ELEMENTDATA;
// Read in size, and any hidden stuff
s.defaultReadObject();
// Read in capacity
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();
}
}
}
总结
- 基础结构为默认大小为10的数组，每次扩容都是原来的1.5倍(向下取整)
- 线程不安全
- 每次循环都会检查modCount是否改变，改变就直接停止循环
- 重写了readObject和writeObject方法，可以更好的序列化
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