ArrayList源码分析

ArrayList源码解析

疑问：1.ArrayList继承自AbstractList，为什么父类和子类都实现了List接口

1. 定义

ArrayList继承AbstractList（这是一个抽象类，对一些基础的list操作进行封装），实现List，RandomAccess，Cloneable，Serializable几个接口，RandomAccess表明其支持快速随机访问，Cloneable表明其支持浅拷贝，Serializable 序列化及反序列化功能。

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable

//这个是干嘛的？？
private static final long serialVersionUID = 8683452581122892189L;

//默认容量
private static final int DEFAULT_CAPACITY = 10;

private static final Object[] EMPTY_ELEMENTDATA = {};

//存放元素的数组，transient修饰
private transient Object[] elementData;

// 存储元素个数，对应的容量用elementData.length来得到
private int size;

注意到存储元素的elementData数组，用transient修饰。序列化时，elementData不会被序列化，序列化时元素具体数据不会被存储？看到后面，我们就会明白，ArrayList自定义了自己的序列化方式，我们来看一下

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();
}
}

/**
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
* deserialize it).
*/
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;
//反序列读取元素顺序应该不会改变，序列化顺序和反序列化读取顺序是相反的？
for (int i=0; i<size; i++) {
a[i] = s.readObject();
}
}
}

然我们来测试一下，

public class ArrayDemo {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<String>();
list.add("hello");
list.add("Hello");
try {
ObjectOutputStream outStream = new ObjectOutputStream(new FileOutputStream(
"d:/Seriatest.out"));
outStream.writeObject(list);
outStream.close();
System.out.println("out success");
} catch (Exception e) {
e.printStackTrace();
}

try {
ObjectInputStream inStream = new ObjectInputStream(new FileInputStream(
"d:/Seriatest.out"));
ArrayList inList = (ArrayList) inStream.readObject();
inStream.close();
System.out.println("in success");
System.out.println(inList.get(0));
System.out.println(inList.get(1));
} catch (Exception e) {
e.printStackTrace();
}
}
}

这样做的好处，序列化时，按照size,元素各个来序列化的，而不是容量，节省了开支。

下面是构造方法，定义了三个构造方法

/**
* 构造一个指定长度的list
*/
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}

/**
* 容量为0，直接将final常量EMPTY_ELEMENTDATA赋予elementData
*/
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}

/**
* 构造一个由Collection转换后的list,list顺序与Collection.iterator一致
* iterator.
*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// 返回若不是Object[]将调用Arrays.copyOf
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}

2.add

public boolean add(E e) {
ensureCapacityInternal(size + 1);  // Increments modCount!!
elementData[size++] = e;
return true;
}

*/
public void add(int index, E element) {
rangeCheckForAdd(index);

ensureCapacityInternal(size + 1);  // Increments modCount!!
//直接用 System.arraycopy方法，而不是逐个后移，这样增加了效率
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
/**
* 添加后的顺序与Collection.iterator顺序一致
*/
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)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);

System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}

/**
* 都是应有System.arraycopy
*/
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;
}

3.get,set

@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}

public E get(int index) {
rangeCheck(index);

return elementData(index);
}

public E set(int index, E element) {
rangeCheck(index);

E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}

4.remove

/**
* 移除以后，并没有减少数list容量，所以在remove多个成员后，可以利用前面定义
*的trimToSize()方法来，减少容量，这个方法时public的哦
*/
public E remove(int index) {
rangeCheck(index);

modCount++;
E oldValue = elementData(index);

int numMoved = size - index - 1;
if (numMoved > 0)
//直接用 System.arraycopy方法，而不是逐个后移，这样增加了效率
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work

return oldValue;
}
public boolean removeAll(Collection<?> c) {
return batchRemove(c, false);
}

private boolean batchRemove(Collection<?> c, boolean complement) {
//final修饰
final Object[] elementData = this.elementData;
int r = 0, w = 0;
boolean modified = false;
try {
//这个循环666
for (; r < size; r++)
if (c.contains(elementData[r]) == complement)
//remove时，complement=false,
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
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;
//修改modCount的值
modCount += size - w;
size = w;
//根据元素个数判断是否remove过元素
modified = true;
}
}
return modified;
}
/**
* 都是应用System.arraycopy
* protected修饰
*/
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;
}

5.sublist

这段代码略长，就不贴了，这个方法放回的是一个this原来list的副本，对副本中做任何改变，都会对原来的list做出相同的变化，副本与原来list区别，就是reference不同吧

6.容量的变化

ArrayList向外部提供了两个改变容量的方法，trimToSize()默认改变为最小容量值，ensureCapacity(int minCapacity)可以传入具体需要设置的容量值

/**
* 改变容量为数组元素个数，用来节约内存
*/
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}

/**
* public
*设置具体的容量值，但可以发现扩展容量不足1.5倍时，自动扩展为1.5倍
*/
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;
//不符合要求时不予理睬
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
/*
*无参构造器生产list，容量扩充时最小值为默认容量10）
*/
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}

ensureExplicitCapacity(minCapacity);
}

private void ensureExplicitCapacity(int minCapacity) {
//用于在迭代器中同步
modCount++;

// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}

/**
* 容量扩充为原来的1.5倍
*/
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
//为什么看到别人的都是1.5+1，版本不同吧
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
//制定容量大于1.5倍时，扩充至制定容量
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);
}

/**
* list容量过大，1.5*容量<MAX_ARRAY_SIZE时，下次扩充容量不再为1.5倍
*/
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}

7.其他

public void clear() {
//modeCount只改变一次，这个方法不是同步安全的哎
modCount++;

// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;

size = 0;//不改变容量？数组长度大是，造成很大浪费
}

注意点：容量扩充时，通常是1.5倍，但根据情况也可能出现意外，如制定容量大于1.5时,容量超过最大值时；ArrayList向外提供了两个改变容量的方法；subList方法返回的是一个副本；add,remove时，采用的是System.arraycopy来增加效率；序列化时，要利用ArrayList自定义的方法