【Java8源码分析】集合框架-ArrayList

一、总结

（1）ArrayList随机元素时间复杂度O(1)，插入删除操作需大量移动元素，效率较低

（2）为了节约内存，当新建容器为空时，会共享

Object[] EMPTY_ELEMENTDATA = {}

和

Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}

空数组

（3）容器底层采用数组存储，每次扩容为1.5倍

（4）ArrayList的实现中大量地调用了

Arrays.copyof()

和

System.arraycopy()

方法，其实

Arrays.copyof()

内部也是调用

System.arraycopy()

。

System.arraycopy()

为Native方法

（5）两个ToArray方法

Object[] toArray()

方法。该方法有可能会抛出java.lang.ClassCastException异常

<T> T[] toArray(T[] a)

方法。该方法可以直接将ArrayList转换得到的Array进行整体向下转型

（6）ArrayList可以存储null值

（7）ArrayList每次修改（增加、删除）容器时，都是修改自身的modCount；在生成迭代器时，迭代器会保存该modCount值，迭代器每次获取元素时，会比较自身的modCount与ArrayList的modCount是否相等，来判断容器是否已经被修改，如果被修改了则抛出异常（fast-fail机制）。

二、源码和注解

package java.util;

public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;

// 默认初始大小
private static final int DEFAULT_CAPACITY = 10;

// 指定大小的空的ArrayList共享此对象
private static final Object[] EMPTY_ELEMENTDATA = {};

// 默认大小的空的ArrayList共享
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

// 存储容器，为数组
transient Object[] elementData;

// 存放容器大小
private int size;

// 传入初始大小
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);
}
}

// 无參构造
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

// 由Collection构造
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
this.elementData = EMPTY_ELEMENTDATA;
}
}

// 将当前容量设为实际元素个数
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}

// 确保容器容量，当容器非空时，不得小于默认大小
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
? 0
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}

private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}

// 注意此方法在每次添加元素时调用，确保容量足够，且更改modCount
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}

// 数组最大元素个数，因为部分虚拟机会为数组预留了头部，故这里减8保证不会内存溢出
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

// 扩容为minCapacity或者旧容器大小*1.5中较大的一个
private void grow(int minCapacity) {
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
elementData = Arrays.copyOf(elementData, newCapacity);
}

private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // 溢出
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;
}

// 遍历所有元素的方法，效率较低
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;
}

// 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);
}
}

// 该方法有可能会抛出java.lang.ClassCastException异常，
// 如果直接用向下转型的方法，将整个ArrayList集合转变为指定类型的Array数组，便会抛出该异常
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}

// 该方法可以直接将ArrayList转换得到的Array进行整体向下转型
@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];
}

// 随机访问
public E get(int index) {
rangeCheck(index);
return elementData(index);
}

// 随机存储
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}

// 末尾添加元素
public boolean add(E e) {
ensureCapacityInternal(size + 1);  // 改变modCount
elementData[size++] = e;
return true;
}

// 随机位置添加元素，需复制后续元素，效率较低
public void add(int index, E element) {
rangeCheckForAdd(index);

ensureCapacityInternal(size + 1);
// 复制插入位置的后续元素
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}

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);
// 设置不访问的元素为null，触发gc
elementData[--size] = null;

return oldValue;
}

// 删除第一个出现的元素
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;
}

// 快速删除，在确保下标正确的情况下，不检查下标范围
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;
}

// 清楚所有元素，设置为null，但存储的elementData容量不变
public void clear() {
modCount++;

for (int i = 0; i < size; i++)
elementData[i] = null;

size = 0;
}

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;
}

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;
}

protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
int newSize = size - (toIndex-fromIndex);
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}

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;
}

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

public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
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)
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;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}

// 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()
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();
}
}

// java.io.Serializable的读取函数：根据写入方式读出
// 先将ArrayList的“容量”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;

// 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();
}
}
}
}

三、参考

http://blog.csdn.net/ns_code/article/details/35568011