本解析源码来自JDK1.7
ArrayList 概要
- 继承Collection接口，实现了随机存取，自动扩容
- 内部使用数组进行存储，size表示List中元素的个数
private transient Object[] elementData;
private int size;- 允许null值
- 与Vector基本相同，但是ArrayList是非同步的，可以使用
List list = Collections.synchronizedList(new ArrayList(...))做同步 - ArrayList的线性操作比LinkedList的操作的常数项系数要小
- 有fast-fail机制
一些常量
- 没有指定ArrayList初始容量，首次添加元素时ArrayList会扩展到长度为10
- 没有指定ArrayList初始容量，且没有添加元素之前，内部数组为空
- ArrayList的长度扩展是不会超过Integer.MAX_VALUE-8
DEFAULT_CAPACITY = 10
Object[] EMPTY_ELEMENTDATA = {}
MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8 实现接口
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable - List 包含了ArrayList的主要操作方法的描述
- RandomAccess 标志接口，表明随机访问效率高
- Cloneable 标志接口，表明可以进行深拷贝
- Serializable 可序列化
构造函数
主要有三种构造方式
- 给定初始大小，创建initialCapacity的数组
- 不给定大小，使用空数组
- 给定另个一Collection c，将集合c的转换为数组，拷贝数组作为成员数组，原集合大小作为size
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<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} List 接口主要方法
public interface List<E> extends Collection<E> {
// Query Operations
int size();
boolean isEmpty();
boolean contains(Object o);
Iterator<E> iterator();
Object[] toArray();
<T> T[] toArray(T[] a);
// Modification Operations
boolean add(E e);
boolean remove(Object o);
// Bulk Modification Operations
boolean containsAll(Collection<?> c);
boolean addAll(Collection<? extends E> c);
boolean addAll(int index, Collection<? extends E> c);
boolean removeAll(Collection<?> c);
boolean retainAll(Collection<?> c);
void clear();
// Comparison and hashing
boolean equals(Object o);
int hashCode();
// Positional Access Operations
E get(int index);
E set(int index, E element);
void add(int index, E element);
E remove(int index);
// Search Operations
int indexOf(Object o);
int lastIndexOf(Object o);
// List Iterators
ListIterator<E> listIterator();
ListIterator<E> listIterator(int index);
// View
List<E> subList(int fromIndex, int toIndex);
}存入元素
主要方法
- set(int index, E element) 更新指定位置元素
- add(E e) 末尾添加元素
- add(int i,E e) 指定位置后添加元素
- addAll(Collection c) 末尾批量添加元素
- addAll(int i, Collection) 指定位置批量添加元素
主要操作
- 检查插入index的合法性 rangecheck（index）
- rangeCheck 只检查index是否**大于等于**size，不检查index是否小于零，因为rangeCheck后总是数组操作，所以会抛出ArrayIndexOutOfBoundsException，而不是IndexOutOfBoundsExcepiton
- rangeCheckForAdd 检查index是否**大于**size或小于0，抛出IndexOutOfBoundsExcepiton
- 异常信息为 Index：index，Size：size
- 确保数组有足够容量 ensureCapacityInternal（minCapacity）
- 将数据拷贝到数组中
利用Arrays.copyof（originalArray，length）来进行扩容，length指定新数组长度，如果新数组大于原数组长度，其余部分补null
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
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!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
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;
}
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;
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
} 容量扩展
- ensureCapacityInternal 如果数组为空，就扩展数组为max（默认大小，给定的minCapacity）大小，否则扩展到给定最小值大小
- 检查是否有足够容量，如果没有调用grow(minCapacity)进行扩容
- 默认扩展到1.5倍
- 如果扩展1.5倍后还是小于给定最小值，就要扩展到的大小为给定最小值
- 如果要扩展到的大小大于MAX_ARRAY_SIZE，就扩展到 Integer.MAX_VALUE
NOTE
- 共有的扩展数组方法 ensureCapacity（minCapacity）
ArrayList提供了共有的保证容量的方法，一般情况下我们是不用管扩容的事情，但是当我们预计要插入的元素个数比较多时，且数量可估计时，手动确保List容量可以避免频繁的扩容，造成的数据频繁的拷贝，从而提高效率
- 默认初始化会将数组初始化为空，存入元素时直接扩展到DefaultCapacity
如果List为空，只有minCapacity大于DefaultCapacity时才需要扩容，如果List不为空，大于零就需要扩容。
- 扩展的相反操作 trimToSize(int size)
如果设定的size小于数组的长度，用Arrays.copyof(array,length)复制创建一个给定大小的数组
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
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 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);
}
} 读取元素 get(index)
- 首先进行index合法性检查，注意不检查小于零的情况，而是由数组抛出ArrayIndexOutOfBoundsException
- 进行类型转换，返回
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
} 查找
- 采用逐个遍历的方法进行查找
- indexOf从前往后找，lastIndexOf从后往前找，
- contains（e） 调用 indexOf进行查找
NOTE
由于允许null值，所以null需要单独处理，而不能用elementData[i].equals(o)
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;
}
public boolean contains(Object o) {
return indexOf(o) >= 0;
} 删除元素
删除元素主要过程：检查参数合法性 其后面元素前移，然后把最后面的元素全部置为null。
- remove（index） 保存原index值用来返回，index后的元素向前拷贝，最后值赋值null以回收
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; // clear to let GC do its work
return oldValue;
} - remove（object） 遍历找到对应元素，不需要进行index合法性检查，直接删除
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 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
} - removeAll（collection），retainAll（collection）
- removeAll（collection） 与 retainAll（collection）是两个相反的过程，一个保留给定集合包含的元素，相当于求交集，一个删除给定集合包含的元素，相当于求差集
- batchRemove 使用双指针的方法，将要保留的值赋值到前面，然后将后面的值置null
NOTE
其中finally语句中，r<size的情况是由于Collection中的contains方法有可能抛出NullPointerException和ClassCastException，如果出现异常，就将当前w之后的值复制到r之后，不再检查是否包含，而后r之后的值依旧赋值为null
public boolean removeAll(Collection<?> c) {
return batchRemove(c, false);
}
public boolean retainAll(Collection<?> 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;
}Fast-Fail机制
- 源码中凡是修改List结构（插入，删除，打乱顺序，调整容量，不包含set更新元素），都会涉及到modCount++
- 在ArrayList类创建迭代器之后，除非通过迭代器自身remove或add对列表结构进行修改，否则在其他线程中以任何形式对列表进行修改，迭代器马上会抛出异常，快速失败。
- 该机制通过检查modCount的值来确定是否迭代过程中有其他线程对列表进行修改
private void checkForComodification() {
if (ArrayList.this.modCount != this.modCount)
throw new ConcurrentModificationException();
} 复制方法 Clone
浅拷贝，拷贝引用，而非创建新对象，如果引用指向的对象改变也会跟着改变
public Object clone() {
try {
@SuppressWarnings("unchecked")
ArrayList<E> v = (ArrayList<E>) 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();
}
} 转换为数组 toArray
- toArray（）返回新的Object数组
- toArray(T[]) 如果实参数组长度小于List，返回一个新的T类型数组，如果实参数组大于List，执行复制拷贝，将紧邻的后一个置为null（在list不含null时，便于检查List长度），返回实参数组
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
} 序列化方法
- elementData声明为transient是不会被序列化写入的，所以序列化分两步，首先执行通用序列化，然后将数组中的元素逐一读入，或写出
- 在进行写入的过程中有fast-fail机制，即这个过程中有对list的修改会造成失败
- 虽然size单独写入了，但是在readObject中这个值并没有用到，为了与clone方法适配
private transient Object[] elementData;
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();
}
}
} 迭代器
- 迭代器作为ArrayList的内部类，可以直接访问修改ArrayList
- ArrayList实现了两种迭代器，其中ListIterator除了实现基本的Iterator方法（hasNext，Next，remove），还包含更丰富的方法
- ListIterator额外实现的方法
- listIterator(index) 可以指定开始遍历的位置
- hasPrevious 有没有前驱
- previous 返回前驱
- add 实现添加元素
- set 更新上次访问的元素
- previousIndex() nextIndex() 返回下一个，和上一个位置
NOTE
remove，add，set方法都是改变上次被访问元素位置进行操作，连续调用两次以上就会出现问题
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}
public ListIterator<E> listIterator() {
return new ListItr(0);
}
public Iterator<E> iterator() {
return new Itr();
}
private class Itr implements Iterator<E> {
int cursor; // index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
int expectedModCount = modCount;
public boolean hasNext() {
return cursor != size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
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();
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();
}
}
private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
super();
cursor = index;
}
public boolean hasPrevious() {
return cursor != 0;
}
public int nextIndex() {
return cursor;
}
public int previousIndex() {
return cursor - 1;
}
@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
int i = cursor - 1;
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[lastRet = i];
}
public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.set(lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
public void add(E e) {
checkForComodification();
try {
int i = cursor;
ArrayList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
} 子视图 SubList
- 首先进行位置检查，如果合法，就调用新建SubList对象
- ArrayList将自身作为参数传递，也就是说，对SubList的操作其实是对原ArrayList的操作，SubList的方法跟ArrayList相近，只是需要进行index的转换，加上fromIndex
- SubList 只会返回ListIterator，其ListIterator对象通过匿名内部类的方式定义
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, 0, fromIndex, toIndex);
}
static void subListRangeCheck(int fromIndex, int toIndex, int size) {
if (fromIndex < 0)
throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
if (toIndex > size)
throw new IndexOutOfBoundsException("toIndex = " + toIndex);
if (fromIndex > toIndex)
throw new IllegalArgumentException("fromIndex(" + fromIndex +
") > toIndex(" + toIndex + ")");
}
private class SubList extends AbstractList<E> implements RandomAccess {
private final AbstractList<E> parent;
private final int parentOffset;
private final int offset;
int size;
SubList(AbstractList<E> parent,
int offset, int fromIndex, int toIndex) {
this.parent = parent;
this.parentOffset = fromIndex;
this.offset = offset + fromIndex;
this.size = toIndex - fromIndex;
this.modCount = ArrayList.this.modCount;
}
public E set(int index, E e) {
rangeCheck(index);
checkForComodification();
E oldValue = ArrayList.this.elementData(offset + index);
ArrayList.this.elementData[offset + index] = e;
return oldValue;
}
public E get(int index) {
rangeCheck(index);
checkForComodification();
return ArrayList.this.elementData(offset + index);
}
public int size() {
checkForComodification();
return this.size;
}
public void add(int index, E e) {
rangeCheckForAdd(index);
checkForComodification();
parent.add(parentOffset + index, e);
this.modCount = parent.modCount;
this.size++;
}
public E remove(int index) {
rangeCheck(index);
checkForComodification();
E result = parent.remove(parentOffset + index);
this.modCount = parent.modCount;
this.size--;
return result;
}
protected void removeRange(int fromIndex, int toIndex) {
checkForComodification();
parent.removeRange(parentOffset + fromIndex,
parentOffset + toIndex);
this.modCount = parent.modCount;
this.size -= toIndex - fromIndex;
}
public boolean addAll(Collection<? extends E> c) {
return addAll(this.size, c);
}
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
int cSize = c.size();
if (cSize==0)
return false;
checkForComodification();
parent.addAll(parentOffset + index, c);
this.modCount = parent.modCount;
this.size += cSize;
return true;
}
public Iterator<E> iterator() {
return listIterator();
}
public ListIterator<E> listIterator(final int index) {
checkForComodification();
rangeCheckForAdd(index);
final int offset = this.offset;
return new ListIterator<E>() {
int cursor = index;
int lastRet = -1;
int expectedModCount = ArrayList.this.modCount;
public boolean hasNext() {
return cursor != SubList.this.size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= SubList.this.size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (offset + i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[offset + (lastRet = i)];
}
public boolean hasPrevious() {
return cursor != 0;
}
@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
int i = cursor - 1;
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (offset + i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[offset + (lastRet = i)];
}
public int nextIndex() {
return cursor;
}
public int previousIndex() {
return cursor - 1;
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
SubList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = ArrayList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.set(offset + lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
public void add(E e) {
checkForComodification();
try {
int i = cursor;
SubList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = ArrayList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
final void checkForComodification() {
if (expectedModCount != ArrayList.this.modCount)
throw new ConcurrentModificationException();
}
};
}
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, offset, fromIndex, toIndex);
}
private void rangeCheck(int index) {
if (index < 0 || index >= this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private void rangeCheckForAdd(int index) {
if (index < 0 || index > this.size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+this.size;
}
private void checkForComodification() {
if (ArrayList.this.modCount != this.modCount)
throw new ConcurrentModificationException();
}
} 