ArrayList类源码解析

ArrayList特点：

1.内部通过数组实现

2.删除、插入元素需要大量移动元素

3.默认存储容量是null，需要增加容量时候，默认增加的容量等于原始ArrayList的容量

4.没有实现同步方法，多线程的时候，线程不安全，效率较高

5.存储元素的顺序就说加入集合中的顺序，方便指定index访问元素

所在包

package java.util;

继承AbstractList抽象类

实现List、RandomAccess、Cloneable、java.io.Serializable

public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable{
// 内部代码下面讲解
}

5个参数

private static final long serialVersionUID = 8683452581122892189L;

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

/**
* 空数组
*/
private static final Object[] EMPTY_ELEMENTDATA = {};

/**
* 数组存储元素
*/
private transient Object[] elementData;

/**
* ArrayList内元素个数
*/
private int size;

三个构造器

/**
* initialCapacity容量大小
*
* @throws initialCapacity 小于0的时候，抛出：IllegalArgumentException
*/
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0) // 抛出异常
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity]; // 新建initialCapacity大小的数组
}

/**
* 无参构造器，容量是空
*/
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}

/**
* 集合c中的元素加入的数组中
* @throws 集合空的时候 抛出：NullPointerException
*/
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) // 集合内的元素复制到element中
elementData = Arrays.copyOf(elementData, size, Object[].class);
}

修剪elementData，保证其内部的元素都是有效元素

elementCount 记录实际元素个数

容量大小等于元素个数

public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}

增加容量的操作

具体分为下面几步：

1.是否需要增加容量

2.增加多少容量

3.增加容量

/**
* @param   minCapacity   最小容量，这个时候容量等于ArrayList内元素的大小
*/
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++;

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

/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

/**
* @param minCapacity the desired minimum capacity
*/
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);// 默认增加原始容量的一半
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;
}

获取ArrayList元素个数

public int size() {
return size;
}

判断是否空

public boolean isEmpty() {
return size == 0;
}

是否包含元素o

public boolean contains(Object o) {
return indexOf(o) >= 0;
}

元素o第一次出现的下标

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;
}

元素o最后一次出现的下标

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

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();
}
}

ArrayList内元素复制到数组，并返回这个数组

public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}

ArrayList内元素更新数组a

/* @throws ArrayStoreException if the runtime type of the specified array
*         is not a supertype of the runtime type of every element in
*         this list
* @throws NullPointerException if the specified array is null
*/
@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;
}

获取index位置元素

不抛出异常

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

获取index位置元素

可抛出异常

/* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
rangeCheck(index);

return elementData(index);
}

更新index位置元素

/* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
rangeCheck(index);

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

尾部加入元素

public boolean add(E e) {
ensureCapacityInternal(size + 1);  // 是否增加容量
elementData[size++] = e;
return true;
}

index位置插入元素

/* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
rangeCheckForAdd(index);

ensureCapacityInternal(size + 1);  // 是否增加容量
System.arraycopy(elementData, index, elementData, index + 1,size - index);
elementData[index] = element;
size++;
}

删除index位置元素

/* @throws IndexOutOfBoundsException
*/
public E remove(int index) {
rangeCheck(index);// 检查index是否合法

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;
}

删除元素o

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;
}

具体删除操作

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
}

清空操作

元素设为null

size设为0

public void clear() {
modCount++;

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

size = 0;
}

添加集合c元素到ArrayList中

/* @throws NullPointerException if the specified collection is null
*/
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;
}

index位置开始插入集合c的元素

/* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);// 检查index是否合法

Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew);  // 是否需要增加容量

int numMoved = size - index;// 需要移动元素数量
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,numMoved); // 后移

System.arraycopy(a, 0, elementData, index, numNew);// 插入
size += numNew;// 更新元素数量
return numNew != 0;
}

删除区间内元素

/* @throws IndexOutOfBoundsException if {@code fromIndex} or
*         {@code toIndex} is out of range
*         ({@code fromIndex < 0 ||
*          fromIndex >= size() ||
*          toIndex > size() ||
*          toIndex < fromIndex})
*/
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;// 移动元素数量
System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved); // 移动

// clear to let GC do its work
int newSize = size - (toIndex-fromIndex); // 更新size
for (int i = newSize; i < size; i++) { // 后面部分设null
elementData[i] = null;
}
size = newSize;
}

index是否合法检查

/**
* 检查index是否合法，没有对负数进行判断，是负数的时候抛出ArrayIndexOutOfBoundsException
*
*/
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

/**
* 添加元素时候index检查
*/
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

/**
* 越界输出信息
*/
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}

差集

/* @throws ClassCastException if the class of an element of this list
*         is incompatible with the specified collection
* @throws NullPointerException if this list contains a null element and the
*         specified collection does not permit null elements
*         or if the specified collection is null
* @see Collection#contains(Object)
*/
public boolean removeAll(Collection<?> c) {
return batchRemove(c, false);
}

交集

/* @throws ClassCastException if the class of an element of this list
*         is incompatible with the specified collection
* @throws NullPointerException if this list contains a null element and the
*         specified collection does not permit null elements
*         or if the specified collection is null
* @see Collection#contains(Object)
*/
public boolean retainAll(Collection<?> c) {
return batchRemove(c, true);
}

差集交集具体实现

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) // true 交集，false差集
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) {
// 垃圾回收
for (int i = w; i < size; i++)
elementData[i] = null;
modCount += size - w;
size = w;
modified = true;
}
}
return modified;
}

输出流和输入流

/**
* Save the state of the <tt>ArrayList</tt> instance to a stream (that
* is, serialize it).
*
* @serialData The length of the array backing the <tt>ArrayList</tt>
*             instance is emitted (int), followed by all of its elements
*             (each an <tt>Object</tt>) in the proper order.
*/
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;
// Read in all elements in the proper order.
for (int i=0; i<size; i++) {
a[i] = s.readObject();
}
}
}

三个不同类型的迭代器

/**
* 获取指定index开始的迭代器
* @throws index非法，抛出 ：IndexOutOfBoundsException
*/
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size) // 检查下标是否合法
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}

/**
* 获取所有元素的迭代器，该迭代器，可以向前也可以先后遍历，可以更新元素
* @see #listIterator(int)
*/
public ListIterator<E> listIterator() {
return new ListItr(0);
}

/**
* 获取所有元素的迭代器 ，只可以向后遍历元素
*/
public Iterator<E> iterator() {
return new Itr();
}

内部类实现Itr迭代器

只能够向后遍历

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() { // 是否结束
return cursor != size;
}

@SuppressWarnings("unchecked")
public E next() { // 下一个位置
checkForComodification(); // 运行记录最大值
int i = cursor; // 当前元素
if (i >= size) // 越界
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length) // 越界
throw new ConcurrentModificationException();
cursor = i + 1; // 下一个位置
return (E) elementData[lastRet = i]; // 返回当前元素
}

public void remove() { // 删除
if (lastRet < 0) // 非法
throw new IllegalStateException();
checkForComodification();

try {
ArrayList.this.remove(lastRet);// 删除
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}

ListItr 迭代器，继承Itr，并增加了前驱遍历、更新元素的方法

private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
super();
cursor = index;
}

public boolean hasPrevious() { // 是否有前驱
return cursor != 0;
}

public int nextIndex() { // 下一个位置
return cursor;
}

public int previousIndex() { // 前驱id

return cursor - 1;
}

@SuppressWarnings("unchecked")
public E previous() { // 前驱元素
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) { // 设置元素
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
ArrayList.this.set(lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

public void add(E e) { // 添加元素
checkForComodification();

try {
int i = cursor;
ArrayList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
}

获取子数组，以及其迭代器代码，和上面很类似

/* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
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();
}
}