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ArrayList源码浅析（jdk1.8）
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ArrayList的实质就是动态数组。所以可以通过下标准确的找到目标元素，因此查找的效率高。但是添加或删除元素会涉及到大量元素的位置移动，所以效率低。
一、构造方法
ArrayList提供了3个构造方法
1、无参的，就是把表示集合的数组赋值为空。
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
2、带初始化大小的构造方法，就是new一个指定大小的数组赋值给表示集合的数组。
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);
}
}
3、带集合的构造方法，就是把传过来的集合转成数组在赋值给表示集合的数组。
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;
}
}
二、添加元素
1、add(E e)方法，添加元素前会先判断数组是否为空，如果为空，则创建一个长度为10的空数组，
然后在把所添加的元素加入到数组中。如果不为空，则判断集合大小是否超出数组的长度，
如果超出则进行数组扩容，不然直接进行赋值。数组扩容的规则是原先数组长度加上其一半。即扩容1.5倍。
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_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);
}
2、add(int index, E element)方法，添加元素到指定位置。添加前先判断所传参数是否超出集合大小或小于0，
如果是则抛出数组越界异常，符合条件则在进行和add(E e)方法一样的数组大小的检查，看看是否扩容。
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++;
}
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++;
// 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);
}
3、addAll(Collection<? extends E> c)方法，添加集合到集合中。把集合转成数组，然后检查数组是否进行扩容，
如果不须就直接进行添加。
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;
}
三、删除元素
1、remove(int index)根据下标删除元素，先检查下标是否越界，然后获取本下标的值用于返回，最后进行数组的变动。
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;
}
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
E elementData(int index) {
return (E) elementData[index];
}
2、remove(Object o)根据元素删除值，for循环整个集合来检查是否有和参数相同的元素，有就根据索引进行删除。
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
}
3、removeRange(int fromIndex, int toIndex)根据下标范围进行删除元素。
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;
}
4、removeAll(Collection<?> c)只要所传集合中的元素在本身集合中就删除，取差集的意思。
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
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;
}
四、其他方法
1、retainAll(Collection<?> c)，只要所传集合中的元素不在本身集合中就删除，取并集的意思。
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;
}
2、set(int index, E element)，取代指定位置的元素。
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
3、trimToSize()，裁剪空间。
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
五、其他部分源码
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L; /**
* 默认集合初始容量为10
*/
private static final int DEFAULT_CAPACITY = 10; private static final Object[] EMPTY_ELEMENTDATA = {}; private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /**
* 存储集合元素的数组
*/
transient Object[] elementData; // non-private to simplify nested class access /**
* 集合大小（元素个数）
*
* @serial
*/
private int size; /**
* 构造一个带具体初始容量的空集合
* 就是new一个指定大小的object数组
* @param initialCapacity 集合的初始容量
*/
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);
}
} /**
* 构造一个初始为10的空集合
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
} /**
* 参数为集合的构造函数，集合元素被复制到数组中
*
* @param c 集合参数*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// 防止c.toArray()返回的不是数组
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// 空数组
this.elementData = EMPTY_ELEMENTDATA;
}
} /**
* 确保容量
*/
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
// any size if not default element table
? 0
// larger than default for default 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 == DEFAULTCAPACITY_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);
} /**
* 最大容量
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; /**
* 容量扩展
* @param minCapacity the desired minimum capacity
*/
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;
} /**
* 返回集合大小
*
* @return the number of elements in this list
*/
public int size() {
return size;
} /**
* 集合为空返回true
*
* @return <tt>true</tt> if this list contains no elements
*/
public boolean isEmpty() {
return size == 0;
} /**
* 判断集合是否包含一个元素
*/
public boolean contains(Object o) {
return indexOf(o) >= 0;
} /**
* 判断集合是否包含一个元素的具体方法
* 就是for循环整个数组进行比较，存在就返回下标，否则返回-1
*/
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;
} /**
* 从集合的尾部开始判断是否含有某个元素
* 规则同indexOf（）方法
*/
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;
} /**
* 克隆
*/
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);
}
} /**
* 集合转数组
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
} // 返回数组指定下标的元素 @SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
} /**
* 根据指定位置获取元素
*/
public E get(int index) {
// 越界检查
rangeCheck(index); return elementData(index);
} /**
* 把某元素塞到集合指定的位置(替换)，然后返回旧的值
*/
public E set(int index, E element) {
// 越界检查
rangeCheck(index); E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
} /**
* 添加元素
*/
public boolean add(E e) {
// 集合容量检查
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
} /**
* 把元素添加到指定位置
*/
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++;
} /**
* 删除指定位置的元素
*/
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;
} /**
* 删除指定元素
*/
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;
} /*
* 删除指定位置的元素
*/
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
} /**
* 清空集合
*/
public void clear() {
modCount++; // clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null; size = 0;
} /**
* 添加集合到集合中
*/
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;
} /**
* 从指定位置添加集合
*/
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;
} /**
* 删除指定范围的集合元素
*/
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;
} /**
* 集合越界检查
*/
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
} /**
* 添加元素时进行越界检查
*/
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
} /**
* 越界信息提示
*/
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
} /**
* 根据所传集合的元素进行删除指定的元素（差集）
*/
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
} /**
* 根据所传集合和本身集合取交集（交集）
*/
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;
}
}
