第一部分 ArrayList簡(jiǎn)介
ArrayList是java.util包下面AbstractList<E>類的一個(gè)子類。涉及到的類和接口具體關(guān)系如下：
ArrayList UML圖
值得一提的是，類AbstractCollection是實(shí)現(xiàn)了Collection接口的，所以ArrayList可以算得上是Collection接口的一個(gè)典型的實(shí)現(xiàn)類（因?yàn)槠匠Ｓ玫帽容^多）。
List接口的大小可變數(shù)組的實(shí)現(xiàn)，實(shí)現(xiàn)了所有可選列表操作，并允許包括null在內(nèi)的所有元素。除了實(shí)現(xiàn)List接口之外，此類(ArrayList)還提供了一些方法來(lái)操作內(nèi)部用來(lái)存儲(chǔ)列表的數(shù)組的大小(此類大致上等同于Vector類，但它是不同步的)。
size、isEmpty、get、set、iterator和listIterator操作都以固定時(shí)間運(yùn)行。add操作以分?jǐn)偟墓潭〞r(shí)間運(yùn)行，也就是說(shuō)添加n個(gè)元素需要O(n)時(shí)間。其他所有操作都以線性時(shí)間運(yùn)行(大體上講)。與用于LinkedList實(shí)現(xiàn)的常數(shù)因子相比，此實(shí)現(xiàn)的常數(shù)因子較低。
每個(gè)ArrayList實(shí)例都有一個(gè)容量。該容量是指用來(lái)存儲(chǔ)列表元素的數(shù)組的大小。它總是至少等于列表的大小。隨著向ArrayList中不斷添加元素，其容量也自動(dòng)增長(zhǎng)，并未指定增長(zhǎng)策略的細(xì)節(jié)，因?yàn)檫@不只是添加元素會(huì)帶來(lái)分?jǐn)偣潭〞r(shí)間開(kāi)銷(xiāo)那么簡(jiǎn)單。
在添加大量元素之前，應(yīng)用程序可以使用ensureCapacity操作來(lái)增加ArrayList實(shí)例的容量，這可以減少遞增式再分配的容量。
要注意的是，ArrayList不是同步的
如果多個(gè)線程同時(shí)訪問(wèn)一個(gè)ArrayList實(shí)例，而其中至少一個(gè)線程從結(jié)構(gòu)上修改了列表，那么它必須保持外部同步(結(jié)構(gòu)上的修改是指任何添加或刪除一個(gè)或多個(gè)元素的操作，或者顯式調(diào)整底層數(shù)組的大?。粌H僅設(shè)置元素的值不是結(jié)構(gòu)上的修改)。這一般通過(guò)對(duì)自然封裝該列表的對(duì)象進(jìn)行同步操作來(lái)完成。如果不存在這樣的對(duì)象，則應(yīng)該使用Collections.synchronizedList方法將該列表"包裝"起來(lái)。這最好在創(chuàng)建時(shí)完成，以防止意外對(duì)列表進(jìn)行不同步的訪問(wèn)：
List list = Collections.synchronizedList(new ArrayList(...));
此類的iterator和listIterator方法返回的迭代器是快速失敗的：在創(chuàng)建迭代器之后，除非通過(guò)迭代器自身的remove或add方法從結(jié)構(gòu)上對(duì)列表進(jìn)行修改，否則在任何時(shí)間以任何方式對(duì)列表進(jìn)行修改，迭代器都會(huì)拋出ConcurrentModificationException。因此，面對(duì)并發(fā)的修改，迭代器很快就會(huì)完全失敗，而不是冒著將來(lái)某個(gè)不確定時(shí)間發(fā)生任意不確定行為的風(fēng)險(xiǎn)。
需要注意的是，迭代器的快速失敗行為無(wú)法得到保證，因?yàn)橐话銇?lái)說(shuō)，不可能對(duì)是否拋出不同步并發(fā)修改做出任何硬性保證。快速失敗迭代器會(huì)盡最大努力拋出ConcurrentModificationException。因此，為提高這類迭代器的正確性而編寫(xiě)一個(gè)依賴此程序的程序是錯(cuò)誤的做法：迭代器的快速失敗行為應(yīng)該僅用于檢測(cè)bug。
ArrayList 是一個(gè)數(shù)組隊(duì)列，相當(dāng)于 動(dòng)態(tài)數(shù)組。與Java中的數(shù)組相比，它的容量能動(dòng)態(tài)增長(zhǎng)。它繼承于AbstractList，實(shí)現(xiàn)了List, RandomAccess, Cloneable, java.io.Serializable這些接口。
ArrayList 繼承了AbstractList，實(shí)現(xiàn)了List。它是一個(gè)數(shù)組隊(duì)列，提供了相關(guān)的添加、刪除、修改、遍歷等功能。ArrayList 實(shí)現(xiàn)了RandmoAccess接口，即提供了隨機(jī)訪問(wèn)功能。RandmoAccess是java中用來(lái)被List實(shí)現(xiàn)，為L(zhǎng)ist提供快速訪問(wèn)功能的。在ArrayList中，我們即可以通過(guò)元素的序號(hào)快速獲取元素對(duì)象；這就是快速隨機(jī)訪問(wèn)。稍后，我們會(huì)比較List的“快速隨機(jī)訪問(wèn)”和“通過(guò)Iterator迭代器訪問(wèn)”的效率。
ArrayList 實(shí)現(xiàn)了Cloneable接口，即覆蓋了函數(shù)clone()，能被克隆。
ArrayList 實(shí)現(xiàn)java.io.Serializable接口，這意味著ArrayList支持序列化，能通過(guò)序列化去傳輸。
和Vector不同，ArrayList中的操作不是線程安全的！所以，建議在單線程中才使用ArrayList，而在多線程中可以選擇Vector或者CopyOnWriteArrayList。
第二部分 ArrayList源碼探究（基于jdk1.8.0_74）
package java.util;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
/** 概述： List接口可調(diào)整大小的數(shù)組實(shí)現(xiàn)。實(shí)現(xiàn)所有可選的List操作，并允許所有元素，包括null，
元素可重復(fù)。 除了列表接口外，該類提供了一種方法來(lái)操作該數(shù)組的大小來(lái)存儲(chǔ)該列表中的數(shù)組的大小。
時(shí)間復(fù)雜度： 方法size、isEmpty、get、set、iterator和listIterator的調(diào)用是常數(shù)時(shí)間的。
添加刪除的時(shí)間復(fù)雜度為O(N)。其他所有操作也都是線性時(shí)間復(fù)雜度。
容量： 每個(gè)ArrayList都有容量，容量大小至少為L(zhǎng)ist元素的長(zhǎng)度，默認(rèn)初始化為10。
容量可以自動(dòng)增長(zhǎng)。 如果提前知道數(shù)組元素較多，
可以在添加元素前通過(guò)調(diào)用ensureCapacity()方法提前增加容量以減小后期容量自動(dòng)增長(zhǎng)的開(kāi)銷(xiāo)。
也可以通過(guò)帶初始容量的構(gòu)造器初始化這個(gè)容量。
線程不安全： ArrayList不是線程安全的。如果需要應(yīng)用到多線程中，需要在外部做同步
modCount： 定義在AbstractList中：protected transient int modCount = 0;
已從結(jié)構(gòu)上修改此列表的次數(shù)。從結(jié)構(gòu)上修改是指更改列表的大小，或者打亂列表，
從而使正在進(jìn)行的迭代產(chǎn)生錯(cuò)誤的結(jié)果。
此字段由iterator和listiterator方法返回的迭代器和列表迭代器實(shí)現(xiàn)使用。
如果意外更改了此字段中的值，則迭代器（或列表迭代器）將拋出concurrentmodificationexception
來(lái)響應(yīng)next、remove、previous、set或add操作。
在迭代期間面臨并發(fā)修改時(shí)，它提供了快速失敗 行為，而不是非確定性行為。
子類是否使用此字段是可選的。 如果子類希望提供快速失敗迭代器（和列表迭代器），
則它只需在其 add(int,e)和remove(int)方法
（以及它所重寫(xiě)的、導(dǎo)致列表結(jié)構(gòu)上修改的任何其他方法）中增加此字段。
對(duì)add(int, e)或remove(int)的單個(gè)調(diào)用向此字段添加的數(shù)量不得超過(guò) 1，
否則迭代器（和列表迭代器）將拋出虛假的 concurrentmodificationexceptions。
如果某個(gè)實(shí)現(xiàn)不希望提供快速失敗迭代器，則可以忽略此字段。
transient： 默認(rèn)情況下,對(duì)象的所有成員變量都將被持久化.
在某些情況下,如果你想避免持久化對(duì)象的一些成員變量,
你可以使用transient關(guān)鍵字來(lái)標(biāo)記他們,transient也是java中的保留字(JDK 1.8)
*/
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
private static final long serialVersionUID = 8683452581122892189L;
private static final int DEFAULT_CAPACITY = 10;//默認(rèn)初始容量
private static final Object[] EMPTY_ELEMENTDATA = {};//用于空實(shí)例的共享空數(shù)組對(duì)象實(shí)例
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData; // non-private to simplify nested class access
private int size;
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);
}
}
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
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;
}
}
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
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);
}
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
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 int size() {
return size;
}
public boolean isEmpty() {
return size == 0;
}
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;
}
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);
}
}
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;
}
// Positional Access Operations
@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;
}
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 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;
}
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
}
public void clear() {
modCount++;
// clear to let GC do its work
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);
// clear to let GC do its work
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));
}
/**
* A version of rangeCheck used by add and addAll.
*/
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;
}
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();
}
}
}
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}
/**
* Returns a list iterator over the elements in this list (in proper
* sequence).
*
* <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
*
* @see #listIterator(int)
*/
public ListIterator<E> listIterator() {
return new ListItr(0);
}
public Iterator<E> iterator() {
return new Itr();
}
/**
* An optimized version of AbstractList.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();
}
}
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
Objects.requireNonNull(consumer);
final int size = ArrayList.this.size;
int i = cursor;
if (i >= size) {
return;
}
final Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length) {
throw new ConcurrentModificationException();
}
while (i != size && modCount == expectedModCount) {
consumer.accept((E) elementData[i++]);
}
// update once at end of iteration to reduce heap write traffic
cursor = i;
lastRet = i - 1;
checkForComodification();
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
/**
* An optimized version of AbstractList.ListItr
*/
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();
}
}
}
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)];
}
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
Objects.requireNonNull(consumer);
final int size = SubList.this.size;
int i = cursor;
if (i >= size) {
return;
}
final Object[] elementData = ArrayList.this.elementData;
if (offset + i >= elementData.length) {
throw new ConcurrentModificationException();
}
while (i != size && modCount == expectedModCount) {
consumer.accept((E) elementData[offset + (i++)]);
}
// update once at end of iteration to reduce heap write traffic
lastRet = cursor = i;
checkForComodification();
}
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();
}
public Spliterator<E> spliterator() {
checkForComodification();
return new ArrayListSpliterator<E>(ArrayList.this, offset,
offset + this.size, this.modCount);
}
}
@Override
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
final int expectedModCount = modCount;
@SuppressWarnings("unchecked")
final E[] elementData = (E[]) this.elementData;
final int size = this.size;
for (int i=0; modCount == expectedModCount && i < size; i++) {
action.accept(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
/**
* Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
* and <em>fail-fast</em> {@link Spliterator} over the elements in this
* list.
*
* <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
* {@link Spliterator#SUBSIZED}, and {@link Spliterator#ORDERED}.
* Overriding implementations should document the reporting of additional
* characteristic values.
*
* @return a {@code Spliterator} over the elements in this list
* @since 1.8
*/
@Override
public Spliterator<E> spliterator() {
return new ArrayListSpliterator<>(this, 0, -1, 0);
}
/** Index-based split-by-two, lazily initialized Spliterator */
static final class ArrayListSpliterator<E> implements Spliterator<E> {
private final ArrayList<E> list;
private int index; // current index, modified on advance/split
private int fence; // -1 until used; then one past last index
private int expectedModCount; // initialized when fence set
/** Create new spliterator covering the given range */
ArrayListSpliterator(ArrayList<E> list, int origin, int fence,
int expectedModCount) {
this.list = list; // OK if null unless traversed
this.index = origin;
this.fence = fence;
this.expectedModCount = expectedModCount;
}
private int getFence() { // initialize fence to size on first use
int hi; // (a specialized variant appears in method forEach)
ArrayList<E> lst;
if ((hi = fence) < 0) {
if ((lst = list) == null)
hi = fence = 0;
else {
expectedModCount = lst.modCount;
hi = fence = lst.size;
}
}
return hi;
}
public ArrayListSpliterator<E> trySplit() {
int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
return (lo >= mid) ? null : // divide range in half unless too small
new ArrayListSpliterator<E>(list, lo, index = mid,
expectedModCount);
}
public boolean tryAdvance(Consumer<? super E> action) {
if (action == null)
throw new NullPointerException();
int hi = getFence(), i = index;
if (i < hi) {
index = i + 1;
@SuppressWarnings("unchecked") E e = (E)list.elementData[i];
action.accept(e);
if (list.modCount != expectedModCount)
throw new ConcurrentModificationException();
return true;
}
return false;
}
public void forEachRemaining(Consumer<? super E> action) {
int i, hi, mc; // hoist accesses and checks from loop
ArrayList<E> lst; Object[] a;
if (action == null)
throw new NullPointerException();
if ((lst = list) != null && (a = lst.elementData) != null) {
if ((hi = fence) < 0) {
mc = lst.modCount;
hi = lst.size;
}
else
mc = expectedModCount;
if ((i = index) >= 0 && (index = hi) <= a.length) {
for (; i < hi; ++i) {
@SuppressWarnings("unchecked") E e = (E) a[i];
action.accept(e);
}
if (lst.modCount == mc)
return;
}
}
throw new ConcurrentModificationException();
}
public long estimateSize() {
return (long) (getFence() - index);
}
public int characteristics() {
return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
}
}
@Override
public boolean removeIf(Predicate<? super E> filter) {
Objects.requireNonNull(filter);
// figure out which elements are to be removed
// any exception thrown from the filter predicate at this stage
// will leave the collection unmodified
int removeCount = 0;
final BitSet removeSet = new BitSet(size);
final int expectedModCount = modCount;
final int size = this.size;
for (int i=0; modCount == expectedModCount && i < size; i++) {
@SuppressWarnings("unchecked")
final E element = (E) elementData[i];
if (filter.test(element)) {
removeSet.set(i);
removeCount++;
}
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
// shift surviving elements left over the spaces left by removed elements
final boolean anyToRemove = removeCount > 0;
if (anyToRemove) {
final int newSize = size - removeCount;
for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
i = removeSet.nextClearBit(i);
elementData[j] = elementData[i];
}
for (int k=newSize; k < size; k++) {
elementData[k] = null; // Let gc do its work
}
this.size = newSize;
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
return anyToRemove;
}
@Override
@SuppressWarnings("unchecked")
public void replaceAll(UnaryOperator<E> operator) {
Objects.requireNonNull(operator);
final int expectedModCount = modCount;
final int size = this.size;
for (int i=0; modCount == expectedModCount && i < size; i++) {
elementData[i] = operator.apply((E) elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
@Override
@SuppressWarnings("unchecked")
public void sort(Comparator<? super E> c) {
final int expectedModCount = modCount;
Arrays.sort((E[]) elementData, 0, size, c);
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
}
以上是ArrayList的源碼，下面我將對(duì)其源碼進(jìn)行深入探究
- 繼承的類以及實(shí)現(xiàn)的接口(上面已經(jīng)提過(guò)了)
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
- 成員變量
private static final long serialVersionUID = 8683452581122892189L;//版本號(hào)
private static final int DEFAULT_CAPACITY = 10;//默認(rèn)容量
/*
*默認(rèn)空Object數(shù)組， 用于定義空的ArrayList
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/* 也是空數(shù)組，
* 但是是在新增元素的時(shí)候使用
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData;//ArrayList中用來(lái)存儲(chǔ)元素的地方，數(shù)組長(zhǎng)度就是它的容量
private int size;//ArrayList所包含的元素的數(shù)量，即ArrayList的大小
數(shù)組elementData為ArrayList存儲(chǔ)元素的Buffer，ArrayList的容量即為該數(shù)組的長(zhǎng)度。當(dāng)ArrayList為空時(shí)，此時(shí)elementData==EMPTY_ELEMENTDATA，當(dāng)往ArrayList添加一個(gè)元素時(shí)，elementData的長(zhǎng)度就被初始化為10,也就是DEFAULT_CAPACITY的值。
注意到elementData數(shù)組被transient修飾，因?yàn)锳rrayList實(shí)現(xiàn)了Serializable接口，這意味著ArrayList可以被序列化，也就是說(shuō)，ArrayList的屬性可以通過(guò)網(wǎng)絡(luò)傳輸，或者被存儲(chǔ)到磁盤(pán)持久化。但是在很多情況下，我們并不希望有些屬性被傳輸或者存儲(chǔ)，比如一些敏感信息。這時(shí)使用trainsent標(biāo)記屬性，就可以使得ArrayList在序列化的過(guò)程中elementData不參與序列化。因此，elementData中的數(shù)據(jù)僅存在于調(diào)用者的內(nèi)存中（關(guān)于transient這點(diǎn)在后面還會(huì)提）。
- 構(gòu)造函數(shù)
由于ArrayList是Collection的實(shí)現(xiàn)類，所以它像Collection其它的通用實(shí)現(xiàn)類一樣，除了擁有自己特有的構(gòu)造方法之外，還提供兩個(gè)“標(biāo)準(zhǔn)”構(gòu)造方法，故有三個(gè)構(gòu)造方法：
- 無(wú)參數(shù)構(gòu)造方法:
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
無(wú)參構(gòu)造方法直接創(chuàng)建一個(gè)大小為10的數(shù)組
ps：在jdk1.5以前，無(wú)參的構(gòu)造方法是下面這樣的
public ArrayList(){
this(10);
}
- 帶有 Collection 類型單參數(shù)的構(gòu)造方法:
/*
*構(gòu)造一個(gè)包含指定元素的list，這些元素的是按照Collection的迭代器返回的順序排列的
*/
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;
}
}
用于創(chuàng)建一個(gè)具有與其參數(shù)相同元素新的 collection。實(shí)際上，它允許用戶復(fù)制任何 collection，以生成所需實(shí)現(xiàn)類型的一個(gè)等效 collection。
換句話說(shuō)，就是將提供的集合轉(zhuǎn)換成數(shù)組返回給elementData(返回的不是Object[]，那么將使用Arrays.copyOf將其轉(zhuǎn)換成Object[])。如果該集合大小為零，則返回一個(gè)默認(rèn)的數(shù)組。
- ArrayList特有的構(gòu)造方法
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);
}
}
如果傳入的參數(shù)initialCapacity大于零，那么就為elementData生成一個(gè)大小為initialCapacity的數(shù)組。如果initialCapacity等于零，那么就生成一個(gè)默認(rèn)數(shù)組。如果initialCapacity小于零，則拋出一個(gè)非法參數(shù)的錯(cuò)誤。
- trimToSize函數(shù):
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
將此 ArrayList實(shí)例的容量調(diào)整為列表的當(dāng)前大小---如果此ArrayList的size為零，那就返回一個(gè)默認(rèn)的數(shù)組，如果size不為零，那就將容量調(diào)整為列表的當(dāng)前大小。在應(yīng)用中通常使用此方法來(lái)最大限度地降低一個(gè)ArrayList所使用的存儲(chǔ)空間。
下面我將用一個(gè)小程序來(lái)看看trimToSize函數(shù)到底是如何工作的：
import java.util.ArrayList;
public class Eakon{
public static void main(String[] args){
//創(chuàng)建一個(gè)大小為10的NumberList
ArrayList<Integer> NumberList = new ArrayList<Integer>(10);
//往里面添加15個(gè)數(shù)字
for(int i = 0; i < 15; i++){
NumberList.add(i);
}
NumberList.add(1);
//使用trimToSize函數(shù)
NumberList.trimToSize();
}
}
接下來(lái)是Debug的過(guò)程：
carried line 6
如上圖所示，執(zhí)行完第6行代碼之后生成了一個(gè)大小為10的數(shù)組，然后我們繼續(xù)往下執(zhí)行，即進(jìn)入循環(huán)：
往NumberList添加了9個(gè)元素之后
往NumberList添加了11個(gè)元素之后
可以看到，由于ArrayList的自動(dòng)擴(kuò)容，本來(lái)初始化時(shí)大小為10的NumberList在添加到第11個(gè)元素的時(shí)候(此時(shí)modCount值為11)自動(dòng)大小自動(dòng)擴(kuò)容到了15（至于如何實(shí)現(xiàn)自動(dòng)擴(kuò)容我們以后再學(xué)習(xí)）。那么我們繼續(xù)執(zhí)行下去，直到執(zhí)行完trimToSize函數(shù)：
執(zhí)行完第11行代碼之后，NumberList里面只有16個(gè)元素，但是它的大小為22
執(zhí)行完trimToSize函數(shù)之后
我們可以看到，當(dāng)執(zhí)行完trimToSize函數(shù)之后，NumberList的大小變?yōu)榱怂鼘?shí)際存儲(chǔ)的元素的數(shù)量。這樣一來(lái)，我們就不難理解trimToSize函數(shù)所發(fā)揮的作用了。
