ArrayList

ArrayList

ArrayList

概述

成员变量

构造器

成员方法

Summary

概述

ArrayList是List接口的具体实现类，可以存储任何元素，包括null。这个类与Vector类几乎是一样的，仅有的区别就是Vector类是线程安全的(方法有synchronized关键字)，而ArrayList不是。

关于时间复杂度：

constant time

size, isEmpty, get, set, iterator, listIterator

amortized constant time

add(也就是说，添加n个元素，需要O(n)时间)

linear time

other operations

关于线程安全：

ArrayList不是线程安全的

如果多个线程同时访问一个ArrayList实例，并且至少一个线程对ArrayList进行了structural modification(结构化修改，顾名思义，就是修改了结构发生了变化，是说添加或者是删除了一个或多个元素，如果仅仅是修改了元素的值，则不是结构化修改)，那么必须在外部syncronized(使用ArrayList的地方synchronized)。

有时候，也会通过以下用法来达到ArrayList的线程安全

List list = Collections.synchronizedList(new ArrayList(...))

关于fast-fail：

ArrayList的iterator()和listIterator()方法返回的iterator，具有fast-fail的特点，也就是说，在iterator创建后，不允许list的结构发生变化(add/remove)，否则会抛出ConcurrentModificationException异常。

成员变量

private static final long serialVersionUID = 8683452581122892189L;
/* 默认容量 */
private static final int DEFAULT_CAPACITY = 10;

/* 所有的空实例共享的空数组 */
private static final Object[] EMPTY_ELEMENTDATA = {};

/**
* 这个空数组是为了与EMPTY_ELEMENTDATA区别开来，后者是当ArrayList
* 的capacity为0时使用的(比如用来ArrayList(0)这个构造器)，而
* DEFAULTCAPACITY_EMPTY_ELEMENTDATA 是给有默认长度的空数组使用的
* 它还可以用来判断容量需要增加多少(见ensureCapacity()方法)
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

/**
* 这个数组就是ArrayList存储数据的地方啦
* 它的长度就是数组的capacity。
* 任何elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* 的ArrayList，当第一个元素添加进来的时候，capacity自动增大为
* DEFAULT_CAPACITY
*/
transient Object[] elementData;

/*List大小，也即当前数组中元素的数目*/
private int size;

构造器

/**
* 如果initialCapacity为0，就用EMPTY_ELEMENTDATA
*/
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);
}
}

/**
* 没有指定initialCapacity，就用DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* 这样当第一个数据添加进来的时候，size自动变为DEFAULT_CAPACITY
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

/**
* 利用一个Collection构造一个ArrayList
* 先把Collection变为Array
*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
if ((size = elementData.length) != 0) {
// 注意这里，如果elementData的类型不是Object，那么用Array.copyOf变为Object
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// size为0，空数组
this.elementData = EMPTY_ELEMENTDATA;
}
}

成员方法

/**
* 把ArrayList的capacity“剪”成size大小，节省空间
*/
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}

/**
* 增加ArrayList的capacity, 到底能不能增加，还要看情况
*
*/
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
// 如果不相等，那么可以expand任意大小
? 0
// 如果相等，那么说明现在的capacity已经是 DEFAULT_CAPACITY了
: DEFAULT_CAPACITY;

// 如果比需要增大的最小值还小，那么什么都不做
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}

// 内部方法，主要是给add用的
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}

ensureExplicitCapacity(minCapacity);
}

private void ensureExplicitCapacity(int minCapacity) {
modCount++;

// 防止溢出
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

/**
* expand capacity
*/
private void grow(int minCapacity) {
// 注意溢出
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1); // 每次扩大1.5倍
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity); // 比最大数组大小还大！！
elementData = Arrays.copyOf(elementData, newCapacity);
}

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

/**
* 返回元素在List中第一次出现的索引值，或者是-1(如果没找到)
* 就算Object为null，也会进行查找~~
*/
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;
}

/**
* 与indexOf()方向相反
*/
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;
}

/**
* 返回的是ArrayList的浅拷贝！！☆
*/
public Object clone() {
try {
ArrayList<?> v = (ArrayList<?>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
} catch (CloneNotSupportedException e) {
throw new InternalError(e);
}
}

/**
* 浅拷贝 ☆
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}

/**
* 返回一个包含了本ArrayList所有元素的数组，如果a的长度不小于ArrayList的长度
* 那么元素就保存在a中，否则，就重新创建一个新的数组，返回...
*/
@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;
}

@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;
}

/**
* 在给定的index处添加元素，那么index后面的元素都要后移，可见效率不高
*/
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++;
}

/**
* 删除给定index处的元素，index后的元素都要前移，效率不高
*/
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;
}

/**
* 删除在ArrayList中第一次出现o的地方，注意null也合法
*/
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;
}

/*
* 调用这个方法的方法应该都确保了index合法，所以才敢这么干
*/
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++;

// 都设置为null，让垃圾收集器回收
for (int i = 0; i < size; i++)
elementData[i] = null;

size = 0;
}

/**
* 把Collection c变成Array再添加到ArrayList的尾部，如果c中途有变化，则结果不确定
*/
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);
}

/**
* 保留c中的所有元素，也就是删除所有不是c中的元素
*/
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}

/**
* 批量删除
*/
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
// w是符合条件的索引值，r是原始数据的索引
int r = 0, w = 0;
boolean modified = false;
try {
for (; r < size; r++)
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// 发生r != size的情况应该是发生异常了，依然拷贝剩下的size - r个元素
if (r != size) {
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
// w == size说明所有元素都符合条件，不用动，否则要把w后面的元素置为null
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;
}

/**
* 序列化ArrayList实例的状态
* 这里就能看出为什么elementData[] 设置为transient了，因为要保存的是值，而不是数组首地址
*/
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);

// 保存的是元素的值.
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}

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

/**
* 反序列化
*/
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();
}
}
}

/**
* 返回一个list迭代器，迭代器的初始指向位置是index
* index不写，就是0
*/
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();
}

/**
* An optimized version of AbstractList.Itr
*/
private class Itr implements Iterator<E> {
int cursor;       // 下一个元素的索引
int lastRet = -1; // 最后一个返回的元素的索引，没有就是-1
int expectedModCount = modCount;

public boolean hasNext() {
return cursor != size; // cursor == size了，说明已经到底了
}

// 可见next()是先返回当前位置的值，再把cursor往后移动一位
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor; // cursor是下一个元素的索引值
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1; // cursor向后移动一位
return (E) elementData[lastRet = i]; // lastRet设置为i，并且把i处的元素返回
}

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

try {
// 删除的是最后一次返回的值，注意调用的是ArrayList的remove()方法
ArrayList.this.remove(lastRet);
cursor = lastRet; // 下一个元素的位置变成了lastRet
lastRet = -1; // 表明不能连续删除两次
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

@Override
@SuppressWarnings("unchecked")
// jdk1.8特有的，函数式接口Consumer ☆
public void forEachRemaining(Consumer<? super E> consumer) {
Objects.requireNonNull(consumer);
final int size = ArrayList.this.size;
int i = cursor; // 下一个元素的索引
if (i >= size) {
return;
}
final Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length) {
throw new ConcurrentModificationException();
}
// 当前位置后面的所有元素都交给了comsumer接口中的accept抽象方法
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
* 继承Itr，实现了ListIterator，多了一些previous等方法
* 要记得，iterator的操作，与当前指向的位置很重要，切记
*/
private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
super();
cursor = index;
}

public boolean hasPrevious() {
return cursor != 0; // cursor == 0说明没有前一个元素了
}

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();
// previous()方法会让cursor发生移动
cursor = i;
return (E) elementData[lastRet = i];
}

public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();

try {
// 调用的是外部类的set方法，复用
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后移，这个add可不是在尾端添加，只会在iterator指向的当前添加
cursor = i + 1;
// 说明add后不能够remove
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
}

/**
* 返回ArrayList的一部分
*/
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
// this就是subList的parent
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 + ")");
}

// 整个SubList类具有的方法和ArrayList类似
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;
}

// 会修改parent的数据
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;
}

// 会修改parent的数据
public void add(int index, E e) {
rangeCheckForAdd(index);
checkForComodification();
parent.add(parentOffset + index, e);
this.modCount = parent.modCount;
this.size++;
}

// 不知道为什么一会儿用ArrayList.this，一会儿用parent，这两者不是一样的吗？
public E remove(int index) {
rangeCheck(index);
checkForComodification();
E result = parent.remove(parentOffset + index);
this.modCount = parent.modCount;
this.size--;
return result;
}

// 会修改parent的数据
protected void removeRange(int fromIndex, int toIndex) {
checkForComodification();
parent.removeRange(parentOffset + fromIndex,
parentOffset + toIndex);
this.modCount = parent.modCount;
this.size -= toIndex - fromIndex;
}

// 会修改parent的数据
public boolean addAll(Collection<? extends E> c) {
return addAll(this.size, c);
}

// 会修改parent的数据
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)];
}

// 好像与ArrayList#forEachRemaining()不一样？☆
@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();
}
};
}

// SubList类中subList方法
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);
}
}

// forEachRemaining()的特例
@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> {

/*
* If ArrayLists were immutable, or structurally immutable (no
* adds, removes, etc), we could implement their spliterators
* with Arrays.spliterator. Instead we detect as much
* interference during traversal as practical without
* sacrificing much performance. We rely primarily on
* modCounts. These are not guaranteed to detect concurrency
* violations, and are sometimes overly conservative about
* within-thread interference, but detect enough problems to
* be worthwhile in practice. To carry this out, we (1) lazily
* initialize fence and expectedModCount until the latest
* point that we need to commit to the state we are checking
* against; thus improving precision.  (This doesn't apply to
* SubLists, that create spliterators with current non-lazy
* values).  (2) We perform only a single
* ConcurrentModificationException check at the end of forEach
* (the most performance-sensitive method). When using forEach
* (as opposed to iterators), we can normally only detect
* interference after actions, not before. Further
* CME-triggering checks apply to all other possible
* violations of assumptions for example null or too-small
* elementData array given its size(), that could only have
* occurred due to interference.  This allows the inner loop
* of forEach to run without any further checks, and
* simplifies lambda-resolution. While this does entail a
* number of checks, note that in the common case of
* list.stream().forEach(a), no checks or other computation
* occur anywhere other than inside forEach itself.  The other
* less-often-used methods cannot take advantage of most of
* these streamlinings.
*/

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;
}
}

// Predicate中 test方法返回boolean值，为true则remove
@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;
// 使用BitSet保存filter.test()为true的index，巧妙
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); // 如果符合过滤条件，则index位置为true
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; // 剩下的元素数目
for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
// 寻找从i开始第一个bit位为false的位置，为false，表明不需要删除，保存起来
i = removeSet.nextClearBit(i);
elementData[j] = elementData[i];
}
for (int k=newSize; k < size; k++) {
elementData[k] = null;  // 剩余的元素都设为null，垃圾回收
}
this.size = newSize;
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}

return anyToRemove;
}

// UnaryOperator中的apply方法应用到所有元素上
@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++;
}

// 利用Arrays工具类排序
@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++;
}
}

Summary☆☆☆☆☆

ArrayList#clone() 和 ArrayList#toArray()

package com.stephen.collection;
import java.util.ArrayList;
public class ArrayListTest {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
ArrayList<Person> al = new ArrayList<>();
al.add(new Person("zhangsan", 1));
al.add(new Person("lisi", 2));
al.add(new Person("wangwu", 3));
ArrayList<Person> al2 = (ArrayList<Person>) al.clone();
// Object[] al2 =  al.toArray();
System.out.println(al.get(0) == al2.get(0));
// System.out.println(al2[0] == al.get(0));
}
}
class Person {
private String name;
public Person(String name, int age) {
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Person [name=" + name + "]";
}
}
// output,shallow copy
true

Arrays#copyOf()和System#arraycopy()

/**
* copyOf()调用了arraycopy()
* copyOf()的参数是: 源数组，新数组长度，以及新数组中的元素类型；返回新数组
* arraycopy()的参数是: 源数组，源数组起始index，目的数组，目的数组index；无返回值
* 能用arraycopy()尽量用arraycopy()
*/
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
@SuppressWarnings("unchecked")
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}

SubList类

可以看出SubList类中很多方法都会修改parent List，所以使用的时候要小心。

FunctionalInterface

JDK8新增了FunctionalInterface，ArrayList中就使用了Predict, Consume, UnaryOperator等函数式编程接口。

modCount

在Iterator中定义了以下函数

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

可以看出modCount主要是用来检查在迭代器操作List的过程中，List的结构是否发生了结构性的变化，防止多个对象共同操作一个List，会抛出异常。

package com.stephen.collection;
import java.util.ArrayList;
import java.util.Iterator;
public class ArrayListTest {
public static void main(String[] args) {
ArrayList<String> al = new ArrayList<>();
al.add("one");
al.add("two");
al.add("three");
Iterator<String> iter = al.iterator();
while(iter.hasNext()) {
System.out.println(iter.next());
al.add("four");
}
}
}

//output

Exception in thread “main” java.util.ConcurrentModificationException

at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)

at java.util.ArrayList$Itr.next(ArrayList.java:851)

at com.stephen.collection.ArrayListTest.main(ArrayListTest.java:16)

one

forEachRemaining()

在SubList和ArrayList中这个方法最后的cursor和lastRet的值不一样，Why?

Spliterator

Quite BIG ? – 有待继续学习