ArrayList 源码精读（JDK 21 中文注释版）
返回阅读本页时，可以重点留意：
- 与 JDK 8/11 相比，
ArrayList在 JDK 21 中新增的 API（例如getFirst/getLast/addFirst/addLast等）； - 使用
ArraysSupport、SharedSecrets等内部工具类带来的实现差异； removeIf/replaceAll/spliterator等“新风格”集合 API 的实现方式。
/*
* 本文件基于 JDK 21 的 ArrayList 源码，加入中文注释，帮助理解实现原理。
* 代码结构与官方源码保持一致，仅在关键位置补充中文说明。
*/
package java.util;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
import jdk.internal.access.SharedSecrets;
import jdk.internal.util.ArraysSupport;
/**
* ArrayList：基于可扩容数组实现的 {@code List}。
*
* 核心特性：
* - 底层使用 Object[] 顺序存储元素，支持 O(1) 的随机访问（get/set）。
* - 支持动态扩容，默认容量 10，扩容策略通过 {@link ArraysSupport#newLength} 计算。
* - 非线程安全，多线程并发修改必须外部同步或使用 Collections.synchronizedList。
* - 迭代器 / Spliterator / forEach / removeIf / replaceAll / sort 都是 fail-fast 的，
* 通过 modCount 检测结构性修改。
*
* JDK21 主要新增/变化：
* - 基于 {@link ArraysSupport#newLength} 的统一扩容策略。
* - 新增 Deque 风格方法：getFirst/getLast、addFirst/addLast、removeFirst/removeLast。
* - SubList 的 Spliterator 实现更严谨，显式说明对 SubList 的延迟绑定语义。
*/
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
@java.io.Serial
private static final long serialVersionUID = 8683452581122892189L;
/**
* 默认初始容量：10
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 共享空数组：用于“显式容量为 0”的 ArrayList（如 new ArrayList(0)）。
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 默认空数组：用于无参构造，首次添加元素时才扩容到 DEFAULT_CAPACITY。
* 与 EMPTY_ELEMENTDATA 区分开，主要是为了知道“第一次扩容扩到多少”。
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* 存储元素的真实数组。
* - 容量 = elementData.length
* - 当 elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA 且首次添加元素时，会被扩容到 DEFAULT_CAPACITY。
*
* 非 private，方便内部类（SubList、ArrayListSpliterator）直接访问。
*/
transient Object[] elementData;
/**
* 实际元素个数（逻辑大小）
*
* @serial
*/
private int size;
// ====================== 构造方法 ======================
/**
* 指定初始容量的构造方法。
*
* - initialCapacity > 0：直接 new Object[initialCapacity]
* - initialCapacity == 0：使用 EMPTY_ELEMENTDATA
* - initialCapacity < 0：抛 IllegalArgumentException
*/
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);
}
}
/**
* 无参构造：延迟分配真实容量，首次 add 时再扩容到 DEFAULT_CAPACITY。
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 用另一个集合 c 的全部元素构造 ArrayList。
*
* 注意：
* - 先调用 c.toArray() 得到 Object[]；
* - 若 c 本身是 ArrayList，则直接复用其数组（浅拷贝语义）；
* - 否则复制为 Object[]；
* - 若 size == 0，则退回到 EMPTY_ELEMENTDATA。
*/
public ArrayList(Collection<? extends E> c) {
Object[] a = c.toArray();
if ((size = a.length) != 0) {
if (c.getClass() == ArrayList.class) {
elementData = a;
} else {
elementData = Arrays.copyOf(a, size, Object[].class);
}
} else {
elementData = EMPTY_ELEMENTDATA;
}
}
// ====================== 容量管理 ======================
/**
* trimToSize：将容量裁剪到当前 size，释放多余空间。
*/
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
/**
* ensureCapacity：如有必要，增加容量到不少于 minCapacity。
*
* 条件：minCapacity > elementData.length 且
* 不属于“默认空表但 minCapacity <= 默认容量”的情况。
*/
public void ensureCapacity(int minCapacity) {
if (minCapacity > elementData.length
&& !(elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
&& minCapacity <= DEFAULT_CAPACITY)) {
modCount++;
grow(minCapacity);
}
}
/**
* grow(minCapacity)：扩容，保证容量至少为 minCapacity。
*
* 使用 ArraysSupport.newLength 统一计算扩容后的大小：
* - oldLength：旧容量
* - minGrowth：至少需要增长的数量（minCapacity - oldCapacity）
* - prefGrowth：偏好增长（oldCapacity >> 1，即 1.5 倍策略）
*
* 若当前是“默认空表”，则直接分配 max(DEFAULT_CAPACITY, minCapacity)。
*/
private Object[] grow(int minCapacity) {
int oldCapacity = elementData.length;
if (oldCapacity > 0 || elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
int newCapacity = ArraysSupport.newLength(oldCapacity,
minCapacity - oldCapacity, /* minimum growth */
oldCapacity >> 1 /* preferred growth */);
return elementData = Arrays.copyOf(elementData, newCapacity);
} else {
return elementData = new Object[Math.max(DEFAULT_CAPACITY, minCapacity)];
}
}
/**
* grow() 无参版本：默认按 size+1 来计算最小容量，多用于 add 时自动扩容。
*/
private Object[] grow() {
return grow(size + 1);
}
// ====================== 基本查询 ======================
public int size() {
return size;
}
public boolean isEmpty() {
return size == 0;
}
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
/**
* indexOf：从前往后查找第一次出现的位置。
*/
public int indexOf(Object o) {
return indexOfRange(o, 0, size);
}
/**
* indexOfRange：在 [start, end) 区间查找元素 o。
* 为支持 SubList 等内部调用而抽出的通用方法。
*/
int indexOfRange(Object o, int start, int end) {
Object[] es = elementData;
if (o == null) {
for (int i = start; i < end; i++) {
if (es[i] == null) {
return i;
}
}
} else {
for (int i = start; i < end; i++) {
if (o.equals(es[i])) {
return i;
}
}
}
return -1;
}
/**
* lastIndexOf：从后往前查找最后一次出现的位置。
*/
public int lastIndexOf(Object o) {
return lastIndexOfRange(o, 0, size);
}
/**
* lastIndexOfRange：在 [start, end) 区间倒序查找。
*/
int lastIndexOfRange(Object o, int start, int end) {
Object[] es = elementData;
if (o == null) {
for (int i = end - 1; i >= start; i--) {
if (es[i] == null) {
return i;
}
}
} else {
for (int i = end - 1; i >= start; i--) {
if (o.equals(es[i])) {
return i;
}
}
}
return -1;
}
/**
* clone：浅拷贝，复制结构和底层数组，但不复制内部元素对象。
*/
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);
}
}
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
// ====================== 按索引访问/修改 ======================
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}
@SuppressWarnings("unchecked")
static <E> E elementAt(Object[] es, int index) {
return (E) es[index];
}
public E get(int index) {
Objects.checkIndex(index, size);
return elementData(index);
}
/**
* 新增：获取第一个元素，空列表时抛 NoSuchElementException。
*/
public E getFirst() {
if (size == 0) {
throw new NoSuchElementException();
} else {
return elementData(0);
}
}
/**
* 新增：获取最后一个元素，空列表时抛 NoSuchElementException。
*/
public E getLast() {
int last = size - 1;
if (last < 0) {
throw new NoSuchElementException();
} else {
return elementData(last);
}
}
public E set(int index, E element) {
Objects.checkIndex(index, size);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
/**
* add(E e) 的核心逻辑拆分，便于 JIT 内联优化，减少方法体字节码大小。
*/
private void add(E e, Object[] elementData, int s) {
if (s == elementData.length)
elementData = grow();
elementData[s] = e;
size = s + 1;
}
/**
* 尾插一个元素。
*/
public boolean add(E e) {
modCount++;
add(e, elementData, size);
return true;
}
/**
* 在指定下标 index 插入一个元素，后面的元素整体右移一位。
*/
public void add(int index, E element) {
rangeCheckForAdd(index);
modCount++;
final int s;
Object[] elementData;
if ((s = size) == (elementData = this.elementData).length)
elementData = grow();
System.arraycopy(elementData, index,
elementData, index + 1,
s - index);
elementData[index] = element;
size = s + 1;
}
/**
* 新增：在头部插入元素（等价于 add(0, e)）。
*/
public void addFirst(E element) {
add(0, element);
}
/**
* 新增：在尾部插入元素（等价于 add(e)）。
*/
public void addLast(E element) {
add(element);
}
/**
* 删除指定下标的元素，并返回旧值。
*/
public E remove(int index) {
Objects.checkIndex(index, size);
final Object[] es = elementData;
@SuppressWarnings("unchecked") E oldValue = (E) es[index];
fastRemove(es, index);
return oldValue;
}
/**
* 新增：删除第一个元素，空列表时抛 NoSuchElementException。
*/
public E removeFirst() {
if (size == 0) {
throw new NoSuchElementException();
} else {
Object[] es = elementData;
@SuppressWarnings("unchecked") E oldValue = (E) es[0];
fastRemove(es, 0);
return oldValue;
}
}
/**
* 新增：删除最后一个元素，空列表时抛 NoSuchElementException。
*/
public E removeLast() {
int last = size - 1;
if (last < 0) {
throw new NoSuchElementException();
} else {
Object[] es = elementData;
@SuppressWarnings("unchecked") E oldValue = (E) es[last];
fastRemove(es, last);
return oldValue;
}
}
// ====================== equals / hashCode（支持 fail-fast） ======================
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (!(o instanceof List)) {
return false;
}
final int expectedModCount = modCount;
boolean equal = (o.getClass() == ArrayList.class)
? equalsArrayList((ArrayList<?>) o)
: equalsRange((List<?>) o, 0, size);
checkForComodification(expectedModCount);
return equal;
}
boolean equalsRange(List<?> other, int from, int to) {
final Object[] es = elementData;
if (to > es.length) {
throw new ConcurrentModificationException();
}
var oit = other.iterator();
for (; from < to; from++) {
if (!oit.hasNext() || !Objects.equals(es[from], oit.next())) {
return false;
}
}
return !oit.hasNext();
}
private boolean equalsArrayList(ArrayList<?> other) {
final int otherModCount = other.modCount;
final int s = size;
boolean equal;
if (equal = (s == other.size)) {
final Object[] otherEs = other.elementData;
final Object[] es = elementData;
if (s > es.length || s > otherEs.length) {
throw new ConcurrentModificationException();
}
for (int i = 0; i < s; i++) {
if (!Objects.equals(es[i], otherEs[i])) {
equal = false;
break;
}
}
}
other.checkForComodification(otherModCount);
return equal;
}
private void checkForComodification(final int expectedModCount) {
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
public int hashCode() {
int expectedModCount = modCount;
int hash = hashCodeRange(0, size);
checkForComodification(expectedModCount);
return hash;
}
int hashCodeRange(int from, int to) {
final Object[] es = elementData;
if (to > es.length) {
throw new ConcurrentModificationException();
}
int hashCode = 1;
for (int i = from; i < to; i++) {
Object e = es[i];
hashCode = 31 * hashCode + (e == null ? 0 : e.hashCode());
}
return hashCode;
}
// ====================== 删除 / 清空 ======================
/**
* 删除第一次出现的指定元素。
*/
public boolean remove(Object o) {
final Object[] es = elementData;
final int size = this.size;
int i = 0;
found: {
if (o == null) {
for (; i < size; i++)
if (es[i] == null)
break found;
} else {
for (; i < size; i++)
if (o.equals(es[i]))
break found;
}
return false;
}
fastRemove(es, i);
return true;
}
/**
* fastRemove：内部快速删除实现，不做下标检查。
*/
private void fastRemove(Object[] es, int i) {
modCount++;
final int newSize;
if ((newSize = size - 1) > i)
System.arraycopy(es, i + 1, es, i, newSize - i);
es[size = newSize] = null;
}
public void clear() {
modCount++;
final Object[] es = elementData;
for (int to = size, i = size = 0; i < to; i++)
es[i] = null;
}
// ====================== addAll / removeRange ======================
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
modCount++;
int numNew = a.length;
if (numNew == 0)
return false;
Object[] elementData;
final int s;
if (numNew > (elementData = this.elementData).length - (s = size))
elementData = grow(s + numNew);
System.arraycopy(a, 0, elementData, s, numNew);
size = s + numNew;
return true;
}
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
modCount++;
int numNew = a.length;
if (numNew == 0)
return false;
Object[] elementData;
final int s;
if (numNew > (elementData = this.elementData).length - (s = size))
elementData = grow(s + numNew);
int numMoved = s - index;
if (numMoved > 0)
System.arraycopy(elementData, index,
elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size = s + numNew;
return true;
}
/**
* 删除区间 [fromIndex, toIndex) 的元素。
*/
protected void removeRange(int fromIndex, int toIndex) {
if (fromIndex > toIndex) {
throw new IndexOutOfBoundsException(
outOfBoundsMsg(fromIndex, toIndex));
}
modCount++;
shiftTailOverGap(elementData, fromIndex, toIndex);
}
/**
* shiftTailOverGap：把 [hi, size) 区间整体左移到 lo，并清空尾部。
*/
private void shiftTailOverGap(Object[] es, int lo, int hi) {
System.arraycopy(es, hi, es, lo, size - hi);
for (int to = size, i = (size -= hi - lo); i < to; i++)
es[i] = null;
}
// ====================== 下标检查与错误消息 ======================
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
private static String outOfBoundsMsg(int fromIndex, int toIndex) {
return "From Index: " + fromIndex + " > To Index: " + toIndex;
}
// ====================== 批量删除 / retainAll ======================
public boolean removeAll(Collection<?> c) {
return batchRemove(c, false, 0, size);
}
public boolean retainAll(Collection<?> c) {
return batchRemove(c, true, 0, size);
}
/**
* 批量删除/保留的核心实现：
* - complement = false：删除在 c 中的元素；
* - complement = true ：仅保留在 c 中的元素。
*/
boolean batchRemove(Collection<?> c, boolean complement,
final int from, final int end) {
Objects.requireNonNull(c);
final Object[] es = elementData;
int r;
// 优化：先跳过前面一段“幸存者”
for (r = from;; r++) {
if (r == end)
return false;
if (c.contains(es[r]) != complement)
break;
}
int w = r++;
try {
for (Object e; r < end; r++)
if (c.contains(e = es[r]) == complement)
es[w++] = e;
} catch (Throwable ex) {
// 行为保持与 AbstractCollection 一致，即便 c.contains 抛异常
System.arraycopy(es, r, es, w, end - r);
w += end - r;
throw ex;
} finally {
modCount += end - w;
shiftTailOverGap(es, w, end);
}
return true;
}
// ====================== 自定义序列化 ======================
@java.io.Serial
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
int expectedModCount = modCount;
s.defaultWriteObject();
// 写出 size 作为“容量”，与 clone 行为保持一致
s.writeInt(size);
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
@java.io.Serial
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// 读取 size、modCount 等字段
s.defaultReadObject();
// 读取 capacity（忽略），仅为兼容序列化协议
s.readInt(); // ignored
if (size > 0) {
// 根据 size 分配数组，而不是 capacity
SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, size);
Object[] elements = new Object[size];
for (int i = 0; i < size; i++) {
elements[i] = s.readObject();
}
elementData = elements;
} else if (size == 0) {
elementData = EMPTY_ELEMENTDATA;
} else {
throw new java.io.InvalidObjectException("Invalid size: " + size);
}
}
// ====================== 迭代器 ======================
public ListIterator<E> listIterator(int index) {
rangeCheckForAdd(index);
return new ListItr(index);
}
public ListIterator<E> listIterator() {
return new ListItr(0);
}
public Iterator<E> iterator() {
return new Itr();
}
/**
* Itr：单向迭代器。
*/
private class Itr implements Iterator<E> {
int cursor; // 下一个元素的下标
int lastRet = -1; // 上一个返回元素的下标
int expectedModCount = modCount;
Itr() {}
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
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
final int size = ArrayList.this.size;
int i = cursor;
if (i < size) {
final Object[] es = elementData;
if (i >= es.length)
throw new ConcurrentModificationException();
for (; i < size && modCount == expectedModCount; i++)
action.accept(elementAt(es, i));
cursor = i;
lastRet = i - 1;
checkForComodification();
}
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
/**
* ListItr：双向迭代器，在 Itr 的基础上增加 previous / set / add 等方法。
*/
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();
}
}
}
// ====================== SubList 视图 ======================
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList<>(this, fromIndex, toIndex);
}
private static class SubList<E> extends AbstractList<E> implements RandomAccess {
private final ArrayList<E> root;
private final SubList<E> parent;
private final int offset;
private int size;
/**
* 构造基于 ArrayList 的根 SubList。
*/
public SubList(ArrayList<E> root, int fromIndex, int toIndex) {
this.root = root;
this.parent = null;
this.offset = fromIndex;
this.size = toIndex - fromIndex;
this.modCount = root.modCount;
}
/**
* 构造基于另一个 SubList 的嵌套 SubList。
*/
private SubList(SubList<E> parent, int fromIndex, int toIndex) {
this.root = parent.root;
this.parent = parent;
this.offset = parent.offset + fromIndex;
this.size = toIndex - fromIndex;
this.modCount = parent.modCount;
}
public E set(int index, E element) {
Objects.checkIndex(index, size);
checkForComodification();
E oldValue = root.elementData(offset + index);
root.elementData[offset + index] = element;
return oldValue;
}
public E get(int index) {
Objects.checkIndex(index, size);
checkForComodification();
return root.elementData(offset + index);
}
public int size() {
checkForComodification();
return size;
}
public void add(int index, E element) {
rangeCheckForAdd(index);
checkForComodification();
root.add(offset + index, element);
updateSizeAndModCount(1);
}
public E remove(int index) {
Objects.checkIndex(index, size);
checkForComodification();
E result = root.remove(offset + index);
updateSizeAndModCount(-1);
return result;
}
protected void removeRange(int fromIndex, int toIndex) {
checkForComodification();
root.removeRange(offset + fromIndex, offset + toIndex);
updateSizeAndModCount(fromIndex - toIndex);
}
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();
root.addAll(offset + index, c);
updateSizeAndModCount(cSize);
return true;
}
public void replaceAll(UnaryOperator<E> operator) {
root.replaceAllRange(operator, offset, offset + size);
}
public boolean removeAll(Collection<?> c) {
return batchRemove(c, false);
}
public boolean retainAll(Collection<?> c) {
return batchRemove(c, true);
}
private boolean batchRemove(Collection<?> c, boolean complement) {
checkForComodification();
int oldSize = root.size;
boolean modified =
root.batchRemove(c, complement, offset, offset + size);
if (modified)
updateSizeAndModCount(root.size - oldSize);
return modified;
}
public boolean removeIf(Predicate<? super E> filter) {
checkForComodification();
int oldSize = root.size;
boolean modified = root.removeIf(filter, offset, offset + size);
if (modified)
updateSizeAndModCount(root.size - oldSize);
return modified;
}
public Object[] toArray() {
checkForComodification();
return Arrays.copyOfRange(root.elementData, offset, offset + size);
}
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
checkForComodification();
if (a.length < size)
return (T[]) Arrays.copyOfRange(
root.elementData, offset, offset + size, a.getClass());
System.arraycopy(root.elementData, offset, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (!(o instanceof List)) {
return false;
}
boolean equal = root.equalsRange((List<?>)o, offset, offset + size);
checkForComodification();
return equal;
}
public int hashCode() {
int hash = root.hashCodeRange(offset, offset + size);
checkForComodification();
return hash;
}
public int indexOf(Object o) {
int index = root.indexOfRange(o, offset, offset + size);
checkForComodification();
return index >= 0 ? index - offset : -1;
}
public int lastIndexOf(Object o) {
int index = root.lastIndexOfRange(o, offset, offset + size);
checkForComodification();
return index >= 0 ? index - offset : -1;
}
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
public Iterator<E> iterator() {
return listIterator();
}
public ListIterator<E> listIterator(int index) {
checkForComodification();
rangeCheckForAdd(index);
return new ListIterator<E>() {
int cursor = index;
int lastRet = -1;
int expectedModCount = SubList.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 = root.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 = root.elementData;
if (offset + i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[offset + (lastRet = i)];
}
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
final int size = SubList.this.size;
int i = cursor;
if (i < size) {
final Object[] es = root.elementData;
if (offset + i >= es.length)
throw new ConcurrentModificationException();
for (; i < size && root.modCount == expectedModCount; i++)
action.accept(elementAt(es, offset + i));
cursor = i;
lastRet = i - 1;
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 = SubList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
root.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 = SubList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
final void checkForComodification() {
if (root.modCount != expectedModCount)
throw new ConcurrentModificationException();
}
};
}
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new SubList<>(this, fromIndex, toIndex);
}
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 (root.modCount != modCount)
throw new ConcurrentModificationException();
}
/**
* 更新当前 SubList 以及所有祖先 SubList 的 size/modCount。
*/
private void updateSizeAndModCount(int sizeChange) {
SubList<E> slist = this;
do {
slist.size += sizeChange;
slist.modCount = root.modCount;
slist = slist.parent;
} while (slist != null);
}
public Spliterator<E> spliterator() {
checkForComodification();
// 对 SubList 进行延迟绑定的 Spliterator：绑定的是 SubList 而不是根 ArrayList
return new Spliterator<E>() {
private int index = offset; // 当前下标
private int fence = -1; // -1 表示尚未绑定，首次使用时绑定
private int expectedModCount;
private int getFence() {
int hi;
if ((hi = fence) < 0) {
expectedModCount = modCount;
hi = fence = offset + size;
}
return hi;
}
public ArrayList<E>.ArrayListSpliterator trySplit() {
int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
// 这里可以安全地使用根 ArrayList 的 ArrayListSpliterator
return (lo >= mid) ? null :
root.new ArrayListSpliterator(lo, index = mid, expectedModCount);
}
public boolean tryAdvance(Consumer<? super E> action) {
Objects.requireNonNull(action);
int hi = getFence(), i = index;
if (i < hi) {
index = i + 1;
@SuppressWarnings("unchecked") E e = (E)root.elementData[i];
action.accept(e);
if (root.modCount != expectedModCount)
throw new ConcurrentModificationException();
return true;
}
return false;
}
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
int i, hi, mc;
ArrayList<E> lst = root;
Object[] a;
if ((a = lst.elementData) != null) {
if ((hi = fence) < 0) {
mc = modCount;
hi = offset + 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 getFence() - index;
}
public int characteristics() {
return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
}
};
}
}
// ====================== JDK8+ Lambda 增强 ======================
@Override
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
final int expectedModCount = modCount;
final Object[] es = elementData;
final int size = this.size;
for (int i = 0; modCount == expectedModCount && i < size; i++)
action.accept(elementAt(es, i));
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
@Override
public Spliterator<E> spliterator() {
return new ArrayListSpliterator(0, -1, 0);
}
/**
* ArrayListSpliterator：基于下标的延迟绑定 Spliterator，用于 stream 遍历。
*/
final class ArrayListSpliterator implements Spliterator<E> {
private int index; // 当前索引
private int fence; // -1 表示尚未绑定，首次使用后变为 size
private int expectedModCount;
ArrayListSpliterator(int origin, int fence, int expectedModCount) {
this.index = origin;
this.fence = fence;
this.expectedModCount = expectedModCount;
}
private int getFence() {
int hi;
if ((hi = fence) < 0) {
expectedModCount = modCount;
hi = fence = size;
}
return hi;
}
public ArrayListSpliterator trySplit() {
int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
return (lo >= mid) ? null :
new ArrayListSpliterator(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)elementData[i];
action.accept(e);
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
return true;
}
return false;
}
public void forEachRemaining(Consumer<? super E> action) {
int i, hi, mc;
Object[] a;
if (action == null)
throw new NullPointerException();
if ((a = elementData) != null) {
if ((hi = fence) < 0) {
mc = modCount;
hi = 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 (modCount == mc)
return;
}
}
throw new ConcurrentModificationException();
}
public long estimateSize() {
return getFence() - index;
}
public int characteristics() {
return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
}
}
// ====================== removeIf / replaceAll / sort ======================
// 一个简单的 bitset 实现，用 long[] 表示要删除的元素位置
private static long[] nBits(int n) {
return new long[((n - 1) >> 6) + 1];
}
private static void setBit(long[] bits, int i) {
bits[i >> 6] |= 1L << i;
}
private static boolean isClear(long[] bits, int i) {
return (bits[i >> 6] & (1L << i)) == 0;
}
@Override
public boolean removeIf(Predicate<? super E> filter) {
return removeIf(filter, 0, size);
}
/**
* removeIf：在区间 [i, end) 上删除所有满足 filter 的元素。
*
* 两阶段策略：
* 1）扫描并用 bitset 标记要删的元素，期间如果 filter 抛异常，集合不变；
* 2）根据 bitset 将未删除的元素向左压缩覆盖。
*/
boolean removeIf(Predicate<? super E> filter, int i, final int end) {
Objects.requireNonNull(filter);
int expectedModCount = modCount;
final Object[] es = elementData;
// 优化：先跳过前面一段不需要删除的元素
for (; i < end && !filter.test(elementAt(es, i)); i++)
;
if (i < end) {
final int beg = i;
final long[] deathRow = nBits(end - beg);
deathRow[0] = 1L; // bit 0 表示 beg 要删除
for (i = beg + 1; i < end; i++)
if (filter.test(elementAt(es, i)))
setBit(deathRow, i - beg);
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
modCount++;
int w = beg;
for (i = beg; i < end; i++)
if (isClear(deathRow, i - beg))
es[w++] = es[i];
shiftTailOverGap(es, w, end);
return true;
} else {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
return false;
}
}
@Override
public void replaceAll(UnaryOperator<E> operator) {
replaceAllRange(operator, 0, size);
// TODO(8203662): 将来可能去掉这里额外的 modCount++
modCount++;
}
/**
* replaceAllRange：在 [i, end) 区间对每个元素执行 operator。
*/
private void replaceAllRange(UnaryOperator<E> operator, int i, int end) {
Objects.requireNonNull(operator);
final int expectedModCount = modCount;
final Object[] es = elementData;
for (; modCount == expectedModCount && i < end; i++)
es[i] = operator.apply(elementAt(es, i));
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
@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++;
}
void checkInvariants() {
// 用于内部调试不变式：
// assert size >= 0;
// assert size == elementData.length || elementData[size] == null;
}
}