类的方法(续)
(16)add方法
public boolean add(E e) {
// 确定ArrayList的容量大小(modCount++)
ensureCapacityInternal(size + 1);
// 添加 e 到 ArrayList 中,然后 size 自增 1
elementData[size++] = e;
return true;
}
注:size + 1是为保证资源空间不被浪费。
public void add(int index, E element) {
rangeCheckForAdd(index);
// modCount值会自增1
ensureCapacityInternal(size + 1);
//在原来 index 位置的值往后移动一位
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
其中rangeCheckForAdd方法的源码如下:
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
其中outOfBoundsMsg方法返回异常消息,用于传给 IndexOutOfBoundsException
源码如下:
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
(17)remove方法
public E remove(int index) {
// 越界检查
rangeCheck(index);
modCount++;
// 旧值
E oldValue = elementData(index);
// 需要左移的元素个数
int numMoved = size - index - 1;
if (numMoved > 0)
// 将源数组从下标为 index+1 开始的元素,拷贝到目标数组下标为 index的位置,一共拷贝numMoved个
System.arraycopy(elementData, index+1, elementData, index, numMoved);
// 将最后一个元素置空
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
注:该方法利用了 System.arraycopy() 进行左移一位的操作
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
// 判断是否存储了 null
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
// 遍历ArrayList,找到o,则删除,并返回true
for (int index = 0; index < size; index++)
// 利用 equals 判断两对象值是否相等
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
注:该方法删除的某个元素,若有多个相同值,则删除的是符合条件的结果中索引号最小的那个元素,若不包含要删除的元素,则返回 false,相比 remove(index),该方法并没有进行越界检查,即调用 rangeCheck()。
其中fastRemove方法的源码如下:
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
// 左移操作
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
//将最后一个元素设为null
elementData[--size] = null; // clear to let GC do its work
}
(18)clear方法(清空所有存储元素)
public void clear() {
//记录修改次数
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
注:该方法会将数组缓冲区所以元素置为 null,清空后,若打印 list,却只会看见一个 [], 而不是 [null, null, ….],这是因为迭代器进行了处理。
(19)addAll方法
public boolean addAll(Collection<? extends E> c) {
// 若 c 为 null,此行将抛出空指针异常
Object[] a = c.toArray();
// 要添加的元素个数
int numNew = a.length;
//modCount的值会自增1
ensureCapacityInternal(size + numNew);
//添加集合元素到list中
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
注:c不能null,否则会报NPE,由于ArrayList 是线程不安全的,该方法没有加锁,所以当一个线程正在将 c 中的元素加入 list 中,但同时有另一个线程在更改 c 中的元素,就会抛出ConcurrentModificationException(并发修改异常)
public boolean addAll(int index, Collection<? extends E> c) {
// 越界检查
rangeCheckForAdd(index);
// 若c为null,抛出NPE
Object[] a = c.toArray();
int numNew = a.length;
//modCount的值会自增1
ensureCapacityInternal(size + numNew); // Increments modCount
// 要移动的元素个数
int numMoved = size - index;
//先移动,然后拷贝(以免被覆盖)
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
注:该方法并不会覆盖掉在 index 位置原有的值(类似于 insert 方法)
(20)removeRange ( int fromIndex, int toIndex ) 方法 : 删除fromIndex到toIndex之间的全部元素
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
//需要移动元素的个数
int numMoved = size - toIndex;
//进行元素拷贝后,需要删除的几个元素就复制到了最后几个位置
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// clear to let GC do its work
// 删除后新的长度
int newSize = size - (toIndex-fromIndex);
// 将需要删除的元素(index在最后几个)置为 null
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}
注:该方法的区间:fromIndex(包括)和 toIndex(不包括),助记:左闭右开
(21)removeAll方法 (移除 list 中和 c 中共有的元素)
public boolean removeAll(Collection<?> c) {
// 当 c == null,则抛出NPE
Objects.requireNonNull(c);
//批量删除c
return batchRemove(c, false);
}
注:该方法是从类 java.util.AbstractCollection 继承的方法
批量删除batchRemove方法源码如下:
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;
}
(22)retainAll方法(只保留 list 和 集合 c 中公有的元素)
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
注:该方法是从类 java.util.AbstractCollection 继承的方法,与removeAll() 功能相反
(23)writeObject方法(写序列化)
private void writeObject(ObjectOutputStream s) throws IOException{
int expectedModCount = modCount;
s.defaultWriteObject();
// 写入ArrayList大小
s.writeInt(size);
// 写入存储的元素
for (int i=0; i<size; i++) {
s.writeObject(elementData[i]);
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
注:若在执行该方法的同时,有另一个线程也对elementData对象进行修改,这时候modCount(记录修改次数)的值会改变,所以就抛出ConcurrentModificationException。
(24)readObject方法(读序列化)
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
elementData = EMPTY_ELEMENTDATA;
// 读取s(包括隐藏的值)
s.defaultReadObject();
// 从输入流中读取ArrayList的size
s.readInt(); // ignored
if (size > 0) {
ensureCapacityInternal(size);
Object[] a = elementData;
// 从输入流中将所有的元素值读出
for (int i=0; i<size; i++) {
a[i] = s.readObject();
}
}
}
(25)listIterator方法
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}
注:该方法中创建的ListItr类是AbstractList.ListItr 的优化版本
源码如下:
private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
super();
cursor = index; // cursor 还是指向下一个返回元素的索引位置
}
//是否有上一个元素
public boolean hasPrevious() {
return cursor != 0;
}
//获取下一个元素的索引
public int nextIndex() {
return cursor;
}
//获取 cursor 前一个元素的索引(不是当前元素前一个的索引)
public int previousIndex() {
return cursor - 1;
}
//返回 cursor 的前一元素
@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];
}
//指定元素(将游标当前指向的索引位置的值设置为指定元素)
public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.set(lastRet, e);
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
//添加元素(在游标当前指向的索引位置插入一个元素)
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 ListIterator<E> listIterator() {
return new ListItr(0);
}
(26)iterator方法 (返回以恰当顺序在此列表的元素上进行迭代的迭代器)
public Iterator<E> iterator() {
return new Itr();
}
注:该方法返回一个 Iterator 迭代器,该迭代器是 fail-fast 机制的,Itr类是AbstractList.Itr类 的优化版本。
Itr类源码如下:
private class Itr implements Iterator<E> {
// 下一个返回元素的索引,默认值为 0
int cursor;
// 上一个返回元素的索引,若没有上一个元素,则为 -1。每次调用 remove(),lastRet 都会重置为 -1
int lastRet = -1;
int expectedModCount = modCount;
public boolean hasNext() {
// 判断是否有下一元素
return cursor != size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
// 临时变量 i,指向游标当前位置。
int i = cursor;
// 第一次检查
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
// 第二次检查
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);
// 移除元素
cursor = lastRet;
// 指针回移
//lastRet 不一定就等于 cursour - 1
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
// 非空判断
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();
}
}
注: Iterator 与ListIterator 的区别:ListIterator可以进行双向移动,而Iterator只能单向移动,ListIterator还可以添加元素(用add方法实现),而Iterator不能添加元素。
(27)subList方法 : 获取从 fromIndex 到 toIndex 之间的子集合(左闭右开)
public List<E> subList(int fromIndex, int toIndex) {
// 合法性检查
subListRangeCheck(fromIndex, toIndex, size);
return new SubList(this, 0, fromIndex, toIndex);
}
注:若 fromIndex == toIndex,则返回的空集合,对该子集合的操作,会影响原有集合
static void subListRangeCheck(int fromIndex, int toIndex, int size) {
//越界检查
if (fromIndex < 0)
throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
if (toIndex > size)
throw new IndexOutOfBoundsException("toIndex = " + toIndex);
//非法参数检查
if (fromIndex > toIndex)
throw new IllegalArgumentException("fromIndex(" + fromIndex +
") > toIndex(" + toIndex + ")");
}
private class SubList extends AbstractList<E> implements RandomAccess {
private final AbstractList<E> parent;
// 相对于父集合的偏移量(即fromIndex)
private final int parentOffset;
// 偏移量,默认是 0
private final int offset;
// SubList 中存储元素的个数
int size;
//因为属性是final的,所以在取子集合后,父集合不能删除 SubList 中的首个元素(offset不会变)
SubList(AbstractList<E> parent,
int offset, int fromIndex, int toIndex) {
// 子集合的处理,仅给出了一个映射到父集合相应区间的引用
this.parent = parent;
this.parentOffset = fromIndex;
this.offset = offset + fromIndex;
this.size = toIndex - fromIndex;
this.modCount = ArrayList.this.modCount;
}
// 设置新值,返回旧值
public E set(int index, E e) {
rangeCheck(index);
// 越界检查
checkForComodification();
E oldValue = ArrayList.this.elementData(offset + index);
ArrayList.this.elementData[offset + index] = e;
return oldValue;
}
// 取值
public E get(int index) {
rangeCheck(index);
// 越界检查
checkForComodification();
return ArrayList.this.elementData(offset + index);
}
//获取SubList大小
public int size() {
checkForComodification();
return this.size;
}
// 添加元素
public void add(int index, E e) {
rangeCheckForAdd(index);
checkForComodification();
// 对子类添加元素,是直接操作父类添加的
parent.add(parentOffset + index, e);
this.modCount = parent.modCount;
this.size++;
}
// 删除元素
public E remove(int index) {
rangeCheck(index);
checkForComodification();
// 对子类删除元素,是直接操作父类删除的
E result = parent.remove(parentOffset + index);
this.modCount = parent.modCount;
this.size--;
return result;
}
// 范围删除
protected void removeRange(int fromIndex, int toIndex) {
checkForComodification();
parent.removeRange(parentOffset + fromIndex,
parentOffset + toIndex);
this.modCount = parent.modCount;
this.size -= toIndex - fromIndex;
}
//将一个集合的所有元素顺序添加到 lits 末尾
public boolean addAll(Collection<? extends E> c) {
return addAll(this.size, c);
}
//从 index 位置开始,将集合 c 中的元素添加到ArrayList
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;
}
//返回一个Iterator迭代器,但底层实现的是ListIterator
public Iterator<E> iterator() {
return listIterator();
}
// 返回一个 ListIterator迭代器
public ListIterator<E> listIterator(final int index) {
checkForComodification();
// 越界检查(使用了add方法专用的越界检查方法)
rangeCheckForAdd(index);
final int offset = this.offset;
// 匿名内部类
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);
}
}
注:SubList仅给出了一个映射到父集合相应区间的引用。
(28)forEach方法(用于函数式编程)
@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();
}
}
(29)spliterator方法 (获取一个分割器)
@Override
public Spliterator<E> spliterator() {
return new ArrayListSpliterator<>(this, 0, -1, 0);
}
分割器类ArrayListSpliterator的源码如下:
// 基于索引的、二分的、懒加载的分割器
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;
}
}
(30)removeIf方法
@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;
}
(31)replaceAll方法
@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++;
}
(32)sort方法
@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++;
}
(1) ArrayList 基于数组实现,其内存储元素的数组为 elementData,声明为:transient Object[] elementData;
(2) ArrayList 中EMPTY_ELEMENTDATA 和 DEFAULTCAPACITY_EMPTY._ELEMENTDATA 的使用
这两个常量,使用场景不同。前者是用在用户通过 ArrayList(int initialCapacity) 该构造方法直接指定初试容量为 0 时,后者是用户直接使用无参构造创建 ArrayList 时。
(3) ArrayList 默认容量为 10。
(4) ArrayList 的扩容计算为 newCapacity = oldCapacity + (oldCapacity >> 1); 且扩容并非是无限制的,有内存限制、虚拟机限制。
(5) ArrayList 的 toArray() 方法和 subList() 方法,在源数据和子数据之间的区别
toArray():对该方法返回的数组,进行操作(增删改查)都不会影响源数据(ArrayList中elementData)。二者之间是不会相互影响的
subList():对返回的子集合,进行操作(增删改查)都会影响父集合。而且若是对父集合中进行删除操作(仅仅在删除子集合的首个元素)时,会抛出异常 java.util.ConcurrentModificationException
(6) ArrayList 中涉及到用 index 进行访问,就需要进行越界检查。
本人才疏学浅,若有错,请指出
谢谢!