Arraylist介绍
ArrayList数据结构
ArrayList源码解析
ArrayList遍历方式
toArray（）异常
第一部分：Arraylist介绍
- ArrayList 的简介
ArrayList是一个数组队列，底层使用的数据结构存储数据就是数组，相当于动态数组。与Java中的数组相比，它的容量能动态扩容，它继承于AbstractList，实现了List, RandomAccess, Cloneable, java.io.Serializable这些接口。
ArrayList 继承了AbstractList，实现了List。它是一个数组队列，提供了相关的添加、删除、修改、遍历等功能。
ArrayList 实现了RandmoAccess接口，即提供了随机访问功能。RandmoAccess是java中用来被List实现，为List提供快速访问功能的。在ArrayList中，我们即可以通过元素的序号快速获取元素对象；这就是快速随机访问。稍后，我们会比较List的“快速随机访问”和“通过Iterator迭代器访问”的效率。
ArrayList 实现了Cloneable接口，即覆盖了函数clone()，能被克隆。因为想要使用Object的clone（）方法，必须要实现Cloneable接口。
ArrayList 实现java.io.Serializable接口，这意味着ArrayList支持序列化，能通过序列化去传输。
ArrayList和Vector不同，Arraylist中的操作是线程不安全的！当然线程不安全都是针对于多线程来讲才会出现，所以建议在单线程的情况下才使用Arraylist，如果在多线程情况下使用，要在业务当中保证ArrayList的线程安全。在多线程的情况下可以使用Vector和CopyWriteArrayList，后续会讲解为什么要在多线程情况下使用Vector或者CopyWriteArrayList。
- ArrayList 的构造函数:
// 默认构造函数
ArrayList()
// capacity是ArrayList的默认容量大小。当由于增加数据导致容量不足时，容量会添加上一次容量大小的一半。
ArrayList(int capacity)
// 创建一个包含collection的ArrayList
ArrayList(Collection<? extends E> collection)- ArrayList的API
// Collection中定义的API
boolean add(E object)
boolean addAll(Collection<? extends E> collection)
void clear()
boolean contains(Object object)
boolean containsAll(Collection<?> collection)
boolean equals(Object object)
int hashCode()
boolean isEmpty()
Iterator<E> iterator()
boolean remove(Object object)
boolean removeAll(Collection<?> collection)
boolean retainAll(Collection<?> collection)
int size()
<T> T[] toArray(T[] array)
Object[] toArray()
// AbstractCollection中定义的API
void add(int location, E object)
boolean addAll(int location, Collection<? extends E> collection)
E get(int location)
int indexOf(Object object)
int lastIndexOf(Object object)
ListIterator<E> listIterator(int location)
ListIterator<E> listIterator()
E remove(int location)
E set(int location, E object)
List<E> subList(int start, int end)
// ArrayList新增的API
Object clone()
void ensureCapacity(int minimumCapacity)
void trimToSize()
void removeRange(int fromIndex, int toIndex)第二部分 ArrayList数据结构
- ArrayList的继承关系
java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractList<E>
java.util.ArrayList<E>
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable {}- ArrayList和Collection的关系如下图所示：
ArrayList包含了两个重要的对象：elementData和size
1.elementData是Object[]类型的数组。它保存了添加到ArrayList中的元素。实际上，elementData是个动态数组，我们可以通过构造函数Arraylist（int initialCapacity）来执行它的初始化容量；如果通过有无参构造Arrlist()来进行创建list，则elementData的初始化容量设置为10.elementData数组的大小会根据ArrayList容量的增长而动态的增长，具体的增长方式，可以参考下面提供的源码分析的ensureCapacity()方法；
2.size则是动态数组的实际大小，也就是elementData的实际大小（实际长度）
第三部分：ArrayList源码解析
这一部分也是比较重要的一部分，为了更了解Arraylist的原理，以及作者的思想，对源码做出分析。ArrayList的底层数据结构是数组，通过数组进行实现的，从看源码还是比较容易去理解的。
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
private static final long serialVersionUID = 8683452581122892189L;
//1.RandomAccess：（标记接口）代表支持随机访问
// 2.Cloneable：（标记接口）代表 Object.clone() 方法可以合法地对该类实例进行按字段复制。（
//3.没有实现 Cloneable 接口的实例上调用 Object 的 clone 方法，则会导致抛出 CloneNotSupportedException 异常）
//4.java.io.Serializable（标记接口）
/**
* 默认初始化的容量
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 指定该ArrayList容量为0时，返回该空数组
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 用于默认大小的空实例的共享空数组实例。
* 这个空数组的实例用来给无参构造使用。当调用无参构造方法，返回的是该数组。
* 将此与EMPTY_ELEMENTDATA区分开来，以便了解在添加第一个元素时要增加多少容量。
* Shared empty array instance used for default sized empty instances. We
* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
* first element is added.
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* 存储ArrayList元素的数组缓冲区
* ArrayList的容量(capacity)就是是此数组缓冲区的长度。
* 声明为transient 不会被序列化
* 非私有 是为了方便内部类调用
* <p>
* <p>
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* will be expanded to DEFAULT_CAPACITY when the first element is added.
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;
/**
* 构造具有指定初始容量的空列表
* Constructs an empty list with the specified initial capacity.
* initialCapacity列表的初始容量
*
* @param initialCapacity the initial capacity of the list
* 如果指定的值是负数则抛出异常
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: " +
initialCapacity);
}
}
/**
* 构造一个初始容量为10的空列表
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 构造一个包含指定集合元素的列表
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
* @param c the collection whose elements are to be placed into this list
* @throws NullPointerException if the specified collection is null
*/
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;
}
}
/**
* Trims the capacity of this <tt>ArrayList</tt> instance to be the
* list's current size. An application can use this operation to minimize
* the storage of an <tt>ArrayList</tt> instance.
*/
//将当前容量设为等于实际元素个数
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
/**
* Increases the capacity of this <tt>ArrayList</tt> instance, if
* necessary, to ensure that it can hold at least the number of elements
* specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
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 static int calculateCapacity(Object[] elementData, int minCapacity) {
//因为如果是空的话，minCapacity=size+1；其实就是等于1，空的数组没有长度就存放不了，
// 所以就将minCapacity变成10，也就是默认大小，到这里，还没有真正的初始化这个elementData的大小。
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
private void ensureCapacityInternal(int minCapacity) {
//用来得到一个数组的大小
int num = calculateCapacity(elementData, minCapacity);
//这个方法就是实现真正的判断，确认实际的容量，上面只是将minCapacity=10，这个方法就是真正的判断elementData是否够用
ensureExplicitCapacity(num);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
//判断是否通用，如何添加一个之后的数量大于当前数组的大小，则执行扩容操作
//分为两种情况：1.第一次进行添加的时候，第一添加的时候，minCapacity是1，在上一个方法中，已经更改为了，已经默认返回数量10，，
// 到这一步，还没有改变elementData的大小
//第二种情况:elementData已经不是空数组了，那么在add的时候，minCapacity=size+1，也就是minCapacity代表着要和数组的大小进行比较，看minCapacity
//和数组的长度进行比较，看数组的长度是否够用，如果够用直接返回添加就好了，如果不够用，需要执行扩容操作，不然增加的这个元素就会溢出。
if (minCapacity - elementData.length > 0) {
//扩容的关键方法所在
grow(minCapacity);
}
}
/**
The maximum size of array to allocate.
Some VMs reserve some header words in an array.
Attempts to allocate larger arrays may result in
OutOfMemoryError: Requested array size exceeds VM limit
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
private void grow(int minCapacity) {
// overflow-conscious code
//扩容前的数组长度赋值给oldCapacity
int oldCapacity = elementData.length;
//newCapacity 后面的运算就是扩容前的数组长度1.5倍进行
int newCapacity = oldCapacity + (oldCapacity >> 1);
//如果新的长度-默认容量<0;则把初始化的容量赋值给newCapacity
if (newCapacity - minCapacity < 0) {
newCapacity = minCapacity;
}
//如果扩容后的长度已经超过了最大的数组容量
if (newCapacity - MAX_ARRAY_SIZE > 0) {
newCapacity = hugeCapacity(minCapacity);
}
// minCapacity is usually close to size, so this is a win:
//往下追究最后就到了native
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) {
throw new OutOfMemoryError();
}// overflow
//Arraylist可以进行扩容（针对JDK1.8 数组扩容后的容量是扩容前的1.5倍），Arraylist源码中最大的数组容量是Integer.MAX_VALUE-8，对于空出的8位，目前解释是 ：①存储Headerwords；②避免一些机器内存溢出，减少出错几率，所以少分配③最大还是能支持到Integer.MAX_VALUE（当Integer.MAX_VALUE-8依旧无法满足需求时）
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
/**
* Returns the number of elements in this list.
*
* @return the number of elements in this list
*/
public int size() {
return size;
}
/**
* Returns <tt>true</tt> if this list contains no elements.
*
* @return <tt>true</tt> if this list contains no elements
*/
public boolean isEmpty() {
return size == 0;
}
/**
* Returns <tt>true</tt> if this list contains the specified element.
* More formally, returns <tt>true</tt> if and only if this list contains
* at least one element <tt>e</tt> such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*
* @param o element whose presence in this list is to be tested
* @return <tt>true</tt> if this list contains the specified element
*/
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
/**
* Returns the index of the first occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the lowest index <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
* or -1 if there is no such index.
*/
//正向查找，返回元素的索引值
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;
}
/**
* Returns the index of the last occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the highest index <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
* or -1 if there is no such index.
*/
//反向查找，返回元素索引值
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;
}
/**
* Returns a shallow copy of this <tt>ArrayList</tt> instance. (The
* elements themselves are not copied.)
*
* @return a clone of this <tt>ArrayList</tt> instance
*/
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);
}
}
/**
* Returns an array containing all of the elements in this list
* in proper sequence (from first to last element).
* <p>
* <p>The returned array will be "safe" in that no references to it are
* maintained by this list. (In other words, this method must allocate
* a new array). The caller is thus free to modify the returned array.
* <p>
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this list in
* proper sequence
*/
//返回Arraylist的Object数组
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
/**
* Returns an array containing all of the elements in this list in proper
* sequence (from first to last element); the runtime type of the returned
* array is that of the specified array. If the list fits in the
* specified array, it is returned therein. Otherwise, a new array is
* allocated with the runtime type of the specified array and the size of
* this list.
* <p>
* <p>If the list fits in the specified array with room to spare
* (i.e., the array has more elements than the list), the element in
* the array immediately following the end of the collection is set to
* <tt>null</tt>. (This is useful in determining the length of the
* list <i>only</i> if the caller knows that the list does not contain
* any null elements.)
*
* @param a the array into which the elements of the list are to
* be stored, if it is big enough; otherwise, a new array of the
* same runtime type is allocated for this purpose.
* @return an array containing the elements of the list
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in
* this list
* @throws NullPointerException if the specified array is null
*/
// 返回ArrayList的模板数组。所谓模板数组，即可以将T设为任意的数据类型
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
// 若数组a的大小 < ArrayList的元素个数；
// 则新建一个T[]数组，数组大小是“ArrayList的元素个数”，并将“ArrayList”全部拷贝到新数组中
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
// 若数组a的大小 >= ArrayList的元素个数；
// 则将ArrayList的全部元素都拷贝到数组a中。
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];
}
/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
/**
* Replaces the element at the specified position in this list with
* the specified element.
*
* @param index index of the element to replace
* @param element element to be stored at the specified position
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
//确定内部容量是否够用，size是数组中数据的个数，因为要添加一个元素，所以size+1，
//先判断size+1这个数组能否放得下，就在这个方法中去判断是否Object[].length是否够用。
ensureCapacityInternal(size + 1); // Increments modCount!!
/* elementData[size]=e;
size++;*/
//执行赋值操作，size并进行加1，增加数组的大小
elementData[size++] = e;
return true;
}
/**
* Inserts the specified element at the specified position in this
* list. Shifts the element currently at that position (if any) and
* any subsequent elements to the right (adds one to their indices).
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
//将e添加到ArrayList中，也很容易去理解，在添加的过程中还有扩容的操作，
//在ensureCapacityInternal中，每次add方法，都要modcount++操作，其目的主要是遍历时，
//迭代器可以有效检查数据结构是否发生变化，简单来说就是数据是否发生变化
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++;
}
/**
* Removes the element at the specified position in this list.
* Shifts any subsequent elements to the left (subtracts one from their
* indices).
*
* @param index the index of the element to be removed
* @return the element that was removed from the list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
//删除指定位置的元素
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index + 1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
return oldValue;
}
/**
* Removes the first occurrence of the specified element from this list,
* if it is present. If the list does not contain the element, it is
* unchanged. More formally, removes the element with the lowest index
* <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>
* (if such an element exists). Returns <tt>true</tt> if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
* @param o element to be removed from this list, if present
* @return <tt>true</tt> if this list contained the specified element
*/
//删除list的指定元素
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
//快速删除第index个元素
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
//从index+1位置开始，后面的元素替换前面的所有元素
if (numMoved > 0)
System.arraycopy(elementData, index + 1, elementData, index,
numMoved);
//将最后一个元素设为null，当发生GC的时候，可以把这个null元素清除掉，因为这里没有任何引用关系，GCRoots不可达（为什么GC roots不可达还需要进行详细进行分析，明白的可以互相交流一下）
elementData[--size] = null; // clear to let GC do its work
}
/**
* Removes all of the elements from this list. The list will
* be empty after this call returns.
*/
//清空所有元素，并把所有元素都设置为null
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
/**
* Appends all of the elements in the specified collection to the end of
* this list, in the order that they are returned by the
* specified collection's Iterator. The behavior of this operation is
* undefined if the specified collection is modified while the operation
* is in progress. (This implies that the behavior of this call is
* undefined if the specified collection is this list, and this
* list is nonempty.)
*
* @param c collection containing elements to be added to this list
* @return <tt>true</tt> if this list changed as a result of the call
* @throws NullPointerException if the specified collection is null
*/
//将集合C追加到arraylist后面
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;
}
/**
* Inserts all of the elements in the specified collection into this
* list, starting at the specified position. Shifts the element
* currently at that position (if any) and any subsequent elements to
* the right (increases their indices). The new elements will appear
* in the list in the order that they are returned by the
* specified collection's iterator.
*
* @param index index at which to insert the first element from the
* specified collection
* @param c collection containing elements to be added to this list
* @return <tt>true</tt> if this list changed as a result of the call
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
// 从index位置开始，将集合c添加到ArrayList
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;
}
/**
* Removes from this list all of the elements whose index is between
* {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
* Shifts any succeeding elements to the left (reduces their index).
* This call shortens the list by {@code (toIndex - fromIndex)} elements.
* (If {@code toIndex==fromIndex}, this operation has no effect.)
*
* @throws IndexOutOfBoundsException if {@code fromIndex} or
* {@code toIndex} is out of range
* ({@code fromIndex < 0 ||
* fromIndex >= size() ||
* toIndex > size() ||
* toIndex < fromIndex})
*/
//删除fromIndex到toIndex之间的全部元素。
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// clear to let GC do its work
int newSize = size - (toIndex - fromIndex);
for (int i = newSize; i < size; i++) {
elementData[i] = null;
}
size = newSize;
}
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;
}
/**
* Save the state of the <tt>ArrayList</tt> instance to a stream (that
* is, serialize it).
*
* @serialData The length of the array backing the <tt>ArrayList</tt>
* instance is emitted (int), followed by all of its elements
* (each an <tt>Object</tt>) in the proper order.
*/
// java.io.Serializable的写入函数
// 将ArrayList的“容量，所有的元素值”都写入到输出流中
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();
}
}
/**
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
* deserialize it).
*/
// java.io.Serializable的读取函数：根据写入方式读出
// 先将ArrayList的“容量”读出，然后将“所有的元素值”读出
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 从输入流中入读arraylist的容量
s.readInt(); // ignored
if (size > 0) {
// be like clone(), allocate array based upon size not capacity
int capacity = calculateCapacity(elementData, size);
SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);
ensureCapacityInternal(size);
Object[] a = elementData;
// Read in all elements in the proper order. 从输入流将所有的元素值读出
for (int i = 0; i < size; i++) {
a[i] = s.readObject();
}
}
}
/**
* Returns a list iterator over the elements in this list (in proper
* sequence), starting at the specified position in the list.
* The specified index indicates the first element that would be
* returned by an initial call to {@link ListIterator#next next}.
* An initial call to {@link ListIterator#previous previous} would
* return the element with the specified index minus one.
* <p>
* <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
*
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
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>
* <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);
}
/**
* Returns an iterator over the elements in this list in proper sequence.
* <p>
* <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
*
* @return an iterator over the elements in this list in proper sequence
*/
public Iterator<E> iterator() {
return new Itr();
}
}总结：
1、ArrayList实际上是通过一个数组去保存数据的，所以底层的数据结构就是数组。当我们使用无参构造去构造list的时候，则arraylist的默认容量是10。
2、当arraylist容量不足以容纳全部元素的时候，会执行扩容操作，arraylist会重新设置容量：新的容量大小为oldCapacity + (oldCapacity >> 1)，使用>>1方式和jdk1.7相比有一些改变，从我的理解角度来讲，这种效率比1.7 新的容量=(原始容量x3)/2 + 1效率更高，因为在JVM进行解析和计算的过程中，还是会转化为二进制进行计算，在1.8中，直接使用二进制移位运算进行了计算，也就是1.5倍扩容操作。
3.ArrayList的克隆函数，即是将全部元素克隆到另外一个数组中，使用Array.copy()的方式效率会更高一些，因为这是直接通过最底层的native方法通过C库的方式进行操作的,具体如何选型还是看自己的业务需求，以及list中的数据量
4.ArrayList实现java.io.Serializable的方式。当写入到输出流时，先写入“容量”，再依次写入“每一个元素”；当读出输入流时，先读取“容量”，再依次读取“每一个元素”。
第四部分 ArrayList遍历方式
public class ForTst {
public static void main(String[] args) {
List<Integer> ints = new ArrayList();
long start = System.currentTimeMillis();
for (int i = 0; i < 10000000; i++) {
ints.add(i);
}
int size = ints.size();
for (int i = 0; i < size; i++) {
ints.get(i);
}
System.out.println("fori cost time : " + String.valueOf(System.currentTimeMillis() - start));
System.out.println("-----------------------------------------------------");
long start2 = System.currentTimeMillis();
for (Integer i: ints) {
ints.get(i);
}
System.out.println("foreach cost time : " + String.valueOf(System.currentTimeMillis() - start2));
System.out.println("-----------------------------------------------------");
long start3 = System.currentTimeMillis();
Iterator iterator = ints.iterator();
while (iterator.hasNext()) {
iterator.next();
}
System.out.println("iterator cost time : " + String.valueOf(System.currentTimeMillis() - start3));
System.out.println("-----------------------------------------------------");
long start4 = System.currentTimeMillis();
ints.stream().forEach(integer -> {int a = integer;});
System.out.println("steam cost time : " + String.valueOf(System.currentTimeMillis() - start4));
System.out.println("-----------------------------------------------------");
long start5 = System.currentTimeMillis();
ints.forEach(integer -> {int a = integer;});
System.out.println("InterfaceForEach cost time : " + String.valueOf(System.currentTimeMillis() - start5));
System.out.println("-----------------------------------------------------");
}
}
数据量不同，每种遍历方式的性能各不相同，有兴趣的话，可以从1000-100000000等范围去测试一下。
自己测试，建议使用迭代器的遍历方式，性能是比较好，数据量越大，迭代器优势越明显，具体如何使用还是要看场景。
第五部分 toArray()异常
当我们调用ArrayList中的 toArray()，可能遇到过抛出“java.lang.ClassCastException”异常的情况。下面我们说说这是怎么回事。
ArrayList提供了2个toArray()函数：
Object[] toArray()
<T> T[] toArray(T[] contents)调用 toArray() 函数会抛出“java.lang.ClassCastException”异常，但是调用 toArray(T[] contents) 能正常返回 T[]。
toArray() 会抛出异常是因为 toArray() 返回的是 Object[] 数组，将 Object[] 转换为其它类型(如如，将Object[]转换为的Integer[])则会抛出“java.lang.ClassCastException”异常，因为Java不支持向下转型。具体的可以参考前面ArrayList.java的源码介绍部分的toArray()。
解决该问题的办法是调用 <T> T[] toArray(T[] contents) ， 而不是 Object[] toArray()。
调用 toArray(T[] contents) 返回T[]的可以通过以下几种方式实现。
// toArray(T[] contents)调用方式一
public static Integer[] toArray1(ArrayList<Integer> v) {
Integer[] newText = new Integer[v.size()];
v.toArray(newText);
return newText;
}
// toArray(T[] contents)调用方式二。最常用！
public static Integer[] toArray2(ArrayList<Integer> v) {
Integer[] newText = (Integer[])v.toArray(new Integer[0]);
return newText;
}
// toArray(T[] contents)调用方式三
public static Integer[] toArray3(ArrayList<Integer> v) {
Integer[] newText = new Integer[v.size()];
Integer[] newStrings = (Integer[])v.toArray(newText);
return newStrings;
}