世上的事，只要肯用心去学，没有一件是太晚的。请你一定不要停下来，成为你想成为的人。
前言
在learn from collection framework design中提到，collection framework分为两部分，分别为Collection和Map，其中Collection又分为三类分别为List，Set和Queue,本篇文章先来分析ArrayList的实现。
ArrayList继承关系
如上图所示，它实现了RandomAccess（可随机访问），Cloneable（可克隆），Serializable（支持序列化和反序列化）接口以及List接口，并且它还继承了List的抽象模板类AbstractList。
其中，前三个接口都是marker interface，没有可以让实现类实现的方法。
下面直接来看ArrayList内部的一些实现机制。
内部实现
数据结构
其内部维护了一个Object类型的数组，即elementData成员变量，成员变量size记录list的大小。。
初始化
ArrayList的构造方法有如下三种重载，分别是：
第一种方式：根据初始容量初始化ArrayList。
/**
* Constructs an empty list with the specified initial capacity.
*
* @param initialCapacity the initial capacity of the list
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) { // 根据传入的初始的容量大小初始化List，其内部维护的是
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA; // 是一个长度为0的空数组，即{}
} else { // 因数组长度不能小于0，故抛出异常
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
第二种：使用默认大小，默认内部数组长度为0。
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; // DEFAULTCAPACITY_EMPTY_ELEMENTDATA默认为长度为0的空数组
}
第三种：根据传入的集合构建ArrayList
/**
* 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(); // 注意，先构造一个新的数组，然后使用数组拷贝，将旧数据拷贝到新数组，这样效率并不高，并且还浪费内存
if ((size = elementData.length) != 0) { // collection包含元素
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else { // collection不包含元素，使用内部预定义的长度为0的数组。
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
内部数组扩容机制
java.util.ArrayList#ensureCapacityInternal是专门用于扩容的私有方法，具体如下：
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
一共有两个步骤，分别为计算所需容量以及扩容两个步。
计算所需容量
calculateCapacity源码如下：
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity); // 如果刚开始是空数组，则第一次扩容，数组长度需扩容到 max(10,需要的最小容量)
}
return minCapacity;
}
扩容
private void ensureExplicitCapacity(int minCapacity) {
modCount++; // 记录内部数组扩容次数
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
这里为什么要用减法而不直接比较？
因为minCapacity这个是由原始的大小 + 需要插入的元素的个数得到的，在加法运算后可能会出现溢出，变为负数，变为负数了就不能继续扩容了。grow具体如下：
/**
* 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
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0) // 这里之所以用减法还是考虑到新的数组长度可能会溢出
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
huge源码如下：
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
扩容倍数是1.5，最大数组长度为 MAX_ARRAY_SIZE，即Integer.MAX_VALUE - 8,之所以要取这个值是因为，有的JVM在实现数组的时候，刚开始会保留一些header的信息，这些信息会占8个字节。在扩展数组时，长度一旦超过这个大小，会抛出OutOfMemoryError异常。
也就是说，如果当前数组不足以容纳新的元素，则需要1.5倍扩容，最终容量最大为Integer.MAX_VALUE - 8
缩容
/**
* 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.
*/
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = (size == 0)
? EMPTY_ELEMENTDATA
: Arrays.copyOf(elementData, size);
}
}
缩容效率其实不高，因为又重新创建了一个长度等于size的数组，然后再逐一拷贝旧数组上元素到新的数组上。
单个元素插入
有两种方式，分别如下：
方式一，默认在结尾插入，如下：
/**
* 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) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
方式二，在指定位置插入元素，如下：
/**
* 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}
*/
public void add(int index, E element) {
rangeCheckForAdd(index); // 注意，检查下标的合法性，这个下标是跟ArrayList的长度比较的，不是跟内部数据的capacity比较的！
ensureCapacityInternal(size + 1); // Increments modCount!!
// 把指定下标后（包括该下标）的数据整体后移一位
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
多个元素插入
也有两种方式。
方式一，在结尾插入，如下：
/**
* 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
*/
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;
}
方式二，在指定位置插入，如下：
/**
* 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
*/
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; // 计算需要index后（包括index）空出的元素的个数
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
移除单个元素
主要有两种方式，分别为：
方式一，移出指定下标对应位置的元素，如下：
/**
* 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}
*/
public E remove(int index) {
rangeCheck(index); // index 有效性校验，跟 内部元素个数 size 比较
modCount++;
E oldValue = elementData(index); // 获取指定下标下的元素
int numMoved = size - index - 1; // 计算需要移动的元素的个数
if (numMoved > 0) // 指定index后的所有元素统一向前一个索引距离
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work 设置为null，允许gc回收不用的对象，并更新list的大小
return oldValue;
}
方式二，移出左边第一个出现的指定元素
/**
* 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
*/
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;
}
其中，fastRemove方法如下：
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index) {
modCount++; // 修改次数+1
int numMoved = size - index - 1; // 计算需要向前移动的元素的个数
if (numMoved > 0) // 如果需要移动，则将index后的元素统一向前移动一个元素大小位置，并把最后的元素的引用设为null，便于gc回收不再使用的对象,并更新list的大小。
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
移除多个元素
方式一，移除所有元素
/**
* Removes all of the elements from this list. The list will
* be empty after this call returns.
*/
public void clear() {
modCount++; // 修改次数 + 1
// clear to let GC do its work
for (int i = 0; i < size; i++) // 所有索引下标下的元素引用设置为null
elementData[i] = null;
size = 0; // 重置list的大小为0
}
方式二，移出指定范围内的元素，包括开始索引不包括结束索引
/**
* 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})
*/
protected void removeRange(int fromIndex, int toIndex) {
modCount++; // 修改次数 + 1
int numMoved = size - toIndex; // 计算需要移动的元素的个数
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
// clear to let GC do its work
int newSize = size - (toIndex-fromIndex); // 计算list新的大小
for (int i = newSize; i < size; i++) { // 从后往前依次清除指定位置上的元素
elementData[i] = null;
}
size = newSize; // 更新list的大小
}
方式三，批量移出给定集合内的元素或不在给定集合内的元素
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++) // 从前向后遍历
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
// Preserve behavioral compatibility with AbstractCollection,
// even if c.contains() throws.
if (r != size) { // 剩余的整体前移
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
if (w != size) { // 有元素被移除
// clear to let GC do its work
for (int i = w; i < size; i++) // 移除之后的设置为null
elementData[i] = null;
modCount += size - w; // 修改次数 + 移除的元素的个数
size = w; // 修改list的大小
modified = true; // 设置修改标志位为true
}
}
return modified;
}
方式四，移除指定集合内的所有元素
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
其内部调用的是方式三的方法，不做过多说明。
方式五，移除指定集合外的所有元素
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, true);
}
方式六，移除符合条件的所有数据
@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;
}
对序列化的支持
/**
* 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.
*/
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();
}
}
注意，在序列化的时候，list大小不能修改，序列化的时候把list的大小size也保存下来了。
/**
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
* deserialize it).
*/
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
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.
for (int i=0; i<size; i++) {
a[i] = s.readObject();
}
}
}
反序列化后，list的capacity和size是一样的。
测试代码如下：
package com.company;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;
public class Main {
public static void main(String[] args) throws Exception {
// write your code here
List<Integer> list = new ArrayList<>();
for (int i = 0; i < 12; i++) {
list.add(i);
}
System.out.println(list.size());
System.out.println(list);
ByteArrayOutputStream os = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(os);
oos.writeObject(list);
oos.flush();
byte[] bytes = os.toByteArray();
ObjectInputStream inputStream = new ObjectInputStream(new ByteArrayInputStream(bytes));
List<Integer> o = (List<Integer>)inputStream.readObject();
System.out.println(o.size());
System.out.println(o);
Field elementData1 = o.getClass().getDeclaredField("elementData");
elementData1.setAccessible(true);
Object[] elementData = (Object[]) elementData1.get(list);
System.out.println(elementData.length);
elementData = (Object[]) elementData1.get(o);
System.out.println(elementData.length);
}
}
克隆
/**
* 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(); // Object.clone是浅克隆，只克隆引用，内部数据需要重新拷贝一份
v.elementData = Arrays.copyOf(elementData, size); // 重新建一个新的数组存放数据
v.modCount = 0; // 修改内部数组长度变更次数
return v;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError(e);
}
}
测试代码
ArrayList<Integer> list = new ArrayList<>();
list.add(0);
List<Integer> clone = (List<Integer>) list.clone();
clone.set(0,1000);
System.out.println(list.get(0)); // 0
System.out.println(clone.get(0)); // 1000
替换/变换
替换，本质上就是一个变换，只不过这个是在原数组上修改旧值，原来的数据丢失了，如果还想要原来的数据，需要提前拷贝一份。
@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++;
}
排序
排序，其实现了通用的排序算法(调用Array.sort方法)，排序比较规则交给用户来指定。
@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++;
}
遍历
Itr实现了可以向后遍历和remove操作的迭代器，由iterator方法返回。ListItr实现了可以向前遍历和向后遍历、元素的添加删除修改的迭代器,由listIterator方法返回。
ArrayList的优势和缺点
优势
- 顺序存储，随机存取,数据元素与位置相关联，因此查找效率高，索引遍历快，时间复杂度O(1)
- 尾部插入与删除的速度速度快
缺点
- 线程不安全
- 非尾节点的插入和删除需要移除后续的元素，效率较低
- 虽然支持扩缩容，但是，原数据需逐一拷贝，效率较低
总结
本篇文章，相对来说比较简单，归根结底，对ArrayList的各种操作都是对底层数组的操作，深刻理解数组这种非常简单的数据结构对理解ArrayList的各个操作有很大帮助。