ArrayList就是一个以动态数组形式实现的集合类。所以优势是随机访问元素，但是在list中间插入和移动元素时较慢。特别是插入效率。
由于要维护数组的size，即动态数组的实现就是扩容时将原数组的元素复制到更大数组中。
所以在此之前补充下复制数组元素函数，其中Arrays.copyOf()和System.arraycopy()的原理介绍：
数据结构
底层基于数组实现容量大小动态变化。并且元素允许 null 的存在。
其源码如下：
/**
* 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;
初始化
有三种初始化方式（构造函数）：
- 无参构造函数：
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
- 有参构造函数：指定初始容量
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);
}
}
- 有参构造函数：指定 collection 列表
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;
}
}
我们发现当初始化后集合为空，数组则指向如下的空数组常量：
//①如果使用不带参数的构造函数，则就用默认容量10来进行开辟空间，并且elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
//②如果使用带参数的构造函数，但是初始容量为0，则elementData = EMPTY_ELEMENTDATA
private static final Object[] EMPTY_ELEMENTDATA = {};
两者区别在于如何扩张数组（虽然都指向空数组，但是前者的容量为10，后者的容量为0），具体分析如下。
扩容
简单来说，就是将数组扩容到渴望的数组大小minCapacity，具体流程如下分析。
——>扩容流程：
- 首先从
ensureCapacityInternal(int minCapacity)开始：
/**
* @param minCapacity the desired minimum capacity
*/
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
- 调用
calculateCapacity(elementData, minCapacity)：若数组是DEFAULTCAPACITY_EMPTY_ELEMENTDATA状态（无参初始化状态），则多比较下max(DEFAULT_CAPACITY, minCapacity),即从10开始扩容
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
- 接着，
ensureExplicitCapacity(int minCapacity)：检测扩容容量minCapacity要大于elementData.length
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
grow(int minCapacity)：开始扩容（最关键的部分）：
- （1）每次从
1.5*oldCapacity扩容； - （2）若
minCapacity大于1.5*oldCapacity， 则选minCapacity； - （3）若扩容超过
MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8（为什么选MAX_ARRAY_SIZE？），则调用hugeCapacity(int minCapacity)选择，最大限制的容量minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE : MAX_ARRAY_SIZE; - （4）最后
elementData = Arrays.copyOf(elementData, newCapacity);复制数组到扩容后的新数组
/**
* 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)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
/**
* 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;
查找操作
1、通过索引查找元素（随机访问）：
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
2、直接查找元素：
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
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;
}
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;
}
插入操作
1、插入单个元素：
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
//后移
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}
2、插入集合：
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityInternal(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
删除操作
1、通过索引删除元素：
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;
}
2、删除指定元素：
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.
*/
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
}
3、删除部分集合：
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);
return batchRemove(c, false);
}
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++)
//判断元素是否需要保留
if (c.contains(elementData[r]) == complement)
//保留元素到原数组，从w=0开始
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;
}
转化成数组
// 返回ArrayList的Object数组
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
// 返回ArrayList元素组成的数组
@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;
}
参考：
- Java集合之ArrayList
- 【Java集合源码剖析】ArrayList源码剖析（jdk1.6的源码，与jdk1.8不同）