以下源码基于 Android SDK 23， 与JDK中略有差别，但基本相同；整体源码由 构造、添加(add)、设置(set)、获取(get)、移除(remove)、迭代器(iterator) 和序列化(Serializable)组成，最后我还会把里边一些不常用的方法举例说明下作用，下面我们就一一探究其实现原理。
概述
ArrayList 存储的实质是操作一个数组，这个数组可以根据内容自动扩容，所以让 ArrayList看起来像一个无限大小的容器一样。
属性
/**
* The minimum amount by which the capacity of an ArrayList will increase.
* This tuning parameter controls a time-space tradeoff. This value (12)
* gives empirically good results and is arguably consistent with the
* RI's specified default initial capacity of 10: instead of 10, we start
* with 0 (sans allocation) and jump to 12.
*/
private static final int MIN_CAPACITY_INCREMENT = 12;
/**
* The number of elements in this list.
*/
int size;
/**
* The elements in this list, followed by nulls.
*/
transient Object[] array;
- 既然
ArrayList可以自动扩容，那么就要有一个描述每次扩容的基准，MIN_CAPACITY_INCREMENT就是这个基准，默认值是12。 -
array是ArrayList的核心，所有数据均存储在array这个数组中，发生自动扩容时，array会指向新的数组首地址，但注意了，transient表示它不会参与序列化过程。 -
size始终描述ArrayList中实际的大小。
构造方法
/**
* Constructs a new instance of {@code ArrayList} with the specified
* initial capacity.
*
* @param capacity
* the initial capacity of this {@code ArrayList}.
*/
public ArrayList(int capacity) {
if (capacity < 0) {
throw new IllegalArgumentException("capacity < 0: " + capacity);
}
array = (capacity == 0 ? EmptyArray.OBJECT : new Object[capacity]);
}
/**
* Constructs a new {@code ArrayList} instance with zero initial capacity.
*/
public ArrayList() {
array = EmptyArray.OBJECT;
}
/**
* Constructs a new instance of {@code ArrayList} containing the elements of
* the specified collection.
*
* @param collection
* the collection of elements to add.
*/
public ArrayList(Collection<? extends E> collection) {
if (collection == null) {
throw new NullPointerException("collection == null");
}
Object[] a = collection.toArray();
if (a.getClass() != Object[].class) {
Object[] newArray = new Object[a.length];
System.arraycopy(a, 0, newArray, 0, a.length);
a = newArray;
}
array = a;
size = a.length;
}
ArrayList共含有3个构造方法，EmptyArray.OBJECT是一个length为0的空数组new Object[0]，new ArrayList() 则会创建一个大小为0的数组；你也可以去指定初始的容量capacity，new ArrayList(int capacity) ，避免ArrayList 第一次add 或者其他操作就进行扩容；第三个构造可以传入一个集合，这里要提一下Collection，你可以认为它是 List 、Queue、Set的始祖，这里只要在它们内部实现了 toArray 方法，并且返回一个Object[]类型的数据，就可以成功初始化到 ArrayList中。
添加(add / addAll)
/**
* Adds the specified object at the end of this {@code ArrayList}.
*
* @param object
* the object to add.
* @return always true
*/
@Override public boolean add(E object) {
Object[] a = array;
int s = size;
if (s == a.length) {
Object[] newArray = new Object[s +
(s < (MIN_CAPACITY_INCREMENT / 2) ?
MIN_CAPACITY_INCREMENT : s >> 1)];
System.arraycopy(a, 0, newArray, 0, s);
array = a = newArray;
}
a[s] = object;
size = s + 1;
modCount++;
return true;
}
/**
* Inserts the specified object into this {@code ArrayList} at the specified
* location. The object is inserted before any previous element at the
* specified location. If the location is equal to the size of this
* {@code ArrayList}, the object is added at the end.
*
* @param index
* the index at which to insert the object.
* @param object
* the object to add.
* @throws IndexOutOfBoundsException
* when {@code location < 0 || location > size()}
*/
@Override public void add(int index, E object) {
Object[] a = array;
int s = size;
if (index > s || index < 0) {
throwIndexOutOfBoundsException(index, s);
}
if (s < a.length) {
System.arraycopy(a, index, a, index + 1, s - index);
} else {
// assert s == a.length;
Object[] newArray = new Object[newCapacity(s)];
System.arraycopy(a, 0, newArray, 0, index);
System.arraycopy(a, index, newArray, index + 1, s - index);
array = a = newArray;
}
a[index] = object;
size = s + 1;
modCount++;
}
/**
* This method controls the growth of ArrayList capacities. It represents
* a time-space tradeoff: we don't want to grow lists too frequently
* (which wastes time and fragments storage), but we don't want to waste
* too much space in unused excess capacity.
*
* NOTE: This method is inlined into {@link #add(Object)} for performance.
* If you change the method, change it there too!
*/
private static int newCapacity(int currentCapacity) {
int increment = (currentCapacity < (MIN_CAPACITY_INCREMENT / 2) ?
MIN_CAPACITY_INCREMENT : currentCapacity >> 1);
return currentCapacity + increment;
}
/**
* Adds the objects in the specified collection to this {@code ArrayList}.
*
* @param collection
* the collection of objects.
* @return {@code true} if this {@code ArrayList} is modified, {@code false}
* otherwise.
*/
@Override public boolean addAll(Collection<? extends E> collection) {
Object[] newPart = collection.toArray();
int newPartSize = newPart.length;
if (newPartSize == 0) {
return false;
}
Object[] a = array;
int s = size;
int newSize = s + newPartSize; // If add overflows, arraycopy will fail
if (newSize > a.length) {
int newCapacity = newCapacity(newSize - 1); // ~33% growth room
Object[] newArray = new Object[newCapacity];
System.arraycopy(a, 0, newArray, 0, s);
array = a = newArray;
}
System.arraycopy(newPart, 0, a, s, newPartSize);
size = newSize;
modCount++;
return true;
}
/**
* Inserts the objects in the specified collection at the specified location
* in this List. The objects are added in the order they are returned from
* the collection's iterator.
*
* @param index
* the index at which to insert.
* @param collection
* the collection of objects.
* @return {@code true} if this {@code ArrayList} is modified, {@code false}
* otherwise.
* @throws IndexOutOfBoundsException
* when {@code location < 0 || location > size()}
*/
@Override
public boolean addAll(int index, Collection<? extends E> collection) {
int s = size;
if (index > s || index < 0) {
throwIndexOutOfBoundsException(index, s);
}
Object[] newPart = collection.toArray();
int newPartSize = newPart.length;
if (newPartSize == 0) {
return false;
}
Object[] a = array;
int newSize = s + newPartSize; // If add overflows, arraycopy will fail
if (newSize <= a.length) {
System.arraycopy(a, index, a, index + newPartSize, s - index);
} else {
int newCapacity = newCapacity(newSize - 1); // ~33% growth room
Object[] newArray = new Object[newCapacity];
System.arraycopy(a, 0, newArray, 0, index);
System.arraycopy(a, index, newArray, index + newPartSize, s-index);
array = a = newArray;
}
System.arraycopy(newPart, 0, a, index, newPartSize);
size = newSize;
modCount++;
return true;
}
这里有必要先看一个方法，System.arraycopy()
public static native void arraycopy(Object src, int srcPos,
Object dst, int dstPos, int length);
这是一个 native方法，负责数组拷贝，从 src 的 srcPos 开始，将 length长度的数据拷贝到 dst 中，dstPos中的数据是srcPos位置的数据。
public boolean add(E object)
这是最简单的一个add操作，里边会进行扩容判断，如果当前ArrayList.size与array.length相同，则进行扩容，扩容的策略是s < (MIN_CAPACITY_INCREMENT / 2) ? MIN_CAPACITY_INCREMENT : s >> 1，即 s < 6 ? 6 : s * 2， 最终扩容的大小为 (s + s < 6 ? 6 : s * 2)；newCapacity(int currentCapacity) 方法也是这个作用，返回最终扩容后的大小。
public void add(int index, E object)
这个方法的作用是将 object 插入至 index位置，这里也会有扩容判断，既然是插入一个值，那么size就会 +1，所以 ArrayList.size 小于 array.length是一种情况，数组可以直接从 index处 后移一位，再将 object 放入 index的位置；若是大于等于，则原array需要扩容，扩容后现将old array 数据 复制到 new array中，再进行后移，最终把object插入到index位置。
public boolean addAll(Collection<? extends E> collection)
public boolean addAll(int index, Collection<? extends E> collection)
这两个方法只是批量操作，内部逻辑与add 是一样的，都要先判断 ArrayList.size与array.length的大小关系进行扩容，之后通过 System.arraycopy去操作array。
注：这里你有可能会发现有个变量
modCount，它用来表达ArrayList的修改次数（add、remove），是它导致ArrayList不是线程安全的，等讲到迭代器iterator的时候再来说说这个变量。
设置
/**
* Replaces the element at the specified location in this {@code ArrayList}
* with the specified object.
*
* @param index
* the index at which to put the specified object.
* @param object
* the object to add.
* @return the previous element at the index.
* @throws IndexOutOfBoundsException
* when {@code location < 0 || location >= size()}
*/
@Override public E set(int index, E object) {
Object[] a = array;
if (index >= size) {
throwIndexOutOfBoundsException(index, size);
}
@SuppressWarnings("unchecked") E result = (E) a[index];
a[index] = object;
return result;
}
这个方法没什么，就是把array[index] 替换，并且把原来的数据返回。
获取
@SuppressWarnings("unchecked")
@Override
public E get(int index) {
if (index >= size) {
throwIndexOutOfBoundsException(index, size);
}
return (E) array[index];
}
这个方法也不多说，将array[index]返回。
移除
/**
* Removes the object at the specified location from this list.
*
* @param index
* the index of the object to remove.
* @return the removed object.
* @throws IndexOutOfBoundsException
* when {@code location < 0 || location >= size()}
*/
@Override public E remove(int index) {
Object[] a = array;
int s = size;
if (index >= s) {
throwIndexOutOfBoundsException(index, s);
}
@SuppressWarnings("unchecked") E result = (E) a[index];
System.arraycopy(a, index + 1, a, index, --s - index);
a[s] = null; // Prevent memory leak
size = s;
modCount++;
return result;
}
@Override public boolean remove(Object object) {
Object[] a = array;
int s = size;
if (object != null) {
for (int i = 0; i < s; i++) {
if (object.equals(a[i])) {
System.arraycopy(a, i + 1, a, i, --s - i);
a[s] = null; // Prevent memory leak
size = s;
modCount++;
return true;
}
}
} else {
for (int i = 0; i < s; i++) {
if (a[i] == null) {
System.arraycopy(a, i + 1, a, i, --s - i);
a[s] = null; // Prevent memory leak
size = s;
modCount++;
return true;
}
}
}
return false;
}
@Override protected void removeRange(int fromIndex, int toIndex) {
if (fromIndex == toIndex) {
return;
}
Object[] a = array;
int s = size;
if (fromIndex >= s) {
throw new IndexOutOfBoundsException("fromIndex " + fromIndex
+ " >= size " + size);
}
if (toIndex > s) {
throw new IndexOutOfBoundsException("toIndex " + toIndex
+ " > size " + size);
}
if (fromIndex > toIndex) {
throw new IndexOutOfBoundsException("fromIndex " + fromIndex
+ " > toIndex " + toIndex);
}
System.arraycopy(a, toIndex, a, fromIndex, s - toIndex);
int rangeSize = toIndex - fromIndex;
Arrays.fill(a, s - rangeSize, s, null);
size = s - rangeSize;
modCount++;
}
add方法已经进行了详细的讲解，想必大家都能猜到，remove操作就是讲 index 或者 range的一段数据从array中移除，然后再通过System.arraycopy拷贝之后的数据前移补充空位，下图以移除单个为例，将步骤分解：
image.png
迭代器
@Override public Iterator<E> iterator() {
return new ArrayListIterator();
}
private class ArrayListIterator implements Iterator<E> {
/** Number of elements remaining in this iteration */
private int remaining = size;
/** Index of element that remove() would remove, or -1 if no such elt */
private int removalIndex = -1;
/** The expected modCount value */
private int expectedModCount = modCount;
public boolean hasNext() {
return remaining != 0;
}
@SuppressWarnings("unchecked") public E next() {
ArrayList<E> ourList = ArrayList.this;
int rem = remaining;
if (ourList.modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
if (rem == 0) {
throw new NoSuchElementException();
}
remaining = rem - 1;
return (E) ourList.array[removalIndex = ourList.size - rem];
}
public void remove() {
Object[] a = array;
int removalIdx = removalIndex;
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
if (removalIdx < 0) {
throw new IllegalStateException();
}
System.arraycopy(a, removalIdx + 1, a, removalIdx, remaining);
a[--size] = null; // Prevent memory leak
removalIndex = -1;
expectedModCount = ++modCount;
}
}
迭代器，一个很重要的概念，它的作用就是便利整个ArrayList， for each 的原理其实就是迭代器的使用，上文说到了modCount与迭代器相关，
if (ourList.modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
expectedModCount是iterator初始化时赋予的值，值为modCount，而modCount会根据add或者remove进行++操作，这就表明，当iterator创建好后，只要使用这个iterator实例去进行遍历，就不能使用ArrayList.add或者ArrayList.remove操作，因为如果使用了，modCount会发生变化，这样在next()的时候就会抛出异常ConcurrentModificationException，这也进一步说明ArrayList不是线程安全的。那么在遍历中如何移除元素呢，就是下边实现的remove方法了，remove过程与之前类似，关键在于expectedModCount = ++modCount;，remove需要使modCount 递增，那么我让expectedModCount重新赋值，即可完成删除操作。
序列化
private static final long serialVersionUID = 8683452581122892189L;
private void writeObject(ObjectOutputStream stream) throws IOException {
stream.defaultWriteObject();
stream.writeInt(array.length);
for (int i = 0; i < size; i++) {
stream.writeObject(array[i]);
}
}
private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
stream.defaultReadObject();
int cap = stream.readInt();
if (cap < size) {
throw new InvalidObjectException(
"Capacity: " + cap + " < size: " + size);
}
array = (cap == 0 ? EmptyArray.OBJECT : new Object[cap]);
for (int i = 0; i < size; i++) {
array[i] = stream.readObject();
}
}
这是在代码末尾了，ArrayList是通过stream.writeObject 连续写入 array的内容。
其他
public boolean contains(Object object)
利用 object 的 equals方法判断ArrayList中是否包含object对象。
public int indexOf(Object object)
public int lastIndexOf(Object object)
这两个方法都是获取 object 在 ArrayList中的位置，第一个是正序遍历，找到的第一个返回的index；第二个是倒序遍历，找到第一个返回的index。
/**
* Sets the capacity of this {@code ArrayList} to be the same as the current
* size.
*
* @see #size
*/
public void trimToSize() {
int s = size;
if (s == array.length) {
return;
}
if (s == 0) {
array = EmptyArray.OBJECT;
} else {
Object[] newArray = new Object[s];
System.arraycopy(array, 0, newArray, 0, s);
array = newArray;
}
modCount++;
}
这个方法是将当前的array “精简”一下，比如 array.length 是10，但里边的size是 5个，那么就将 array.length变为 5，把数据通过 System.arraycopy 拷贝到新的 array中。
@Override public boolean equals(Object o) {
if (o == this) {
return true;
}
if (!(o instanceof List)) {
return false;
}
List<?> that = (List<?>) o;
int s = size;
if (that.size() != s) {
return false;
}
Object[] a = array;
if (that instanceof RandomAccess) {
for (int i = 0; i < s; i++) {
Object eThis = a[i];
Object ethat = that.get(i);
if (eThis == null ? ethat != null : !eThis.equals(ethat)) {
return false;
}
}
} else { // Argument list is not random access; use its iterator
Iterator<?> it = that.iterator();
for (int i = 0; i < s; i++) {
Object eThis = a[i];
Object eThat = it.next();
if (eThis == null ? eThat != null : !eThis.equals(eThat)) {
return false;
}
}
}
return true;
}
再来看下这个长长的equals方法，非常好懂，但是乍眼一看有个 RandomAccess，这是什么？寻找了一下它的实现类，发现ArrayList就是它的实现类，再看下这个if(...){}else{}，如果是RandomAccess的实现类，那么直接使用get(index)获取元素，否则需要使用迭代器iterator。以下是对于RandomAccess的一段摘录：
jdk中有个RandomAccess接口，这是一个标记接口(Marker)，它没有任何方法，这个接口被List的实现类（子类）使用。如果List子类实现了RandomAccess接口，那就表示它能够快速随机访问存储的元素。RandomAccess接口的意义在于：在对列表进行随机或顺序访问的时候，访问算法能够选择性能最佳方式。一般的列表访问算法在访问列表元素之前，都被建议先使用instanceof关键字检查一下列表是否是一个RandomAccess子类，然后再决定采用随机还是顺序方式访问列表中的元素，这样可以保证访问算法拥有最佳的性能。对于List的子类，如果：
for (int i=0, n=list.size(); i < n; i++)
list.get(i);
的访问方式比
for (Iterator i=list.iterator(); i.hasNext(); )
i.next();
快，那么它应该实现RandomAccess接口。
