ArrayList
以前学习数据结构的时候，自己通过实现过 ArrayList。
但是 jdk 中的源码没有仔细研读过。
本篇查缺补漏，好好学习一下。
jdk 版本为 11
类定义
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
继承自 AbstractList 抽象类。
实现了几个接口 List, RandomAccess, Cloneable, java.io.Serializable。
RandomAccess 接口
其他几个并不陌生，我们看一下 RandomAccess
public interface RandomAccess {
}
没有任何方法和属性，这个类只是用来标识一个实现方法，是否可以 O(1) 获取属性。
相比较而言，LinkedList 就不支持，也没有实现这个接口。
PS: 感觉这里应该换成注解等更加合理。
AbstractList 抽象父类
大部分实现应该都在 AbstractList 中。
为了保障阅读的流畅性，我们把 AbstractList 单独放在一篇文章中。
便于阅读和复用。
属性
// 默认的容量大小
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
* 空对象使用的数组
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* 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.
*
* 用于默认大小的空实例的共享空数组实例。 我们将其与 EMPTY_ELEMENTDATA 区分开来，以了解添加第一个元素时要膨胀多少。
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* 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.
*
* 存储 ArrayList 元素的数组缓冲区。ArrayList 的容量就是这个数组缓冲区的长度。
* 添加第一个元素时，任何具有 elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA 的空 ArrayList 都将扩展为 DEFAULT_CAPACITY。
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* 数组大小
* @serial
*/
private int size;
/**
* The maximum size of array to allocate (unless necessary).
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*
* 最大的申请内存数量，但是实际上应该不会有这么大，除非程序有问题。
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
构造器
构造器的方法实际上不算多，比如就没有基于数组的。
不过有 Arrays.asList(…) 可以替代。
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);
}
}
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 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) {
// 这里还是挺有趣的，不是遍历这个集合，而是把集合转化为数组。难道是 Arrays.copy 更快吗？
Object[] a = c.toArray();
if ((size = a.length) != 0) {
if (c.getClass() == ArrayList.class) {
elementData = a;
} else {
elementData = Arrays.copyOf(a, size, Object[].class);
}
} else {
// replace with empty array.
elementData = EMPTY_ELEMENTDATA;
}
}
核心方法
其他的方法比较多。我们主要看一下 list 的 add/set/get/remove 这几个方法。
add
public boolean add(E e) {
modCount++;
add(e, elementData, size);
return true;
}
可以发现，这里变化的是 modCount 变量。
modCount
这个是用来记录 list 的变化次数。
/**
* The number of times this list has been <i>structurally modified</i>.
* Structural modifications are those that change the size of the
* list, or otherwise perturb it in such a fashion that iterations in
* progress may yield incorrect results.
*
* <p>This field is used by the iterator and list iterator implementation
* returned by the {@code iterator} and {@code listIterator} methods.
* If the value of this field changes unexpectedly, the iterator (or list
* iterator) will throw a {@code ConcurrentModificationException} in
* response to the {@code next}, {@code remove}, {@code previous},
* {@code set} or {@code add} operations. This provides
* <i>fail-fast</i> behavior, rather than non-deterministic behavior in
* the face of concurrent modification during iteration.
*
* <p><b>Use of this field by subclasses is optional.</b> If a subclass
* wishes to provide fail-fast iterators (and list iterators), then it
* merely has to increment this field in its {@code add(int, E)} and
* {@code remove(int)} methods (and any other methods that it overrides
* that result in structural modifications to the list). A single call to
* {@code add(int, E)} or {@code remove(int)} must add no more than
* one to this field, or the iterators (and list iterators) will throw
* bogus {@code ConcurrentModificationExceptions}. If an implementation
* does not wish to provide fail-fast iterators, this field may be
* ignored.
*/
protected transient int modCount = 0;
add(e, elementData, size);
这个是核心的方法。
private void add(E e, Object[] elementData, int s) {
// 触发长度
if (s == elementData.length)
// 扩容
elementData = grow();
// 设置元素
elementData[s] = e;
// 大小加1
size = s + 1;
}
比较核心的就是 grow 方法。
/**
* 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
* @throws OutOfMemoryError if minCapacity is less than zero
*/
private Object[] grow(int minCapacity) {
return elementData = Arrays.copyOf(elementData, newCapacity(minCapacity));
}
可以发现，其实 java 中并不是使用数组的移动来实现元素的添加。
而是直接通过 Arrays.copy，这样性能会更好。
/**
* Returns a capacity at least as large as the given minimum capacity.
* Returns the current capacity increased by 50% if that suffices.
* Will not return a capacity greater than MAX_ARRAY_SIZE unless
* the given minimum capacity is greater than MAX_ARRAY_SIZE.
*
* @param minCapacity the desired minimum capacity
* @throws OutOfMemoryError if minCapacity is less than zero
*/
private int newCapacity(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity <= 0) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
return Math.max(DEFAULT_CAPACITY, minCapacity);
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return minCapacity;
}
return (newCapacity - MAX_ARRAY_SIZE <= 0)
? newCapacity
: hugeCapacity(minCapacity);
}
newCapacity 默认应该是 oldCapacity 的 3 倍。（原始 + 原始 *2）
但是这里就需要考虑一些越界的问题。
如果 newCapacity 没超过 MAX_ARRAY_SIZE，则直接返回 newCapacity；
或者执行 hugeCapacity(minCapacity) 方法。
hugeCapacity(minCapacity)
也就是如果超过了最大的值，怎么办？
private static int hugeCapacity(int minCapacity) {
// 扩容以后，超过了 int 上限
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
// 超过了 int 的限制-8，默认为整形的最大值。
return (minCapacity > MAX_ARRAY_SIZE)
? Integer.MAX_VALUE
: MAX_ARRAY_SIZE;
}
set-修改
修改方法特别简单。
public E set(int index, E element) {
// 校验指定元素的范围
Objects.checkIndex(index, size);
// 获取元素
E oldValue = elementData(index);
// 设置元素
elementData[index] = element;
// 返回就值
return oldValue;
}
get 读取
public E get(int index) {
Objects.checkIndex(index, size);
return elementData(index);
}
remove
/**
* 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) {
// 范围校验
Objects.checkIndex(index, size);
final Object[] es = elementData;
// 获取删除元素
@SuppressWarnings("unchecked") E oldValue = (E) es[index];
// 快速删除
fastRemove(es, index);
// 范围旧元素
return oldValue;
}
fastRemove
private void fastRemove(Object[] es, int i) {
//变化次数+1
modCount++;
final int newSize;
// 这里有一个判断，如果删除的不是最后一个，直接数组拷贝即可。
if ((newSize = size - 1) > i)
System.arraycopy(es, i + 1, es, i, newSize - i);
// 最后一个直接值为空
es[size = newSize] = null;
}
看的出来，缩容并不会降低数组。
也就是一直扩容到最大，删除完，依然会占用这么多内存。
小结
jdk 的源码，其实跟以前学习 c 的时候数据结构的移动差异还是很大的。
参考资料
jdk11
