ArrayList 原码解析

基于java1.7
1.ArrayList 是基于数组实现的
通过源码 我们可以知道 ArrayList 提供了三种构造器用于实例化

/**
* 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) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}

/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
super();
this.elementData = 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) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
这三个构造器 都是赋值给 elementData 变量，由源码可知 elementData 是一个数组对象，同时我们会发现这个变量是由transient修饰的，用于反序列化
/**
* 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 == EMPTY_ELEMENTDATA will be expanded to
* DEFAULT_CAPACITY when the first element is added.
*/
private transient Object[] elementData;
2. 数组初始化大小及动态扩容
如果 我们调用一个无参的构造器 实例化一个ArrayList,elementData 是一个未初始化大小的数组。那何时会给这个数组初始化大小，初始化大小的值又是多少，继续看源码。
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
当我们调用 add(E e) 方法时，方法内部调用了ensureCapacityInternal(int minCapacity) 方法，方法内部做了一个判断 如果 elementData 时{} 则 对 minCapacity 赋值 取 DEFAULT_CAPACITY和minCapacity 中较大的一个。DEFAULT_CAPACITY的值为10，所以初始化的数组大小为10.
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 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;
}
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}

ensureExplicitCapacity(minCapacity);
}
当往ArrayList 中加入的值超过10 ，数组就需要动态扩容，具体如何动态扩容，扩多大 ，我们看下面的源码。首先判断了 当前ArrayList的大小+1(minCapacity=size+1) 和 数组的大小。如果 当前ArrayList的大小+1 大于 数组大小 就需要调用 grow(int minCapacity) 进行数组扩容 ，扩容公式：oldCapacity + (oldCapacity >> 1) 也就是用原先的 数组大小 + 原先数组大小右移一位。例：初始化大小 为 10 第一次扩容后数组大小即为 10 +5 = 15.
private void ensureExplicitCapacity(int minCapacity) {
modCount++;

// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
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);
}
那数组大小最大可以支持多少。数组肯定不可能无限扩展，所以数组的最大为Integer.MAX_VALUE 0x7fffffff

private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}