Java ArrayList
属性
/**
* Default initial capacity.
*
* 如初始化时不指定，则默认容量为10
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
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.
*/
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.
*
* 用来存储所有元素
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
* 当前 elementData 已经有的元素的长度，新元素会放在 elementData[size] 下
*/
private int size;构造器
默认构造器
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}指定长度
/**
* 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) {
// 新建长度为 initialCapacity 的数组
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}传入集合
/**
* 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 c) {
elementData = c.toArray();
// 如果传入的集合长度不为 0 ，则把集合中的元素 copy 到 elementData 中
if ((size = elementData.length) != 0) {
// defend against c.toArray (incorrectly) not returning Object[]
// (see e.g. https://bugs.openjdk.java.net/browse/JDK-6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}扩容
最大长度
根据注释可以看到，有些虚拟机需要在数组中保存额外信息（保存的额外信息）需要占用一定的空间，所以 ArrayList 的最大长度为 2^32 - 8
/**
* 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
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;每次扩容时候，newCapacity 为 oldCapacity + (oldCapacity >> 1)，相当于旧容量的 1.5 倍
/**
* 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);
// 按照扩容规则计算的新容量小于预期容量
if (newCapacity - minCapacity <= 0) {
// 当使用默认构造器进行初始化时，在此处才进行底层数组的初始化
// 新的底层数组长度为默认容量（10）和期望容量之间最大的值
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
return Math.max(DEFAULT_CAPACITY, minCapacity);
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return minCapacity;
}
// 按照扩容规则计算出的容量是否超过默认最大容量？
// 不超过：则直接返回
// 超过：根据 hugeCapacity() 计算出新的长度
return (newCapacity - MAX_ARRAY_SIZE <= 0)
? newCapacity
: hugeCapacity(minCapacity);
}当通过默认规则扩大为原来 1.5 后如果超过默认最大容量时，则通过比较预期容量和默认最大容量来判断最大容量；
当预期容量比默认的 MAX_ARRAY_SIZE 大时，则返回 Integer.MAX_VALUE
否则返回默认的 MAX_ARRAY_SIZE
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE)
? Integer.MAX_VALUE
: MAX_ARRAY_SIZE;
}/**
* 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));
}
private Object[] grow() {
return grow(size + 1);
}add()
add 方法一共有两个公开方法，一个是在尾部添加元素的 add(E e) 和在任意位置插入元素的 add(int index, E element)
尾部插入 add(E e)
该公开方法调用了 add(E e, Object[] elementData, int s)
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return {@code true} (as specified by {@link Collection#add})
*/
public boolean add(E e) {
modCount++;
add(e, elementData, size);
return true;
}如果当前容量已满则进行扩容，否则直接把元素放到 s (= size) 的位置，数组元素长度 size + 1
/**
* This helper method split out from add(E) to keep method
* bytecode size under 35 (the -XX:MaxInlineSize default value),
* which helps when add(E) is called in a C1-compiled loop.
*/
private void add(E e, Object[] elementData, int s) {
if (s == elementData.length)
elementData = grow();
elementData[s] = e;
size = s + 1;
}任意位置插入 add(int index, E element)
/**
* 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) {
// 检查要插入的位置是否越界
// 检查的范围为 [0, size]
rangeCheckForAdd(index);
modCount++;
final int s;
Object[] elementData;
// 数组已经放满了，需要进行扩容
if ((s = size) == (elementData = this.elementData).length)
elementData = grow();
// 把原来元素拷贝一份到新数组
System.arraycopy(elementData, index,
elementData, index + 1,
s - index);
elementData[index] = element;
size = s + 1;
}