成員變量
- elementData爲實際存儲數據的數組
- size爲ArrayList的實際大小,和elementData的長度有所區別
- 有一個特殊的地方,elementData使用transient修飾,即不可以被序列化,這麼做的原因是:
elementData數組的長度在大多數情況下,是大於實際數據的數量size的,所以不需要將空的元素也進行序列化操作。對elementData的序列化操作其實是由writeObject()和readObject()實現的。
private static final long serialVersionUID = 8683452581122892189L;
private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
private transient Object[] elementData;
private int size;
構造函數
- 三個構造函數
- 最後一個函數由於c.toArray可能不會反回一個Object數組,所以特殊處理一下
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
}
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
關於擴展容量的相關操作
- 方法調用關係:ensureCapacity() -> ensureExplicitCapacity() -> grow() -> hugeCapacity()
- 核心的方法是grow()方法,如果minCapacity大於原來容量的1.5倍,容量增加到minCapacity;否則增加到1.5倍。
- 對於ensureCapacity()這個方法,如果事先知道ArrayList所需存儲的數據量大小,我們可以直接調用來設置,避免每次拷貝數組花費的額外時間,提升效率。
/**
* 按照size的大小縮減elementData的長度
*/
public void trimToSize() {
modCount++;
if (size < elementData.length) {
elementData = Arrays.copyOf(elementData, size);
}
}
/**
* 增加容量到minCapacity大小
*/
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != EMPTY_ELEMENTDATA)
? 0
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
/**
* 主要爲add方法所用
*/
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
/**
* 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;
/**
* 增加容量的核心函數。
* 如果minCapacity大於原來容量的1.5倍,容量增加到minCapacity;否則增加到1.5倍。
*/
private void grow(int minCapacity) {
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
elementData = Arrays.copyOf(elementData, newCapacity);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0)
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}