源码分析——new ArrayList() 和 new ArrayList(0)区别

​1、分析

初始化

/**
* 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 = {};

/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};

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

都是初始化为一个空的Object[]，看着没差别，实际主要区别在扩容上

扩容

/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;

/**
* Increases the capacity of this <tt>ArrayList</tt> instance, if
* necessary, to ensure that it can hold at least the number of elements
* specified by the minimum capacity argument.
*
* @param   minCapacity   the desired minimum capacity
*/
public void ensureCapacity(int minCapacity) {
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
// any size if not default element table
? 0
// larger than default for default empty table. It's already
// supposed to be at default size.
: DEFAULT_CAPACITY;

if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}

private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}

ensureExplicitCapacity(minCapacity);
}

private void ensureExplicitCapacity(int minCapacity) {
modCount++;

// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}

ArrayList()：ensureCapacity方法计算出来的minCapacity是10

ArrayList(0)：ensureCapacity方法计算出来的minCapacity是0

/**
* 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
*/
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);
}

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

扩容规则：newCapacity = oldCapacity + (oldCapacity / 2)，即扩为原来的1.5倍

ArrayList()：0、1、2、3、4、6、9...Integer.MAX_VALUE - 8

ArrayList(0)：10、15、22、33、49...Integer.MAX_VALUE - 8

为什么最大容量为 Integer.MAX_VALUE - 8？

数组对象有一个额外的元数据，用于表示数组的大小（8bytes）

2、总结

都初始化为一个空的Object[]，但对象不同，区别在扩容规则不一样

在JDK7中，ArrayList()实际上是调用了ArrayList(10)，因为没有指定动态数组的初始容量，JDK会自动给动态数组初始化一个容量，以减少插入少量元素时动态数组的扩容开销

在JDK8中，基本差不多，只不过采用延迟初始化策略

​