ArrayList 类有三个构造方法,分别为无参构造方法,传int构造方法,和传集合构造方法
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_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);
}
}
/**
* 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();
if ((size = elementData.length) != 0) {
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;
}
}
其中无参构造方法初始化完成之后 创建一个空的Object数组 名为 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 = {};
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
在第一次调用add的时候将 elementData 扩容为10;
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);//拿到默认值 f返回最大值
}
return minCapacity;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) { //这里传入为 10
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0) //这时候初始化的时候elementData.length为0 minCapacity为10 调用 grow方法扩容
grow(minCapacity);
}
此时elementData 长度为 10 可以存放 10个对象,当第11个对象被 add的时候,判断是否需要扩容为true 则进入扩容逻辑,扩容jdk8 逻辑为 将elementData 数组长度扩容为 原有长度的1.5倍,代码中提现为
int newCapacity = oldCapacity + (oldCapacity >> 1);
之后扩容逻辑不变
有参传int构造方法int值为 初始化 elementData 数组长度
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;
/**
* 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);
}
}
读源码可以看到,传入值 大于0 则创建一个 长度为 该值的 Object数组 并且赋值给elementData,如果传入值为 0 则创建一个长度为10的Object数组 并且赋值给elementData.
还有一个构造放法, 啊 我累了 不看了