ArrayList简介
ArrayList称为数组链表，它是继承AbstractList，并实现了List、RandomAccess、Cloneable和Serializable接口
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
}
ArrayList一共有三个构造方法，三个构造方法如下：
/**
* 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 an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_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();
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;
}
}
下面逐步分析ArrayList的构造方法中做了哪些事情。
1 无参构造方法
/**
* 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.
* ArrayList的缓冲数组，里面存储的是ArrayList数据。ArrayList的容量长度就是这个数组的长度，任何空的使用elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA初始化的ArrayList在添加第一个元素时都会被扩容为DEFAULT_CAPACITY （10）
*/
transient Object[] 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.
* 用于默认大小的空的共享空数组实例。我们将此与EMPTY_ELEMENTDATA区分开来，以便在添加第一个元素时知道需要扩张多少。
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
/**
* Constructs an empty list with an initial capacity of ten.
* 构造一个初始容量为10的空list。
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
看了上面的源码的注释，才知道，ArrayList的无参构造函数里ArrayList的初始容量为空，真正初始化容量的地方是在添加第一个元素的时候。
2 一个参数的构造方法，参数为初始的容量
/**
* Shared empty array instance used for empty instances.
* 空的共享数组实例
*/
private static final Object[] EMPTY_ELEMENTDATA = {};
/**
* Constructs an empty list with the specified initial capacity.
* 构造一个指定容量的空List
*
* @param initialCapacity the initial capacity of the list
* initialCapacity是list的初始容量
* @throws IllegalArgumentException if the specified initial capacity is negative
* 如果指定容量为负数，则抛出异常
*/
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {//#1
this.elementData = new Object[initialCapacity];//#2
} else if (initialCapacity == 0) {//#3
this.elementData = EMPTY_ELEMENTDATA;//#4
} else {
throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);//#5
}
}
- 第#1行如果指定容量大于0，则进行#2操作
- 第#2行使用指定容量初始化ArrayList的数组。
- 第#3行如果指定容量为0，则进行#4
- 第#4行初始化elementData为空数组
- 第#5行如果指定容量小于0，则抛出异常
3 参数为集合的构造方法
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*按照集合的迭代器返回的顺序构造一个包含指定集合元素的列表。
* @param c the collection whose elements are to be placed into this list
* 参数是要放入列表中的集合
* @throws NullPointerException if the specified collection is null
* 如果指定集合为空则报空指针异常
*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();//#1
if ((size = elementData.length) != 0) {//#2
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)//#3
elementData = Arrays.copyOf(elementData, size, Object[].class);//#4
} else {
// replace with empty array.
this.elementData = EMPTY_ELEMENTDATA;//#5
}
}
- 第#1行将参数集合转化为数组并赋值给elementData
- 第#2行如果elementData的长度（size指的是数组中数据的个数）不等于0，则进行#3，否则进行#5
- 第#3行如果elementData不是Object数组类型，则执行#4
- 第#4行将数据赋值到Object数组类型的elementData
- 第#5行如果elementData长度为0，初始化elementData为空数组
4 总结
以下针对构造函数进行总结
- ArrayList就是一个数组，在ArrayList里是通过Object[] elementData来存储数据的。
- ArrayList初始化时容量为0（使用空构造方法初始化），它在添加第一个元素（调用add方法）时如果ArrayList容量为0，则会扩容默认值10。
