java源码_ArrayList(一)

阅读ArrayList源码,学习到以下:

1.ArrayList是动态数组,底层是数组,有以下核心成员变量

a.存储元素的数组

transient Object[] elementData;

b.记录元素的个数

private int size;

c.初始容量

private static final int DEFAULT_CAPACITY = 10;

d.默认空数组,为了首次添加元素时候判断需要扩容多少用

/**
 * 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 = {};

e.结构更改次数,add,remove,扩容等结构性修改的时候,modCount会+1,主要为了fail-fast

protected transient int modCount = 0;

f.最大容量,由于一些vm需要保留一些空间作为数组头,所以需要留一些空间

/**
 * 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;

2.ArrayList支持序列化,但elementData却为transient

因为elementData的长度可能超过size的长度,如果把elementData全部序列化,会拷贝很多空值,序列化时候,重写writeObject方法

private void writeObject(java.io.ObjectOutputStream s)
    throws java.io.IOException{
    // Write out element count, and any hidden stuff
    int expectedModCount = modCount;
    s.defaultWriteObject();

    // Write out size as capacity for behavioural compatibility with clone()
    s.writeInt(size);

    // Write out all elements in the proper order.
    for (int i=0; i<size; i++) {
        s.writeObject(elementData[i]);
    }

    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
}