ArrayList源码分析

1. 源码分析

　　

public ArrayList() {.   //默认构造函数，若初始值指定为null则将赋值为一个空的数组
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

public ArrayList(int initialCapacity) {      //指定初始值
    if (initialCapacity > 0) {               //若大于零则初始化一个Object数组
        this.elementData = new Object[initialCapacity];
    } else if (initialCapacity == 0) {        //赋值为一个空的数组
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    }
}


public boolean add(E e) {      //添加元素
    modCount++;                //modCount表示容器被改变的次数
    add(e, elementData, size); 
    return true;
}

private void add(E e, Object[] elementData, int s) {
    if (s == elementData.length)           //如果说数组已满则会进行扩容操作
        elementData = grow();            
    elementData[s] = e;                //将元素插入到当前位置
    size = s + 1;
}

private Object[] grow() {
    return grow(size + 1);
}

private Object[] grow(int minCapacity) {          //将元素复制到新的数组
    return elementData = Arrays.copyOf(elementData,
                                       newCapacity(minCapacity));
}

private int newCapacity(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;        
    int newCapacity = oldCapacity + (oldCapacity >> 1);    //扩容后的容量1.5倍
    if (newCapacity - minCapacity <= 0) {                //扩容后的小于最小要求的容量
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA)    //没有指定初始容量
            return Math.max(DEFAULT_CAPACITY, minCapacity);  
        if (minCapacity < 0) // overflow  
            throw new OutOfMemoryError();
        return minCapacity;
    }
    return (newCapacity - MAX_ARRAY_SIZE <= 0)
        ? newCapacity
        : hugeCapacity(minCapacity);
}

public E remove(int index) {
    Objects.checkIndex(index, size);   //检测要移除的是否超过容器长度
    final Object[] es = elementData;    

    @SuppressWarnings("unchecked") E oldValue = (E) es[index];
    fastRemove(es, index);

    return oldValue;
}

private void fastRemove(Object[] es, int i) {
    modCount++;                    //modCount为容器改变的次数
    final int newSize;            //新的容器大小为size-1
    if ((newSize = size - 1) > i)        
        System.arraycopy(es, i + 1, es, i, newSize - i);    //将要删除的元素直接覆盖掉
    es[size = newSize] = null;   //最后一个元素则直接设置为null
}




//一些成员变量
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;   //默认的长度，方初始化长度为null是在扩容是会指定                                        
                                                    为10

/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {};   //在初始化时将长度指定为0

/**
* 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 = {};   //不指定初始化容量时的大小

/**
* 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.
*/
transient Object[] elementData; // non-private to simplify nested class access //用来存储容器的，transient不会再序列化时被写入
        

/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;        //存储的元素的数量（不是elementData的长度）

/**
* 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
*/

2. 为什么要使用transient来修饰elementData?

　　由于elementData是一个可缓冲的数组，在一般情况下该数组可能不为满（因为每次数组满后都会进行1.5被扩容），因此不会直接使用该数组进行序列化。

3. ArrayList如何进行序列化？（关于序列化的具体细节可看另一篇博客）

　　arrayList通过继承Serializable接口，重写了readObject()，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++) {       //size为实际元素的数量，遍历数组将不为空的元素进行序列化
            s.writeObject(elementData[i]);
        }

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