ArrayList源码分析

ArrayList是一个拥有变长能力的数据结构，它的初始长度是根据构造方法的不同而发生变化的；

首先看看它的无参构造方法：

private static final long serialVersionUID = 8683452581122892189L;    /**     * Default initial capacity.     */    private static final int DEFAULT_CAPACITY = 10;    /**     * Shared empty array instance used for empty instances.     */    private static final Object[] EMPTY_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.//它跟EMPTY_ELEMENTDATA的主要区别在于插入第一个元素的时候，     */    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};    /**     * 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    /**     * 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);        }    }    /**     * Constructs an empty list with an initial capacity of ten.     */    public ArrayList() {        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;//无参构造方法中使用的是默认长度的数组，没有添加内容之前

    它的长度为零跟EMPTY_ELEMENTDATA的长度一样    }

刚开始我是有疑问的，我声明一个ArrayList列表数组内部是如何将数组的长度设为10的呢，无参构造方法只是构造了一个空的数组，并没有指定长度，接下来看看它是怎么完成的，这个躲猫猫玩法可以：插入第一个元素的时候就开始对数组的长度进行规定：

public class  TestList {     public static void main(String[] args){          ArrayList a  = new ArrayList();          a.add(3);          a.add(4); }}

单步调试该程序：

进入 add方法：

/**     * Appends the specified element to the end of this list.     *     * @param e element to be appended to this list     * @return <tt>true</tt> (as specified by {@link Collection#add})     */    public boolean add(E e) {        ensureCapacityInternal(size + 1);  // Increments modCount!!        elementData[size++] = e;        return true;    }

每次进入add方法都要看看数组容量是否爆满：进入ensureCapacityInternal方法看看：

private void ensureCapacityInternal(int minCapacity) {        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));    }

这里先是计算容量：

private static int calculateCapacity(Object[] elementData, int minCapacity) {        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {            return Math.max(DEFAULT_CAPACITY, minCapacity);        }        return minCapacity;        }

这个时候elementData.length为0，elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA 为true

这个时候minCapacity = 1; 返回的最大值是10，再进入ensureExplicitCapacity(10);

private void ensureExplicitCapacity(int minCapacity) {        modCount++;        // overflow-conscious code        if (minCapacity - elementData.length > 0)            grow(minCapacity);    }

第一次插入元素的时候minCapacity = 10,elementData.length = 0,会进入grow函数但是不会扩容

 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);    }

这时候newCapacity = 10了，这时候进入Arrays.copyOf(elementData,newCapacity)创建了一个新的数组对象它的长度为10，这就是数组长度为10的答案了，以后在进入calculateCapacity时候直接返回size + 1，当数组长度为10或者超过10的时候进行扩容。

而构造方法：ArrayList(int capacity)就不一样了，它会直接创建一个你指定大小的对象数组，扩容也是根据这个容量值进行的