工厂模式和ArrayList源码解读

一.工厂方法

定义一个创建对象的接口，但让实现这个接口的类来决定实例哪个类，工厂方法让类的实例化推迟到子类中进行，

解决同一产品的生产工厂问题，定义规范契约。

适用场景：

1.创建对象需要大量重复的代码
2.应用层不依赖于产品类实例如何被创建，实现等细节

3.一个类通过其子类来指定创建哪个类
优点：用户只需要关心所需产品对应的工厂，无须关心创建细节。缺点：类的个数容易过多，增加复杂度，增加了系统的抽象性和理解程度。

UML例子：

 例如FEVideoFactory通过继承抽象类VideoFactory，得到了getVideo（）方法，返回了FEVideo（），FEVideo（）就能够产生相应的具体的你想要Video。产生哪类Video具体交给子类去决定和实现。

二.ArrayList源码解读

介绍了上面的工厂方法，ArrayList在JDK源码中就是采用了工厂模式。Collection就是一个工厂，工厂里声明了工厂方法规范，Iterator（ return new Itr（）），具体的实现由子类去实例化，如Arraylist就实现这个Iterator（），和Arraylist同一级别的实现工厂还有很多，Arraylist的产品为Itr。

1.ArrayList的前言介绍

ArrayList的本质是动态数组，是非线程安全的集合，集合扩容时会创建更大的数组空间，把原有数据复制到新数组中，ArrayList支持对元素的随机访问，但是插入与删除时速度会很慢，因为这个过程要移动数组。ArrayList在JDK1.8的源码如下：

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    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.
     */
    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;
    }

    /**
     * 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;
        }
    }

    /**
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the
     * list's current size.  An application can use this operation to minimize
     * the storage of an <tt>ArrayList</tt> instance.
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

    /**
     * Increases the capacity of this <tt>ArrayList</tt> instance, if
     * necessary, to ensure that it can hold at least the number of elements
     * specified by the minimum capacity argument.
     *
     * @param   minCapacity   the desired minimum capacity
     */
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            // any size if not default element table
            ? 0
            // larger than default for default empty table. It's already
            // supposed to be at default size.
            : DEFAULT_CAPACITY;

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

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

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

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * 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;

    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    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);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
}

解读：通过上述源码可以得出，当你设置了ArrayList默认值时（默认为10），如果你往里面添加元素，索引将会检测出当前是否足够容纳，如果不能则调用grow（）方法进行扩容，计算公式为newCapacity=OldCapacity+(OldCapacity>>1)。

如果设置原始容量为13，当新添加一个元素时，根据计算方法，13的二进制数为1101，我们都知道二进制正数加+1必须向右移动一位，操作后就变成二进制数110，即十进制数为6。最终的扩容结果显而易见就是newCapacity=OldCapacity+(OldCapacity>>1)=13+6=19.

适用位运算主要是基于计算效率的考虑。当使用无参构造时，默认容量为10，第一次add的时候分配为10的容量，后面每次扩容都会调用Array.copyOf方法，创建新数组再复制，那假设有1000个元素放置在Arraylist中，如果在Araaylist初始化的时候没有显式的定义Arraylist的大小，则放置1000个元素必须被动扩容13次才能存储。所以在初始化Arraylist的时候便指定了容量，那么在初始化的时候便直接分配1000个存储空间，这样避免复制带来的开销。如果不指定容量，很有可能在扩容的时候产生OOM异常