ArrayList

一、概述

ArrayList底层数据结构为数组，故保持了数组的基本特点：随机访问速度较快，删除和插入(如果是末尾速度也还好)数据速度较慢，因为要用System.arraycopy()来移动。默认初始容量为10，超出容量扩容按50%增加。

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 = {};

    /**
     * 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 == EMPTY_ELEMENTDATA will be expanded to
     * DEFAULT_CAPACITY when the first element is added.
     */
    private transient Object[] elementData;

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;
    .........

其中elementData(真正保存数据的数组)，注意到是transient修饰的，为什么呢？首先，序列化是对象转为字节序列的过程，反序列化则是字节序列回复为对象的过程。transient是用来表示一个域不是该对象序行化的一部分，当一个对象被序行化的时候，transient修饰的变量的值是不包括在序行化中的。elementData用transient修饰的原因在于elementData里面不是所有的元素都有数据，因为容量的问题，elementData里面有一些元素是空的，这种是没有必要序列化的。那ArrayList是可以进行序列化的，那elementData是怎么进行序列化的？ArrayList是通过其中的两个方式实现的：readObject和writeObject。

    /**
     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;

        // Read in size, and any hidden stuff
        s.defaultReadObject();

        // Read in capacity
        s.readInt(); // ignored

        if (size > 0) {
            // be like clone(), allocate array based upon size not capacity
            ensureCapacityInternal(size);

            Object[] a = elementData;
            // Read in all elements in the proper order.
            for (int i=0; i<size; i++) {
                a[i] = s.readObject();
            }
        }
    }

    /**
     * Save the state of the <tt>ArrayList</tt> instance to a stream (that
     * is, serialize it).
     *
     * @serialData The length of the array backing the <tt>ArrayList</tt>
     *             instance is emitted (int), followed by all of its elements
     *             (each an <tt>Object</tt>) in the proper order.
     */
    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();
        }
    }

二、主要实现

1、构造函数

ArrayList(int initialCapacity) ：构造具有初始容量的空列表；
ArrayList()：默认构造函数，提供初始容量为10的空列表；
ArrayList(Collection<? extends E> c)：构造一个包含指定collection 的元素的列表，这些元素是按照该 collection 的迭代器返回它们的顺序排列的。

/**
     * 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) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        super();
        this.elementData = 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();
        size = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    }

2、trimToSize

是为了缩小elementData的大小以达到节约内存的目的，比如某次场景需要扩容至100，而此后size存储量基本在10以内，那么就可以用这个函数了。另外，在ArrayList中常用到Arrays.copyof，其底层实现是通过System.arraycopy来完成的。

public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = Arrays.copyOf(elementData, size);
        }
    }

3、新增元素

add(E e)、add(int index, E element)、addAll(Collection<? extends E> c)、addAll(int index, Collection<? extends E> c)、set(int index, E element) ，这里主要分析add：

/**
     * 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;
    }
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

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

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

上述代码中重点关注grow方法，新增元素如果需要扩容，则增加50%。另外在add和remove中都存在modcount，修改次数，在readObject、writeObject、Itr中的next中都有用到，在这些方法中如果modcount改变了将会抛出异常：ConcurrentModificationException()。意思就是在遍历遍历的过程中如果对数组进行了增删操作将会抛出异常。以Itr举例，因为ArrayList被设计成非同步的，所以如果在遍历集合过程中删除元素不抛出异常，则会抛出：ArrayIndexOutOfBoundsException。

private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor != size;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

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

4、删除

remove(int index)、remove(Object o)、removeRange(int fromIndex, int toIndex)、removeAll()。

/**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

查找很简单，就不分析了。

三、区别与联系

1、与LinkedList

1）、ArrayList是实现了基于动态数组的数据结构，LinkedList基于链表的数据结构。
2）、对于随机访问get和set，ArrayList觉得优于LinkedList，因为LinkedList要移动指针。
3）、对于新增和删除操作add和remove（不是在尾部添加删除），LinkedList比较占优势，因为ArrayList要移动数据。

2、与vector

1）、Vector和ArrayList几乎是完全相同的,唯一的区别在于Vector是同步类(synchronized)，属于强同步类。因此开销就比ArrayList要大，访问要慢。正常情况下,大多数的Java程序员使用ArrayList而不是Vector,因为同步完全可以由程序员自己来控制。
2）、Vector每次扩容请求其大小的2倍空间，而ArrayList是1.5倍。
3）、Vector还有一个子类Stack.