JDK源码阅读--ArrayList

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

ArrayList继承了AbstractList抽象类，实现了List、RandomAccess、Cloneable、Serializable接口。
ArrayList的默认初始容量为10，阅读源码可知--底层数据结构是数组。添加的元素是可以重复的。

底层数据结构是数组，查询快，增加、删除元素慢(因为每增加或删除元素，数组的其余位置的元素都需要进行变动)。

 /**
     * Default initial capacity.
     * 默认初始容量为10
     */
    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; // 非私有，以简化嵌套类访问。被transient修饰的，不能被序列化

    /**
     * The size of the ArrayList (the number of elements it contains).
     * ArrayList的长度(元素个数)
     * @serial
     */
    private int size;

ArrayList的构造方法的源码：

 /**
     * 使用默认容量构造一个数组
     *
     * @param  initialCapacity  这个列表的初始化容量
     * @throws IllegalArgumentException 如果初始化容量initialCapacity是一个负数，则会抛出这个IllegalArgumentException异常
     */
    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.
     * 构造初始容量为10的空数组
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     *  构造包含指定的元素的列表集合，按照顺序返回这个集合的迭代器。
     *
     * @param c 要放到集合中的Collection
     * @throws NullPointerException 如果传入的Collection为null，则会抛出NullPointerException空指针异常
     */
    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;
        }
    }

重写了clone方法。

  /**
     * Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The elements themselves are not copied.)
     * 返回此ArrayList实例的浅拷贝。(元素本身不会被复制。)
     * @return a clone of this <tt>ArrayList</tt> instance
     */
    public Object clone() {
        try {
            ArrayList<?> v = (ArrayList<?>) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

add方法：

    /**
     * 将指定元素追加到列表末尾。所以ArrayList是有序的，也就是元素的存储顺序和添加顺序是一致的。
     *
     * @param e element to be appended to this list 追加到列表中的元素
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        //确保内部的容量
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;//在list末尾追加元素
        return true;
    }

    /**
     * 在list的指定索引处插入指定元素
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index   被插入的索引位置 index at which the specified element is to be inserted
     * @param element 被插入的元素 element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
        //检查索引是否越界
        rangeCheckForAdd(index);
        //确保内部的容量
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                size - index);
        elementData[index] = element;
        size++;
    }

remove方法：

    /**
     * 移除指定位置的元素，将指定位置元素之后的元素左移动。
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index 要被移除的元素的索引 the index of the element to be removed
     * @return 返回ArrayList中被移除的元素 the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        //检查索引是否越界，如果越界，则抛出IndexOutOfBoundsException异常
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);//取出要被移除的元素

        //将被删除的元素的索引位置右边的所有元素往左移动一位
        int numMoved = size - index - 1;
        if (numMoved > 0) {
            System.arraycopy(elementData, index + 1, elementData, index,
                    numMoved);
        }
        //将ArrayList中原先最后位置的索引位置置为null，以便GC完成它的工作
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;//返回被移除的元素
    }

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If the list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

 /**
     * 增加容量，以确保它至少可以持有由最小容量参数指定的元素容量。
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity 期望的最小容量
     */
    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);//
    }

    /**
     * 返回ArrayList中指定位置(索引index处)的元素
     * Returns the element at the specified position in this list.
     *
     * @param  index  索引 index of the element to return
     * @return list中指定索引处的元素 the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        //检查索引是否越界
        rangeCheck(index);
        //返回指定索引处index的元素
        return elementData(index);
    }

 /**
     * 将ArrayList中某个索引index位置上的元素A替换成你指定的元素B
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     *
     * @param index 替换的元素的索引 index of the element to replace
     * @param element 替换后的元素 element to be stored at the specified position
     * @return 该位置上替换前的元素 the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        //检查索引是否越界
        rangeCheck(index);
        //获取index索引处原先的元素
        E oldValue = elementData(index);
        //将index索引处的元素替换成你指定的元素element
        elementData[index] = element;
        //返回index索引处原先的元素
        return oldValue;
    }

   /**
     * A version of rangeCheck used by add and addAll.
     * 进行范围检查，也就是索引是否越界
     */
    private void rangeCheckForAdd(int index) {
        //如果索引越界，则抛出IndexOutOfBoundsException异常
        if (index > size || index < 0) {
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }
    }

    /**
     * 构造IndexOutOfBoundsException异常的详细信息
     * Constructs an IndexOutOfBoundsException detail message.
     * Of the many possible refactorings of the error handling code,
     * this "outlining" performs best with both server and client VMs.
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }