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;
    }