ArrayList源码分析

     /**
     * Default initial capacity.
     */
    //默认的初始容量
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     */
    //一个空数组，当创建无参ArrayList时，底层数组默认的就是这个
    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.
     */
    //存放元素的底层数组，此数组的长度也就是数组列表的容量
    private transient Object[] elementData;

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    //ArrayList的大小，也就是它存储元素的个数
    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
     */
    //构造一个ArrayList，并指定初始容量initialCapacity
    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.
     */
    //构造一个ArrayList，它初始容量是默认的，为10
    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
     */
    //构造一个ArrayList，并将指定的集合中的元素放到这个数组列表中
    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);
    }

public boolean add(E e) {
        //确保ArrayList容量大于等于现有的元素个数+1
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //将元素添加到数组的尾部
        elementData[size++] = e;
        return true;
    }

 private void ensureCapacityInternal(int minCapacity) {
        //获取期望得到的ArrayList容量
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

  private void ensureExplicitCapacity(int minCapacity) {
        modCount++; //修改次数+1

        // overflow-conscious code
        //如果期望的到的ArrayList容量大现在的ArrayList容量，那么就扩容，就是创建一个长度更大的数组，然后将原来数组中的元素拷贝到新数组中，这个新数组作为ArrayList的底层数组
        //需要理解的是：ArrayList的容量也就是它底层数组的长度，ArrayList的大小size是指它存储元素的个数；这两个概念要清楚
        if (minCapacity - elementData.length > 0)
　　　　　　　//扩容操作 　
            grow(minCapacity);
    }

 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:
        //将原来数组中的元素拷贝到新数组中，新数组的长度是newCapacity
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

 public void add(int index, E element) {
        //检查index，是否越界
        rangeCheckForAdd(index);
        //确保数组长度够用
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //通过copy的方式，从下标位置是index开始到最后一个元素，整体后移1个单位
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        //把指定元素插入index位置
        elementData[index] = element;
        size++;
    }

  public E remove(int index) {
        //检查是否越界
        rangeCheck(index);
        //修改次数+1
        modCount++;
        //指定位置的元素
        E oldValue = elementData(index);
        //要移动的元素个数
        int numMoved = size - index - 1;
        if (numMoved > 0)
            //通过copy的方式，将index+1位置开始的元素整体向前移动一个单位（其实是拷贝）
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        //将最后一个元素位置的值设置为null，然后元素个数-1
        elementData[--size] = null; // clear to let GC do its work
    
        return oldValue;
    }

 public E set(int index, E element) {
        rangeCheck(index);
        //获取指定位置的元素，被替换后，并返回
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

 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++)
                //找到指定元素的下标位置
                if (o.equals(elementData[index])) {
                    //移除操作
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

 private void fastRemove(int index) {
        //修改次数+1
        modCount++;
        //需要移动的元素的数量
        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
    }

    public boolean addAll(int index, Collection<? extends E> c) {
        //判断index是否越界
        rangeCheckForAdd(index);
     
        Object[] a = c.toArray();
        int numNew = a.length;
        //确保数组长度够用，不够就扩容 
        ensureCapacityInternal(size + numNew);  // Increments modCount
        //经过以下两次copy操作就可以把collection中的元素全部插入到列表中 
        int numMoved = size - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);

        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }