ArrayList 的源码分析

ArrayList 中最主要的是扩容机制，因为ArrayList 是非线程安全的，所以当多个线程同时操作时，会导致java.util.ConcurrentModificationException异常

1. 新创建的ArrayList对象，如果没有指定长度，则默认为0.

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

/**
     * 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 = {};

 /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

2. add(E e): 添加元素

 /**
     * 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) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

// 计算容量的大小
  private static int calculateCapacity(Object[] elementData, int minCapacity) {
        //如果初始化的容量大小为DEFAULTCAPACITY_EMPTY_ELEMENTDATA，即为0，则取Math.max(DEFAULT_CAPACITY, minCapacity)之间的最大值，即取DEFAULT_CAPACITY的值，为10
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

//调用扩容的方法：
private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        //判断数组当前长度是否大于数组容量长度，如果大于则需要扩容
        if (minCapacity - elementData.length > 0)
            //调用扩容的方法
            grow(minCapacity);
    }


 private void grow(int minCapacity) {
        // overflow-conscious code
        // 获取数组的长度
        int oldCapacity = elementData.length;
        // 在原数组的长度的基础上添加1.5 半， oldCapacity >> 1 表示oldCapacity的一半，
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //如果新的长度小于容量长度10,则新数组长度设置为10.
        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:
        // 将原数组元素复制到新数组中，并赋值给elementData.
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

  private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
// 如果大于 MAX_ARRAY_SIZE ，即Integer.MAX_VALUE -8
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }