ArrayList

初始化

    //初始化一个object数组 长度为0
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

Add第一个元素

    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // size = 0
        elementData[size++] = e;
        return true;
    }

    private void ensureCapacityInternal(int minCapacity) { //0
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {//如果为空的话（空构造函数）
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); //取 DEFAULT_CAPACITY = 10 和 0 最大的值，结果为10
        }

        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {  //10
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0) //如果10 - 0 不大于0
            grow(minCapacity);
    }

 添加第11个元素
 

  public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // size = 10
        elementData[size++] = e;
        return true;
    }

    private void ensureCapacityInternal(int minCapacity) { //11
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {//不会进
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); 
        }

        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {  //11
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0) //如果11 - 10 大于0 扩容
            grow(minCapacity);
    }

    private void grow(int minCapacity) { //10
        // overflow-conscious code
        int oldCapacity = elementData.length; //10
        int newCapacity = oldCapacity + (oldCapacity >> 1); //newCapacity = 10+5 =15
        if (newCapacity - minCapacity < 0) //15-10 >0
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0) // newCapacity（15） 没有超过Integer.MAX_VALUE - 8
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);//复制数组  newCapacity(15)
    }
    
    //复制数组
    public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
        @SuppressWarnings("unchecked")
        //如果新类型是Object[]数组的话 就创建object数组 否则创建 新类型的数组， 长度为扩容后的长度
        T[] copy = ((Object)newType == (Object)Object[].class)
            ? (T[]) new Object[newLength]
            : (T[]) Array.newInstance(newType.getComponentType(), newLength);
        //复制数据到copy，数据终点为新长度和原始长度的最小值
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }

get()

    public E get(int index) {
        rangeCheck(index);//检查越界 IndexOutOfBoundsException

        return elementData(index); //时间复杂度O(1)
    }

 

随机插入

    //随机插入 耗时
    public void add(int index, E element) {
        //越界检查
        rangeCheckForAdd(index);

        //检查(size +1)是否需要扩容 
        ensureCapacityInternal(size + 1);  // Increments modCount!!

        //拷贝后面的数组
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        //index元素赋值
        elementData[index] = element;
        size++;
    }