ArrayList

Arraylist数据结构：

数组   Object[] EMPTY_ELEMENTDATA 

ArrayList初始长度：10

ArrayList扩容：

数组默认长度为10，重新创建一个新的数组，并将原始数据执行Arrays.copyOf()，每次扩容1.5倍长度

 private void add(E e, Object[] elementData, int s) {
        if (s == elementData.length)
            elementData = grow();
        elementData[s] = e;
        size = s + 1;
    }

//2 将原数组的内容copy到新的数组中
   private Object[] grow(int minCapacity) {
        return elementData = Arrays.copyOf(elementData,
                                           newCapacity(minCapacity));
    }
//1 创建一个新的数组，长度为原来的1.5倍
 private int newCapacity(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity <= 0) {
            if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
                return Math.max(DEFAULT_CAPACITY, minCapacity);
            if (minCapacity < 0) // overflow
                throw new OutOfMemoryError();
            return minCapacity;
        }
        return (newCapacity - MAX_ARRAY_SIZE <= 0)
            ? newCapacity
            : hugeCapacity(minCapacity);
    }

ArrayList删除元素：

  public E remove(int index) {
// 索引index是否在数组长度范围内
        Objects.checkIndex(index, size);
        final Object[] es = elementData;

        @SuppressWarnings("unchecked") E oldValue = (E) es[index];
        fastRemove(es, index);

        return oldValue;
    }
//删除元素，需要改元素后面的元素都向前移动一个位置
 private void fastRemove(Object[] es, int i) {
        modCount++;
        final int newSize;
        if ((newSize = size - 1) > i)
            System.arraycopy(es, i + 1, es, i, newSize - i);
//元素置为null 
        es[size = newSize] = null;
    }