public void insert(int i, Item item) {
		// TODO Auto-generated method stub
		if (i < 0 || i > n) 
			throw new IllegalArgumentException("参数i非法");
		if (n >= items.length) resize(items.length*2);
		for (int k = n-1; k >= i; k--) {
			items[k+1] = items[k]; 
		}
		items[i] = item;
		n++;
	}