java怎么编写 JAVA怎么编写自动饮水机

import java.util.*;
public class k28 {
	public static void main(String[] args){
		Scanner in=new Scanner(System.in);
		System.out.println("\t自动售水系统【满20元享9折】");
		System.out.println("=========================================");
		System.out.println("1.矿泉水   2.咖啡   3.百事可乐   \n4.冰红茶   5.脉动    6.雪碧");
		double price;
		System.out.println("请输入购买的商品编号：");
		int i=in.nextInt();
		switch(i){
		case 1:
			price=2;break;
		case 2:
			price=5.5;break;
		case 3:
			price=3;break;
		case 4:
			price=3;break;
		case 5:
			price=4.5;break;
		case 6:
			price=2.5;break;
			default:
				System.out.println("该商品已下架！");
				return;
		}
		System.out.println("请输入购买的数量：");
		int num=in.nextInt();
		double total=price*num;//应付金额
		String s="";
		if(total>=20){
			total=total*0.9;
			s="已享受9折优惠！";
		}
		System.out.println(s+"请付款"+total+"元！");
		System.out.println("请输入付款金额：");
		double fk=in.nextDouble();
		if(fk<total){
			System.out.println("付款金额不足");
			System.out.println("******谢谢惠顾********");
			return;
		}
		else if(fk==total){
			System.out.println("金额已付，请取走商品！");
			System.out.println("******谢谢惠顾********");
		}
		else if(fk>total){
			String z=String.format("%2f",(fk-total));

			System.out.println("找零：" +z+"元！");
			System.out.println("金额已付，请取走商品！");
			System.out.println("******谢谢惠顾********");
		}
	}}