为什么购物车每次只能存一条商品信息？

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package com.imooc.javapor3;
 
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Scanner;
 
//购物车类
public class ShoppingCart {
    private Map<String,GoodsInCart> shoppingCart;
     
    public ShoppingCart() {
         
    }
    public ShoppingCart(Map<String,GoodsInCart> shoppingCart) {
        this.setShoppingCart(shoppingCart);
    }
     
 
    public Map<String, GoodsInCart> getShoppingCart() {
        return shoppingCart;
    }
 
    public void setShoppingCart(Map<String, GoodsInCart> shoppingCart) {
        this.shoppingCart = shoppingCart;
    }
     
    //添加商品到购物车
    public void addGoodsToCart(GoodsManage gm) {
        System.out.println("购物车中的所有商品信息为：");
        gm.displayAllGoods();
        Iterator<Goods> it = gm.getGoodsSet().iterator();
        System.out.println("请输入要添加的商品编号：");
        Scanner sc = new Scanner(System.in);
        String goodsid = sc.nextLine();
         while(it.hasNext()) {
             Goods good = it.next();
             if(good.getGoodsId().equals(goodsid)){
                 System.out.println(good);
                System.out.println("请输入商品数量："); 
                int goodsnum = sc.nextInt();
                GoodsInCart goodsincart = new GoodsInCart(good,goodsnum);
                 
                this.shoppingCart.put(good.getGoodsId(), goodsincart);
                 
                System.out.println("商品添加成功！");
             } 
         }
        }
 
         
        
         
     
    //修改购物车中的商品数量
    public void updateNumInCart() {
         
    }
    //显示购物车中的所有商品
    public void displayAllInCart() {
         
        Iterator<Map.Entry<String, GoodsInCart>> it = this.shoppingCart.entrySet().iterator();
        if(it.hasNext()) {
            while(it.hasNext()){
                System.out.println( it.next().getValue().getGoods().getGoodsName() + ",数量： " + it.next().getValue().getNum() );
            }
        }else {
            System.out.println("购物车是空的哦,快去添加商品吧！");
        }
         
    }
    //结算
    public void settleAccounts() {
         
    }
     
   
     
     
}
//分割---------------------
package com.imooc.javapor3;
 
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Scanner;
import java.util.Set;
 
public class TestDemo {
   public static void main(String[] args){
       //测试Goods类和GoodsManage类
//     Set set = new HashSet();
//     GoodsManage gm = new GoodsManage(set);
//     gm.importGoods();
//     gm.displayAllGoods();
//     Scanner sc = new Scanner(System.in);
//     t.Menu();
//     int a = sc.nextInt();
       //Iterator it = gm.getGoodsSet().iterator();
        
       TestDemo t = new TestDemo(); 
       //
       Set<Goods> set = new HashSet<Goods>();
       GoodsManage gm = new GoodsManage(set);
        
       GoodsInCart  go = new GoodsInCart();
       ShoppingCart sh = new ShoppingCart();
        
       Map<String,GoodsInCart> shoppingCart = new HashMap<String,GoodsInCart>();
       sh.setShoppingCart(shoppingCart);
        
       //主循环结构
       boolean flag = true;
       while(flag) {
           t.Menu();
           Scanner sc = new Scanner(System.in);
           int a = sc.nextInt();
           switch(a) {
           case 1: t.GoodsMenuwhile(gm);
                   break;
           case 2: if(gm.getGoodsSet().size() == 0) {
                       System.out.println("商品信息还未导入！");
                       break;
                   }else{
                       t.CartMenuwhile(gm,sh);
                       break;
                   }
            
           case 0:flag=false;
                  sc.close();
                  System.out.println("谢谢使用！");
                  System.exit(0);
                  break;
           default:System.out.println("输入操作数不合理，请重新输入！");
           }
       }
        
        
   }
    
   //主菜单
   public void Menu() {
       System.out.println("****************************");
       System.out.println("**********主菜单**********");
       System.out.println("1--商品管理");
       System.out.println("2--购物车");
       System.out.println("0--退出");
       System.out.println("****************************");
       System.out.println("请输入对应数字进行操作：");
   }
    
   //商品管理菜单
   public void GoodsMenu() {
       System.out.println("****************************");
       System.out.println("*******商品管理*******");
       System.out.println("1--商品信息导入");
       System.out.println("2--显示所有商品信息");
       System.out.println("9--返回上一级菜单");
       System.out.println("****************************");
       System.out.println("请输入对应数字对商品进行管理：");
   }
     
   //商品管理逻辑结构 
   public void GoodsMenuwhile(GoodsManage gm) {  
       Iterator<Goods> it = gm.getGoodsSet().iterator();
       boolean flag1 = true;
        
       while(flag1) {
           GoodsMenu();
           Scanner sc1 = new Scanner(System.in);
           int a1 = sc1.nextInt();
           switch(a1) {
           case 1:gm.importGoods();
                  it=gm.getGoodsSet().iterator();
                  break;
           case 2:if(it.hasNext()) {
                   gm.displayAllGoods();
                        }else{
                            System.out.println("商品信息未导入，请先导入");
                       }
                   break;
           case 9:flag1 =false;
                 
                  break;
            default : System.out.println("输入操作数不合理，请重新输入");
                     break;
           }   
       }
    
   }
    
   //购物车管理菜单
   public void CartMenu() {
       System.out.println("****************************");
       System.out.println("*******购物车管理*******");
       System.out.println("1--添加商品到购物车");
       System.out.println("2--修改购物车中的商品数量");
       System.out.println("3--显示购物车中的所有商品信息");
       System.out.println("4--结算");
       System.out.println("9--返回上一级菜单");
       System.out.println("****************************");
       System.out.println("请输入对应数字对购物车进行管理：");
   }
    
   //购物车管理逻辑结构
   public void CartMenuwhile(GoodsManage gm,ShoppingCart sh) {
        
       boolean flag2 = true;
       while(flag2) {
           CartMenu();
           Scanner sc = new Scanner(System.in);
           int a = sc.nextInt();
           switch(a) {
             
           case 1: sh.addGoodsToCart(gm);
                   break;
           case 2:break;
           case 3:sh.displayAllInCart();
                   break;
           case 4:break;
           case 9: flag2 = false;
                   break;
           default : System.out.println("输入操作数不合理,请重新输入！");
                     break;
           }
            
            
            
       }
        
   }
}