5-2购物车空指针异常

Question

package com.immo.car;public class Goods {   private   String   goodId;   private   String goodName;   private   double    price;   private   String    goodDesp;      Goods(String  goodId,String  goodName,double price,String goodDesp){    this.goodId=goodId;    this.goodName=goodName;    this.price=price;    this.goodDesp=goodDesp;   }   Goods(){       }public String getGoodId() { return goodId;}public void setGoodId(String goodId) { this.goodId = goodId;}public String getGoodName() { return goodName;}public void setGoodName(String goodName) { this.goodName = goodName;}public double getPrice() { return price;}public void setPrice(double price) { this.price = price;}public String getGoodDesp() { return goodDesp;}public void setGoodDesp(String goodDesp) { this.goodDesp = goodDesp;}@Overridepublic int hashCode() { final int prime = 31; int result = 1; result = prime * result + ((goodDesp == null) ? 0 : goodDesp.hashCode()); result = prime * result + ((goodId == null) ? 0 : goodId.hashCode()); result = prime * result + ((goodName == null) ? 0 : goodName.hashCode()); long temp; temp = Double.doubleToLongBits(price); result = prime * result + (int) (temp ^ (temp >>> 32)); return result;}@Overridepublic boolean equals(Object obj) { if (this == obj)  return true; if (obj == null)  return false; if (getClass() != obj.getClass())  return false; Goods other = (Goods) obj; if (goodDesp == null) {  if (other.goodDesp != null)   return false; } else if (!goodDesp.equals(other.goodDesp))  return false; if (goodId == null) {  if (other.goodId != null)   return false; } else if (!goodId.equals(other.goodId))  return false; if (goodName == null) {  if (other.goodName != null)   return false; } else if (!goodName.equals(other.goodName))  return false; if (Double.doubleToLongBits(price) != Double.doubleToLongBits(other.price))  return false; return true;}@Overridepublic String toString() { return "[商品编号：" + goodId + ", 商品名称：" + goodName + ", 商品价钱："   + price + ", 商品描述：" + goodDesp + "]";}   }package com.immo.car;import java.util.HashSet;import java.util.Iterator;import java.util.Set;public class GoodsManage {        private  Set<Goods>  goodset;  public Set getGoodset() {   return goodset;  }    public GoodsManage() { // TODO Auto-generated constructor stub         }    public void setGoodset(Set goodset) {   this.goodset = goodset;  }        Set   s=new HashSet();  public   void   improtGoods(){   Goods   goods1=new Goods("s1001","矿泉水",90,"娃哈哈私人订制矿泉水");   Goods   goods2=new Goods("s1002","旺仔牛奶",100,"再看再看就把你喝掉");   Goods   goods3=new Goods("s1003","薯片",40,"咔吱脆");   Goods   goods4=new Goods("s1004","苹果",30,"糖心苹果");   s.add(goods1);   s.add(goods4);   s.add(goods3);   s.add(goods2);   setGoodset(s);  }  public  void   displayAllGoods(){  /*Iterator it= s.iterator();  while (it.hasNext()) {    System.out.println("目前："+it.next());     }*/ Set<Goods> set= this.getGoodset();      for (Goods goods : set) {  System.out.println(goods); }  }  }package com.immo.car;public class GoodsInCart {     private  Goods  goods;     private  int num;          GoodsInCart(Goods  goods,int num){      this.goods=goods;      this.num=num;     }          GoodsInCart(){           } public Goods getGoods() {  return goods; } public void setGoods(Goods goods) {  this.goods = goods; } public int getNum() {  return num; } public void setNum(int num) {  this.num = num; } @Override public String toString() {  return "GoodsInCart [goods=" + goods + ", num=" + num + "]"; }          }package com.immo.car;import java.util.Collection;import java.util.Iterator;import java.util.Map;import java.util.Map.Entry;import java.util.Scanner;import java.util.Set;public class ShopingCart {      public   Map<String, GoodsInCart> map;            Scanner   sc=new Scanner(System.in);     Boolean  flag=true;     GoodsInCart   inCa;     int  num=0;      public  void   addGoodsCart(GoodsManage gm){      if (gm.s.isEmpty()) {      System.out.println("请先导入数据"+gm.s.isEmpty());         }else {           System.out.println("友情提示：");             gm.displayAllGoods();                        System.out.println("请输入商品编号");            String  id=sc.next();          Set<Goods> set= gm.getGoodset();           for (Goods  g : set) {         if (g.getGoodId().equals(id)) {      System.out.println("请输入数量");      num=sc.nextInt();         inCa=new  GoodsInCart(g, num);             System.out.println("数量"+inCa.toString());             System.out.println(g);             map.put(id, inCa);    // System.out.println(map.isEmpty());     } }    //System.out.println(it.next());   }   }  public   void   Anset(){   System.out.println("请输入商品编号");   String  Goodsid=sc.next();      Set<String> set=map.keySet();   for (String st:set) {    if (st.contains(Goodsid)) {     System.out.println("请输入数量");     num=sc.nextInt();     if (num==0) {      map.remove(st);     }    }   }        }    public   void   balanceGoods(){  Collection<GoodsInCart> c= map.values();  for (GoodsInCart goodsInCart : c) {  double  price= goodsInCart.getNum()*goodsInCart.getGoods().getPrice();  System.out.println("共计："+price);  }  } }       import java.util.Iterator;import java.util.Set;public class Test {        public static void main(String[] args) {         Goods  sGoods=new Goods();   ShopingCart  s=new ShopingCart();   //GoodsInCart  inCart=new  GoodsInCart();       GoodsManage  manage=new GoodsManage();            manage.improtGoods();  Set<Goods> set= manage.getGoodset();  for (Goods goods : set) {   System.out.println(goods);  }   //manage.displayAllGoods();   s.addGoodsCart(manage);  }}

Answer 1

首先来看报错位置，ShopingCart的31行；

从这里可以看到是你的map出问题了，上下检查之后发现，map是ShopingCart的成员属性，但是，你在Shoping类中仅仅是定义了一个map属性，但是map没有进行初始化，所以不能直接调用put的方法进行添加键值对的操作，请同学对map进行初始化之后，再进行添加操作！祝，学习顺利！