散列表中每个键的值相同 [英] Same values for every key in hashmap

问题描述

我将以我的代码开始

  public void simulateSale（List< IceCream> dailyIceCreamStock）{
 date = LocalDate.now（）。minusDays（6）; 
 DateTimeFormatter fmt = DateTimeFormat.forPattern（yyyy-MM-dd）; （冰淇淋iceCream：dailyIceCreamStock）{
 iceCream.setSoldCount（rand.nextInt（IceCream.AMOUNT））的
 
（int i = 0; i< timeInterval; i ++） + 1））; 
 iceCream.setStockDate（date）; 
} 
 
 //每天应该有不同的ArrayList值
 this.weeklyStats.put（date.toString（fmt），dailyIceCreamStock）; 
 date = date.plusDays（1）; 
} 
  

问题出现在这行： this.weeklyStats。 put（date.toString（fmt），dailyIceCreamStock）;

正如您所看到的，我将随机生成的值添加到hashmap键入：

 映射< String，List< IceCream>> weeklyStats 
  

问题是，当我迭代这个散列表时，每个键都有相同的List值。输出如下：

  [11，3，11，12，20] 
 [11，3，11 ，12,20] 
 [11,3,11,12,20] 
 [11,3,11,12,20] 
 [11，3，11，12，20] 
 [11，3，11，12，20] 
 [11，3，11，12，20] 
  

期望的输出是在每个列表中都有随机值。
我想，有一些问题与范围，我不明白

解决方案

您添加相同的列表实例多次到Map。你应该创建List的副本，以便在Map中拥有不同的值：

  for（int i = 0; i< ; timeInterval; i ++）{
 List< IceCream> copy = new ArrayList<>（dailyIceCreamStock）; //为（IceCream iceCream：copy）创建一个副本
 {//修改副本元素
 iceCream.setSoldCount（rand.nextInt（IceCream.AMOUNT + 1））; 
 iceCream.setStockDate（date）; 
} 
 
 //每天应该有不同的ArrayList值
 this.weeklyStats.put（date.toString（fmt），copy）; //将副本放在地图
 date = date.plusDays（1）; 
 
  

编辑：

实际上，这还不够，你还应该创建IceCream实例的副本。否则所有的列表将会是不同的实例，但仍然会包含相同的IceCream对象。

  for（int i = 0; i < timeInterval; i ++）{
 List< IceCream> copy = new ArrayList<>（）; 
（IceCream iceCream：dailyIceCreamStock）{
 IceCream newIC = new IceCream（）; //不确定是否要从原始IceCream 
复制任何
 //数据newIC.setSoldCount（rand.nextInt（IceCream.AMOUNT + 1））; 
 newIC.setStockDate（date）; 
 copy.add（newIC）; //添加一个新的IceCream实例到新的列表
} 
 
 //每天应该有不同的值列表
 this.weeklyStats.put（date.toString（fmt） ，复制）; 
 date = date.plusDays（1）; 
} 
  

I'll start with my code

 public void simulateSale(List<IceCream> dailyIceCreamStock) {
    date = LocalDate.now().minusDays(6);
    DateTimeFormatter fmt = DateTimeFormat.forPattern("yyyy-MM-dd");

    for (int i = 0; i < timeInterval; i++) {
        for(IceCream iceCream: dailyIceCreamStock){
            iceCream.setSoldCount(rand.nextInt(IceCream.AMOUNT + 1));
            iceCream.setStockDate(date);
        }

        //Every day should have different ArrayList of values
        this.weeklyStats.put(date.toString(fmt), dailyIceCreamStock);
        date = date.plusDays(1);
    }

Problem is on this line: this.weeklyStats.put(date.toString(fmt), dailyIceCreamStock);

As you can see, I'm adding randomly generated values to the hashmap of type:

Map<String, List<IceCream>> weeklyStats

Problem is, that when I iterate this hashmap, every key has the same List of value. Output looks like this:

[11, 3, 11, 12, 20]
[11, 3, 11, 12, 20]
[11, 3, 11, 12, 20]
[11, 3, 11, 12, 20]
[11, 3, 11, 12, 20]
[11, 3, 11, 12, 20]
[11, 3, 11, 12, 20]

Desired output is to have random values in every of this list. I guess, there's some problem with scope, which I don't understand

解决方案

You are adding the same List instance multiple times to the Map. You should create copies of the List in order to have distinct values in your Map :

for (int i = 0; i < timeInterval; i++) {
    List<IceCream> copy = new ArrayList<>(dailyIceCreamStock); // create a copy
    for(IceCream iceCream: copy){ // modify the elements of the copy
        iceCream.setSoldCount(rand.nextInt(IceCream.AMOUNT + 1));
        iceCream.setStockDate(date);
    }

    //Every day should have different ArrayList of values
    this.weeklyStats.put(date.toString(fmt), copy); // put the copy in the Map
    date = date.plusDays(1);
}

EDIT:

Actually, that's not enough, you should create copies of the IceCream instances too. Otherwise all Lists will be different instances, but the would still contain the same IceCream objects.

for (int i = 0; i < timeInterval; i++) {
    List<IceCream> copy = new ArrayList<>();
    for(IceCream iceCream: dailyIceCreamStock){ 
        IceCream newIC = new IceCream(); // not sure if you want to copy any
                                         // data from the original IceCream
        newIC.setSoldCount(rand.nextInt(IceCream.AMOUNT + 1));
        newIC.setStockDate(date);
        copy.add(newIC); // add a new IceCream instance to the new List
    }

    //Every day should have different ArrayList of values
    this.weeklyStats.put(date.toString(fmt), copy); 
    date = date.plusDays(1);
}

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