java - HashMap 中每个键的值相同

Question

我将从我的代码开始

 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);
    }

问题出在这一行:this.weeklyStats.put(date.toString(fmt), dailyIceCreamStock);

如您所见，我将随机生成的值添加到类型的 HashMap 中:

Map<String, List<IceCream>> weeklyStats

问题是，当我迭代这个 HashMap 时，每个键都有相同的值列表。输出如下所示:

[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。我猜，范围有问题，我不明白

Answer 1

您正在将同一个 List 实例多次添加到 Map。您应该创建列表的副本，以便在您的 map 中具有不同的值:

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);
}

编辑:

实际上，这还不够，您还应该创建 IceCream 实例的副本。否则，所有列表都将是不同的实例，但仍将包含相同的 IceCream 对象。

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);
}