如何获得LinkedList的总和(LinkedList中的总和) [英] How to get the sum of an LinkedList (Sum up items in a LinkedList)

问题描述

我想做的是获取从数据库中检索到的所有整数项的总和，然后将其添加到LinkedList中，如下所示:

What I'm trying to do is get the sum of all integer items retrieved from the database then added into a LinkedList, like so:

ManageFeeNotePOJO selectedRecord = (ManageFeeNotePOJO) manageFeeNoteListTableView().getItems().get(selectdIndex);
List toFeeBillListTot = new LinkedList();
toFeeBillListTot.add(selectedRecord.getFeeNoteValue());

int sum = 0;
for (Object sumItem : toFeeBillListTot) {
    sum += sumItem;
}
System.out.println(sum);

有了这个我得到错误:

bad operand types for binary operator '+'
first type:  int
second type: Object

另一种方法是:

ManageFeeNotePOJO selectedRecord = (ManageFeeNotePOJO) manageFeeNoteListTableView().getItems().get(selectdIndex);
List toFeeBillListTot = new LinkedList();
toFeeBillListTot.add(selectedRecord.getFeeNoteValue());

int sum = 0;
for (int i = 0; i < toFeeBillListTot.size(); i++) {
    sum += (int) toFeeBillListTot.get(i);
}
System.out.println(sum);

我得到 java.lang.ClassCastException:java.lang.String无法强制转换为java.lang.Integer 错误

拜托，我将为此提供任何可行的方法.预先谢谢你.

Please, I would be more than greatful for any working approach around this. Thank you in advance.

推荐答案

bad operand types for binary operator '+'
first type:  int
second type: Object

这是说:您不能将 + 运算符与这两种类型之一一起使用"-我敢打赌，您可以猜出哪一个- Object -哪一种，如果您考虑一下，这是有道理的:

This is saying: "You can't use the + operator with one these two types" - and I bet you can guess which one - Object - which, if you think about it, makes sense:

请记住，您将在Java中使用的每种类型的每个类都扩展了 Object .因此，当您使用已强制转换为 Object 的对象时，Java几乎不知道该对象是什么.特别是，没有任何奇怪的线索对应用于一个 + 的内容该怎么做.

Remember that every class of every type you'll ever work with in Java extends Object. So, when you're working with something that has been cast to Object, Java knows almost nothing about what that something is. In particular, it would have no freaking clue what to do with a + applied to one.

例如，假设您有一个 Person 类并且做了

For instance, imagine that you had a Person class and did

 Person a = new Person();
 Person b = new Person();
 a + b; //WTF?? 

那有意义吗?当然不是.

Would that make sense? Of course not.

现在，您可能会说，但是我没有将对象添加到列表中，而是添加了整数/双精度数/+运算符应与之配合使用的其他类型.您是正确的，但是 问题是您将它们添加到了对象列表 ，您在此处创建了这样的对象:

Now, you might say "but I didn't add Objects to my list, I added Integers/Doubles/some other type that the + operator should work with" - and I'm sure you would be right, but the problem is that you added them to a List of Objects, which you created as such right here:

List sum toFeeBillListTot = new LinkedList();

执行此操作时，您正在创建未键入的列表".这意味着您可以向其中添加任何类型的对象-听起来不错，但自然的结果是，您只能从中检索 Object ，这就是您现在正在做的.

When you do this, you're creating an "untyped list". This means you can add objects of any type you like to it - which might sound nice, but as a natural consequence, you can only retrieve Objects from it, which is what you're doing now.

通过使用通用集合"，@ Jeroen建议您改为执行此操作:

By "use a generic collection", @Jeroen is suggesting you do this instead:

List<Integer> toFeeBillListTot = new LinkedList<>();

< Integer> 而不是创建对象列表，这意味着您正在创建一个整数列表，这意味着您只能添加 Integer，并且您从其中提取的每个元素都将是Integer，因此您可以执行以下操作:

Rather than creating a list of Objects, the <Integer> means you're creating a list of integers, which means you can only add Integers to it, and also that every element you pull out of it will be an Integer, so you can do:

int sum = 0;
for (Integer sumItem : toFeeBillListTot) {
  sum += sumItem;
}

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