java中sum方法的使用方法
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how to use the sum method in java
提问by user442471
I am reading in a text file with four columns of data to a HashMap. I want to sum the value columns. Can I get an example on using the sum method?
我正在读取一个包含四列数据的文本文件到HashMap. 我想对值列求和。我可以获得使用 sum 方法的示例吗？
回答by BalusC
There is no such method like that. There's however the additive operator +which can be used on numeric primitives/types like intand Integer.
没有这样的方法。然而，有加法运算符+可用于数字基元/类型，如int和Integer。
Assuming that you've a Map<String, Integer>, here's an example:
假设你有一个Map<String, Integer>，这是一个例子：
int total = 0;
for (Integer value : map.values()) {
total = total + value; // Can also be done by total += value;
}
System.out.println(total); // Should print the total.
See also:
也可以看看：
Update: I just wanted to add one other hint; your coreproblem might be that you've the numbers in flavor of Stringobjects (since you're parsing a text file) and the +of course won't sum them up, but just concatenate them. You'd like to convert each number from Stringto Integerfirst. This can be done with Integer#valueOf(). E.g.
更新：我只想添加另一个提示；您的核心问题可能是您拥有String对象风格的数字（因为您正在解析文本文件）+，当然不会对它们进行总结，而只是将它们连接起来。您想将每个数字从 转换String为Integer第一个。这可以通过Integer#valueOf(). 例如
String numberAsString = "10";
Integer numberAsInteger = Integer.valueOf(numberAsString);
// Now put in map and so on.
This way you can do basic arithmetic with numbers as intended.
通过这种方式，您可以按预期对数字进行基本算术运算。
回答by Carlos
Based on your question, I made the assumption that you actually need to know how to merge three value columns into one value so you can put it into a map. Here's an example which converts value columns of any type into one string and then puts it into a map. It assumes the first column is the key column and makes no assumptions on their data types.
根据您的问题，我假设您实际上需要知道如何将三个值列合并为一个值，以便将其放入地图中。这是一个示例，它将任何类型的值列转换为一个字符串，然后将其放入映射中。它假设第一列是关键列，并且不对它们的数据类型做任何假设。
//These are values for each column. column1 to column4.
Object column1 = ...
Object column2 = ...
Object column3 = ...
Object column4 = ...
//This is the map you are reading the values into.
Map<Object, String> map = new HashMap<Object, String>();
StringBuilder sb = new StringBuilder();
sb.append(column2);
sb.append(column3);
sb.append(column4);
map.put(column1, sb.toString());
