Let's assume I have the following class:

class Transaction{
  Float amount;
  String Id;
}

Now I'd like to implement a function:

public static Float sum(List<Ob,java"/>
	














    

		

        

            
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首页  > 解决方案 > Java - 返回列表特定属性的总和

Let's assume I have the following class:


class Transaction{
  Float amount;
  String Id;
}



Now I'd like to implement a function:


public static Float sum(List<Ob












java - Java - 返回列表特定属性的总和

Let's assume I have the following class:


class Transaction{
  Float amount;
  String Id;
}



Now I'd like to implement a function:


public static Float sum(List<Ob





问题描述



Let's assume I have the following class:


class Transaction{
  Float amount;
  String Id;
}



Now I'd like to implement a function:


public static Float sum(List<Object> list, String attribute)



That returns the sum of all objects in the list for the specified attribute.


How can I achieve this?









i had similar problem. i solved it by trim. Please check, path.resolve may add additional space to end of path.


var path = require('path');
const envPath = path.resolve(process.cwd()+'/config','.env.'+process.env.NODE_ENV).trim()
require('dotenv').config({ path: envPath })



my package script is like this to use multiple .env:


 "scripts": {
    "start": "set NODE_ENV=development && nodemon ./bin/www",
    "prod": "set NODE_ENV=production && node ./bin/www"
  },



标签： java


解决方案









您要求的此类功能只能使用反射来实现。这应该有效：


public static Float sum(List<Object> list, String attribute) {

    Float sum = 0f;

    for (Object object : list) {
        Class<?> c = object.getClass();
        try {
            Field field = c.getDeclaredField(attribute);
            field.setAccessible(true);
            sum += (Float) field.get(object);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }
    return sum;
}



但这可能是一个不好的做法。很可能有人会以不好的方式使用此方法，从而导致抛出异常。考虑比 java 反射更好的其他选项。










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