Java集合-使用Java 8计算总大小，然后不添加重复记录

Question

一个存储容器中大约有500万条记录。我需要在一个集合对象中获取它们。我得到了下面的列表:

[
    {"Record1":"Java","Version":"6","Size":"56"},
    {"Record2":"Java","Version":"6","Size":"58"},
    {"Record3":"Java","Version":"6","Size":"46"},
    {"Record4":"Java","Version":"6","Size":"56"},
    {"Record5":"Java","Version":"6","Size":"29"},
    {"Record6":"Python","Version":"3.8","Size":"15"},
    {"Record7":"Python","Version":"3.8","Size":"20"},
    {"Record8":"Python","Version":"3.8","Size":"23"},
    {"Record9":"Python","Version":"3.8","Size":"32"},
    {"Record10":"Python","Version":"3.8","Size":"31"}   
]

预期结果（需要计算特定“版本”的总大小）：

[
    {"Record1":"Java","Version":"6","Size":"245"},
    {"Record2":"Python","Version":"3.8","Size":"121"}
]

“Java”的总尺寸示例：

"56" "58" "46" "56" "29"="245"

如何使用Java 8实现这一点？

Answer 1

首先，您应该谨慎行事，并考虑将大量记录加载到java应用程序中的内存需求和网络i/o。试着找到一种在存储容器中进行计算的方法。

对于java，您可以使用以下内容：

import lombok.Data;

import java.util.Arrays;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class StackoverflowQuestion {
    @Data
    static class Row {
        private final Info info;
        private final Integer size;
    }

    @Data
    static class Info {
        private final String platform;
        private final String version;
    }

    public static void main(String[] args) {
        List<Row> rows = Arrays.asList(
            new Row(new Info("Java", "8"), 10),
            new Row(new Info("Java", "8"), 20),
            new Row(new Info("Java", "11"), 30),
            new Row(new Info("Java", "11"), 40),
            new Row(new Info("Python", "3.3"), 50),
            new Row(new Info("Python", "3.3"), 60),
            new Row(new Info("Python", "3.8"), 70)
        );

        Map<Info, IntSummaryStatistics> collect = rows.stream()
            .collect(Collectors.groupingBy(Row::getInfo, // Info class will be the key
                Collectors.mapping(Row::getSize, // what to do with Size?
                    Collectors.summarizingInt(Integer::intValue)))); // sum it

        collect.forEach((key, value) -> System.out.println(key + ": " + value.getSum()));
        
        // output:
        // Info(platform=Python, version=3.8): 70
        // Info(platform=Java, version=11): 70
        // Info(platform=Python, version=3.3): 110
        // Info(platform=Java, version=8): 30
    }
}

隆伯克。这里的数据用作所有参数构造函数equals、hashCode和toString的快捷方式。equals，分组需要hashCode。