基于密钥按给定列表分组并在相同列表中收集的有效方法java 8

问题描述

我有以下课程：

class A{
  String property1;
  String property2;
  Double property3;
  Double property4;
}

因此property1和property2是关键。

So the property1 and property2 is the key.

class Key{
      String property1;
      String property2;
}

我已经有以下A的列表：

I already have a list of A like below:

List<A> list=new ArrayList<>();

我想通过使用密钥进行分组并添加到另一个A列表中以避免出现多个列表中具有相同键的项目：

I want to group by using the key and add to another list of A in order to avoid having multiple items with same key in the list:

Function<A, Key> keyFunction= r-> Key.valueOf(r.getProperty1(), r.getProperty2());

但是当我做分组时，我必须得到一个property3和property4的平均值。

But then while doing group by I have to take a sum of property3 and average of property4.

我需要一种有效的方法。

I need an efficient way to do it.

注意：我跳过了给定类的方法。

Note: I have skipped the methods of the given classes.

推荐答案

收集到地图是不可避免的因为你想要 group 的东西。这样做的一种蛮力方式是：

Collecting to a Map is unavoidable since you want to group things. A brute-force way to do that would be :

yourListOfA
      .stream()
      .collect(Collectors.groupingBy(
             x -> new Key(x.getProperty1(), x.getProperty2()),
             Collectors.collectingAndThen(Collectors.toList(),
                   list -> {
                        double first = list.stream().mapToDouble(A::getProperty3).sum();
                        // or any other default
                        double second = list.stream().mapToDouble(A::getProperty4).average().orElse(0D);
                        A a = list.get(0);
                        return new A(a.getProperty1(), a.getProperty2(), first, second);
            })))
     .values();

这可能会略有改善，例如 Collectors.collectingAndThen 只迭代 List 一次，因为需要一个自定义收集器。写一个并不复杂......

This could be slightly improved for example in the Collectors.collectingAndThen to only iterate the List once, for that a custom collector would be required. Not that complicated to write one...

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