在一个lambda表达式中收集复杂对象

本文介绍了在一个lambda表达式中收集复杂对象的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我有一个对象列表。首先，我需要按类型对其进行排序。 比faceValue。最后，总结所有数量：

I have a list of objects. At first, I need to sort it by type. Than by faceValue. In the end, summarize all quantities:

class Coin{ String type; BigInteger faceValue; BigInteger quantity; ... } List<Coin> coins = new ArrayList<>(); coins.add(new Coin("USD", 1, 150)); coins.add(new Coin("USD", 1, 6)); coins.add(new Coin("USD", 1, 60)); coins.add(new Coin("USD", 2, 100)); coins.add(new Coin("USD", 2, 100)); coins.add(new Coin("CAD", 1, 111)); coins.add(new Coin("CAD", 1, 222));

结果列表必须只包含3个新的硬币对象：

Result list must contains only 3 new coin objects:

Coin("USD", 1 , 216) Coin("USD", 2 , 200) Coin("CAD", 1 , 333)

如何仅在一个lambda表达式中编写？

How can this be written only in one lambda expression?

推荐答案

您可以使用 Collectors.toMap 解决这个问题：

You could solve that using Collectors.toMap as :

public List<Coin> groupedCoins(List<Coin> coins) { return new ArrayList<>( coins.stream() .collect(Collectors.toMap( coin -> Arrays.asList(coin.getType(), coin.getFaceValue()), Function.identity(), (coin1, coin2) -> { BigInteger netQ = coin1.getQuantity().add(coin2.getQuantity()); return new Coin(coin1.getType(), coin1.getFaceValue(), netQ); })) .values()); }

或更复杂的一个班轮分组和总和：

or a further complex one liner grouping and sum as :

public List<Coin> groupedAndSummedCoins(List<Coin> coins) { return coins.stream() .collect(Collectors.groupingBy(Coin::getType, Collectors.groupingBy(Coin::getFaceValue, Collectors.reducing(BigInteger.ZERO, Coin::getQuantity, BigInteger::add)))) .entrySet() .stream() .flatMap(e -> e.getValue().entrySet().stream() .map(a -> new Coin(e.getKey(), a.getKey(), a.getValue()))) .collect(Collectors.toList()); }