使用 Java 8 流方法获取最大值 [英] Using Java 8 stream methods to get a max value

问题描述

我想使用 java 8 流方法从列表中获取最大值.

I would like to get the max value out of a list using java 8 stream methods.

结构如下:

• 我读取了一个 csv 文件并将每一行的数据存储在一个 Round 类型的单独对象中.
• 所有这些Round 对象都存储在一个名为arrRound
的ArrayList 中
• 所有 Round 对象都有一个字段:List命中
• a Hit 包含 2 个字段:int numberOfGames 和 int PrizeAmount

• I read a csv file and store the data of every line in a separate object of type Round.
• all these Round objects are stored in an ArrayList called arrRound
• all Round objects have a field: List<Hit> hits
• a Hit consists of 2 fields: int numberOfGames and int prizeAmount

public class Round{
    private List<Hits> hits; 
}
public class Hits{
    private int numberOfGames;
    private int prizeAmount;
}

<小时>

我想做的是遍历 arrRound 的所有元素，获取它们的命中字段的 getPrizeAmount() 方法并从中获取最大值.我从以下开始，但似乎无法做到:

---

What I would like to do is to iterate over all elements of arrRound, get their hits field's getPrizeAmount() method and get the max out of it. I started as the following but can't seem to do it:

public class Main(){
    public void main(String[]args){
        List<Round> arrRound = getRoundFromCSV();
        int maxPrize = arrRound.stream()
                               .forEach(round -> {
                                 round.getHits()
                                      .forEach(hit -> hit.getPrizeAmount());
                                });
    }
}

并且我无法在语句的末尾调用 max().

and I am not able to call max() on the end of the statement.

提前感谢您的帮助！

推荐答案

你可以这样实现:

• 遍历list
的Round
• 从 Round 对象更改为它的 List点击次数,
• 使用flatMap从Stream>到Stream
• 从 Hits 更改为它的 prizeAmount 字段
• 获取max(如果存在)
• 如果没有最大值(如列表为空或其他)返回 -1

• iterate over the Round of the list
• change from Round object to its List<Hit> hits,
• use flatMap to go from Stream<List<Hits>> to Stream<Hits>
• change from Hits to its prizeAmount field
• get the max if exists
• if no max (like if list empty or else) return -1

所以一个使用方法参考

int maxPrize = arrRoundarrRound.stream()                      // Stream<Round>
                               .map(Round::getHits)           // Stream<List<Hits>>
                               .flatMap(List::stream)         // Stream<Hits>
                               .mapToInt(Hit::getPrizeAmount) // IntStream
                               .max()                         // OptionalInt 
                               .orElse(-1);                   // int

lambda 和 map + flatMap 合二为一:

int maxPrize = arrRoundarrRound.stream()    
                               .flatMap(round -> round.getHits().stream())
                               .mapToInt(hits -> hits.getPrizeAmount())
                               .max()                         
                               .orElse(-1); 

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