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类型为Round的单独对象中.
• 所有这些Round对象都存储在名为arrRound
的ArrayList中
• 所有Round对象都有一个字段:List<Hit> hits
• 一个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;
}

---

我想做的是遍历arrRound的所有元素，获取其hits字段的getPrizeAmount()方法，并从中获取最大价值. 我从以下开始，但似乎做不到:

---

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());
                                });
    }
}

并且我无法在语句末尾调用max().

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

谢谢您的帮助！

推荐答案

您可以通过以下方式实现:

You can achieve it by this way :

• 遍历list
的Round
• 从Round对象更改为其List<Hit> hits，
• 使用flatMap从Stream<List<Hits>>转到Stream<Hits>
• 从Hits更改为其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)，则返回-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

所以使用Method reference

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

其中一个类为lambda和map + flatMap:

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

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