Using Java 8 stream methods to get a max value

问题

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

The structure is the following:

• 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;
}

---

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());
                                });
    }
}

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

Thank you for your help in advance!

回答1:

You can achieve it by this way :

• 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

---

So a solution using 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

With class lambda and map + flatMap in one :

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

回答2:

Use flatMap:

int maxPrize = arrRound.stream() // Stream<Round>
                       .flatMap(r -> r.getHits().stream()) // Stream<Hit>
                       .mapToInt(Hit::getPrizeAmount) // IntStream
                       .max()
                       .orElse(0);

回答3:

using reduce

int max1 = arrRound.stream()
        .flatMap(r -> r.getHits().stream())
        .mapToInt(h -> h.getPrizeAmount())
        .reduce(Math::max) //returns OptionalInt
        .orElse(Integer.MIN_VALUE);

or

int max2 = arrRound.stream()
        .flatMap(r -> r.getHits().stream())
        .mapToInt(h -> h.getPrizeAmount())
        .reduce(Integer.MIN_VALUE, Math::max);

回答4:

Because forEach is a terminal operation. And after you call the forEach on a stream the stream terminates and can't be used again.

回答5:

using Comparator:

int max2 = arrRound.stream()
            .flatMap(round -> round.getHits().stream())
            .max(Comparator.comparing(Hits::getPrizeAmount))
            .map(Hits::getPrizeAmount)
            .orElse(-1);

来源：https://stackoverflow.com/questions/48608983/using-java-8-stream-methods-to-get-a-max-value