Using Java 8 stream methods to get a max value

Question

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!

azro · Accepted Answer · 2018-02-04 14:53:56Z

7

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);

edited Feb 4, 2018 at 14:53

answered Feb 4, 2018 at 14:22

azro

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3 Comments

Ousmane D. Over a year ago

a more readable solution -->

int maxPrize = arrRound.stream()                            .map(Round::getHits)                            .flatMap(List::stream)                            .mapToInt(Hit::getPrizeAmount)                            .max().orElse(-1);

. Although, it's a matter of preference to use method references or not ;-)

fps Over a year ago

I think the original solution was more readable :) This just shows that readability is a subjective thing and it depends more on the reader, at least for these subtle cases.

Kokufuu Over a year ago

Thank you very much, it makes sence now... Need to check all methods more deeply one-by-one.

Eran · Accepted Answer · 2018-02-04 14:19:08Z

1

Use flatMap:

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

answered Feb 4, 2018 at 14:19

Eran

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Comments

Saravana · Accepted Answer · 2018-02-04 14:44:21Z

1

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);

edited Feb 4, 2018 at 14:44

answered Feb 4, 2018 at 14:27

Saravana

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Comments

AliOmar62 · Accepted Answer · 2018-02-04 14:23:59Z

0

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

answered Feb 4, 2018 at 14:23

AliOmar62

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Comments

Sajit Gupta · Accepted Answer · 2018-02-04 18:24:26Z

0

using Comparator:

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

edited Feb 4, 2018 at 18:24

answered Feb 4, 2018 at 18:16

Sajit Gupta

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