arg max in Java 8 streams?

问题

I often need the maximum element of a collection according to the maximization of a criterion which produces a double or int value. Streams have the max() function which requires me to implement a comparator, which I find cumbersome. Is there a more concise syntax, such as names.stream().argmax(String::length) in the following example?

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

public class ArgMax
{
    public static void main(String[] args)
    {
        List<String> names = Arrays.asList("John","Joe","Marilyn");
        String longestName = names.stream().max((String s,String t)->(Integer.compare(s.length(),t.length()))).get();
        System.out.println(longestName);
    }
}

回答1:

Use

String longestName = names.stream().max(Comparator.comparing(String::length)).get();

to compare elements on some property (can be more complex than that, but doesn't have to).

As Brian suggests in the comments, using Optional#get() like this is unsafe if there's a possibility that the Stream is empty. You'd be better suited to use one of the safer retrieval methods, such as Optional#orElse(Object) which will give you some default value if there is no max.

回答2:

I think one should consider that while max/min are unique, this is of course not guaranteed for the argMax/argMin; this in particular implies that the type of the reduction should be a collection, such as for instance a List. This requires a bit more work than suggested above.

The following ArgMaxCollector<T> class provides a simple implementation of such a reduction. The main shows an application of such class to compute the argMax/argMin of the set of strings

one two three four five six seven

ordered by their length. The output (reporting the result of the argMax and argMin collectors respectively) should be

[three, seven]
[one, two, six]

that are the two longest and the three shortest strings respectively.

This is my first attempt at using the new Java 8 stream APIs, so any comment will be more than welcome!

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collector;

class ArgMaxCollector<T> {

    private T max = null;
    private ArrayList<T> argMax = new ArrayList<T>();
    private Comparator<? super T> comparator;

    private ArgMaxCollector( Comparator<? super T> comparator ) {
        this.comparator = comparator;
    }

    public void accept( T element ) {
        int cmp = max == null ? -1 : comparator.compare( max, element );
        if ( cmp < 0 ) {
            max = element;
            argMax.clear();
            argMax.add( element );
        } else if ( cmp == 0 )
            argMax.add( element );
    }

    public void combine( ArgMaxCollector<T> other ) {
        int cmp = comparator.compare( max, other.max );
        if ( cmp < 0 ) {
            max = other.max;
            argMax = other.argMax;
        } else if ( cmp == 0 ) {
            argMax.addAll( other.argMax );
        }
    }

    public List<T> get() {
        return argMax;
    }

    public static <T> Collector<T, ArgMaxCollector<T>, List<T>> collector( Comparator<? super T> comparator ) {
        return Collector.of(
            () -> new ArgMaxCollector<T>( comparator ),
            ( a, b ) -> a.accept( b ),
            ( a, b ) ->{ a.combine(b); return a; },
            a -> a.get() 
        );
    }
}

public class ArgMax {

    public static void main( String[] args ) {

        List<String> names = Arrays.asList( new String[] { "one", "two", "three", "four", "five", "six", "seven" } );

        Collector<String, ArgMaxCollector<String>, List<String>> argMax = ArgMaxCollector.collector( Comparator.comparing( String::length ) );
        Collector<String, ArgMaxCollector<String>, List<String>> argMin = ArgMaxCollector.collector( Comparator.comparing( String::length ).reversed() );

        System.out.println( names.stream().collect( argMax ) );
        System.out.println( names.stream().collect( argMin ) );

    }

}

来源：https://stackoverflow.com/questions/27606185/arg-max-in-java-8-streams