An implementation of an argmax collector using the Java 8 stream APIs

ArgMax.java

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 ) );

	}

}

output.txt

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