PropertyStreamTest.java

package qi4j;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collector;
import java.util.stream.Stream;
import org.junit.Test;

import static java.util.stream.Collectors.toList;

public class PropertyStreamTest
{
    interface Property<T>
    {
        T get();
    }

    interface Student
    {
        Property<String> name();
    }

    List<Student> students = Collections.EMPTY_LIST;

    // ========================================================================
    // Here is what works out of the box

    @Test
    public void lambda()
    {
        List<String> names = students.stream()
            .map( student -> student.name().get() )
            .filter( name -> name.startsWith( "A" ) )
            .collect( toList() );
    }

    @Test
    public void double_method_reference()
    {
        List<String> names = students.stream()
            .map( Student::name ).map( Property::get )
            .filter( name -> name.startsWith( "A" ) )
            .collect( toList() );
    }

    // ========================================================================
    // Approach using a custom Collector
    // (its accumulator function call Property::get)

    @Test
    public void customCollector()
    {
        List<String> names = students.stream()
            .map( Student::name )
            .filter( name -> name.get().startsWith( "A" ) )
            .collect( toValuesList() );
    }

    static <T> Collector<Property<T>, ?, List<T>> toValuesList()
    {
        return Collector.of(
            ArrayList::new,
            (left, right) -> left.add( right.get() ),
            (left, right) ->
            {
                left.addAll( right );
                return left;
            }
        );
    }

    // ========================================================================
    // Approach using a custom Stream
    // (adds a mapToValue(..) methods, in the same vein as mapToLong(..) & co ...)

    @Test
    public void customStream()
    {
        List<String> names = stream( students )
            .mapToValue( Student::name )
            .filter( name -> name.startsWith( "A" ) )
            .collect( toList() );
    }

    static <T> CustomStream<T> stream( Collection<T> collection )
    {
        // Should return a wrapped Stream with the added mapToValue method
        return null;
    }

    interface CustomStream<T>
        extends Stream<T>
    {
        <R> Stream<R> mapToValue( Function<? super T, ? extends Property<R>> mapper );
    }
}