Meu set de classes útil para Java

CollectionsUtil.java

package br.com.logann.sgbe.util;

import java.util.Collection;
import java.util.HashSet;
import java.util.Set;
import java.util.function.Function;

/**
 * Example:
 * 
 * Set<Long> ids = CollectionsUtil.extractFieldSet(ItemDto::getId, listOfItems);
 */
public class CollectionsUtil {
    private CollectionsUtil() {
    }

    /**
     * @param mapper     A class field method reference.
     * @param collection Collection to extract field.
     * @param <T>        Set's type.
     * @param <R>        Collection's type.
     * @return A Set of a field of collection.
     */
    public static <T, R> Set<T> extractFieldSet(Function<R, T> mapper, Collection<R> collection) {
        final Set<T> set = new HashSet<>();

        for (R item : collection) {
            set.add(mapper.apply(item));
        }

        return set;
    }

}

MapCollector.java

package br.com.logann.sgbe.util;

import java.util.*;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;

public class MapCollector<A, B> implements Collector<A, Collection<B>, Collection<B>> {
    private final Function<A, B> mapper;
    private final Supplier<Collection<B>> collectionFactory;

    private MapCollector(Function<A, B> mapper, Supplier<Collection<B>> collectionFactory) {
        this.mapper = mapper;
        this.collectionFactory = collectionFactory;
    }

    @Override
    public Supplier<Collection<B>> supplier() {
        return collectionFactory;
    }

    @Override
    public BiConsumer<Collection<B>, A> accumulator() {
        return (bs, a) -> bs.add(mapper.apply(a));
    }

    @Override
    public BinaryOperator<Collection<B>> combiner() {
        return (bs, bs2) -> {
            bs.addAll(bs2);
            return bs;
        };
    }

    @Override
    public Function<Collection<B>, Collection<B>> finisher() {
        return bs -> bs;
    }

    @Override
    public Set<Characteristics> characteristics() {
        return Collections.singleton(Characteristics.UNORDERED);
    }

    public static <A, B> Collector<A, List<B>, List<B>> mapper(Function<A, B> mapper) {
        return new MapCollector(mapper, ArrayList::new);
    }

    public static <A, B> Collector<A, List<B>, List<B>> mapper(Function<A, B> mapper, Supplier<Collection<B>> collectionFactory) {
        return new MapCollector(mapper, collectionFactory);
    }

}