Grouper.java

package com.federicovitale.bookmytrack.util;


import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Optional;
import java.util.function.Function;

public class Grouper<Z, T> {
    private Function<T, Z> getter;
    private List<T> list;

    public static <T, Z> HashMap<Z, List<T>> group(Function<T, Z> extractor, List<T> input) {
        HashMap<Z, List<T>> map = new HashMap<>();

        input.forEach(t -> {
            List<T> list = new ArrayList<>();
            list.add(t);

            Optional
                    .ofNullable(map.putIfAbsent(extractor.apply(t), list))
                    .ifPresent(l -> l.add(t));
        });

        return map;
    }

    public static <T, Z> HashMap<Z, List<T>> group(List<T> input, Function<T, Z> extractor) {
        HashMap<Z, List<T>> map = new HashMap<>();

        input.forEach(t -> {
            List<T> list = new ArrayList<>();
            list.add(t);

            Optional
                    .ofNullable(map.putIfAbsent(extractor.apply(t), list))
                    .ifPresent(l -> l.add(t));
        });

        return map;
    }

    public static <T, Z, M> HashMap<Z, List<M>> group(List<T> input, Function<List<T>, List<M>> mapper, Function<M, Z> extractor) {
        HashMap<Z, List<M>> map = new HashMap<>();

        List<M> mappedList = mapper.apply(input);

        mappedList.forEach(t -> {
            List<M> list = new ArrayList<>();
            list.add(t);

            Optional
                    .ofNullable(map.putIfAbsent(extractor.apply(t), list))
                    .ifPresent(l -> l.add(t));
        });

        return map;
    }

    public Grouper(Function<T, Z> getter, List<T> list) {
        this.getter = getter;
        this.list = list;
    }

    public HashMap<Z, List<T>> get() {
        return Grouper.group(getter, list);
    }
}