Functional.java

package com.berci;

import java.util.ArrayList;
import java.util.Arrays;

record Tuple<T, U>(T one, U other){}

@FunctionalInterface
interface Combine<T> {
    T method(T left, T right);
}

@FunctionalInterface
interface ProcessElement<T> {
    T method(T t);
}

@FunctionalInterface
interface Filter<T> {
    boolean method(T t);
}

class CombineImpl<T> {
    T combine(ArrayList<T> elems, T start, Combine<T> combineFn) {
        for (T elem : elems) {
            start = combineFn.method(start, elem);
        }
        return start;
    }
    
    ArrayList<T> process(ArrayList<T> elems, ProcessElement<T> processFn) {
        ArrayList<T> result = new ArrayList<>();
        for (T elem : elems) {
            T processedElem = processFn.method(elem); 
            result.add(processedElem);
        }
        return result;
    }
    
    Tuple<ArrayList<T>, ArrayList<T>> partition(ArrayList<T> elems, Filter<T> filterFn) {
        Tuple<ArrayList<T>, ArrayList<T>> result = new Tuple<>(new ArrayList<>(), new ArrayList<>());
        for (T elem : elems) {
            if (filterFn.method(elem)) {
                result.one().add(elem);
            } else {
                result.other().add(elem);
            }
        }
        return result;
    }
}

public class Main {
    
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5, 6, 7));
        ArrayList<String> strings = new ArrayList<>(Arrays.asList("a", "aa", "aaa", "aaaa", "aaaaa"));

        CombineImpl<Integer> intCombiner = new CombineImpl<>();

        Integer sum = new CombineImpl<Integer>().combine(numbers, 0, (a, b) -> a + b);
        Integer product = new CombineImpl<Integer>().combine(numbers, 1, (a, b) -> a * b);

        ArrayList<Integer> plusOnes = new CombineImpl<Integer>().process(numbers, (a) -> a + 1);
        ArrayList<String> exclamations = new CombineImpl<String>().process(strings, s -> s.concat("!"));

        Tuple<ArrayList<String>, ArrayList<String>> result = new CombineImpl<String>().partition(strings, e -> e.length() % 2 == 0);

        System.out.println("evens:");
        System.out.println(result.one());

        System.out.println("odds:");
        System.out.println(result.other());
    }
}