import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class Reduce {
    public static void main(String[] args) {
        Random rnd = new Random();
        List<Integer> input = new ArrayList<>();
        for (int i = 0; i < 5; i++) {
            input.add(rnd.nextInt(100));
        }

        Data result = input.stream().reduce(new Data('a', new ArrayList<>()),
                (data, num) -> {
                    data.list.add("" + num + data.letter);
                    data.letter = (char) (data.letter + 1);
                    return data;
                },
                (data1, data2) -> data1);
        System.out.println(result.list);
    }
}

class ReduceImmutable {
    public static void main(String[] args) {
        Random rnd = new Random();
        List<Integer> input = new ArrayList<>();
        for (int i = 0; i < 5; i++) {
            input.add(rnd.nextInt(100));
        }

        Data result = input.stream().reduce(new Data('a', new ArrayList<>()),
                (data, num) -> {
                    //consider class Data is immutable
                    Data newAcc = new Data();
                    List<String> newList = new ArrayList<>();
                    newList.addAll(data.list);
                    newList.add("" + num + data.letter);
                    newAcc.letter = (char) (data.letter + 1);
                    newAcc.list = newList;
                    return newAcc;
                },
                (data1, data2) -> data1);
        System.out.println(result.list);
    }
}

class Data {
    char letter;
    List<String> list;

    Data() {}

    Data(char letter, List<String> list) {
        this.letter = letter;
        this.list = list;
    }
}