Nikita is the best Nikita of all Nikitas

package aniskov.ns;

import java.util.Arrays;
import java.util.List;

class NumbersSet {
    private final List<Number> values;

    private boolean indexIsValid(int i) {
        return 0 <= i && i < values.size();
    }

    public NumbersSet(Number... numbers) {
        values = Arrays.asList(numbers);
    }

    public int size() {
        return values.size();
    }

    public Number get(int i) {
        return indexIsValid(i) ? values.get(i) : null;
    }

    public int set(int i, Number newValue) {
        if (!indexIsValid(i) || newValue.getClass() != this.values.get(i).getClass()) {
            return -1;
        }
        values.set(i, newValue);
        return 0;
    }
}

public class Main {
    static void foo(NumbersSet numbersSet) {
        for (int i = 0; i < numbersSet.size(); i++) {
            Number oldValue = numbersSet.get(i);
            System.out.println("---\n Old value: " + oldValue + " (" + oldValue.getClass() + ")");
            for (Number newValue : new Number[]{1, 1L, 1D, 1F, (byte) 1, (short) 1}) {
                System.out.println("Setting new value: setter returned " + numbersSet.set(i, newValue) + " (" + newValue.getClass() + ")");
            }
        }
    }

    public static void main(String[] args) {
        foo(new NumbersSet(1, 1L, 1D, 1F, (byte) 1, (short) 1));
    }
}