FlowDesign EBC impl via Java 8

StringCalculator.java

package stringcalculator;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Function;

public class StringCalculator {

    public static final String SEPARATORS = "(,|\\n)";
    public List<Consumer<Integer>> onResult = new ArrayList<>();

    public void add(String numbers) {
        validate(numbers,
            this::sum,
            (Void) -> { throw new IllegalArgumentException(); });
    }

    private void validate(
        String numbers,
        Function<String, Integer> onSuccess,
        Consumer<Void> onError) {

        if (numbers.length() > 0) {
            onResult.stream().forEach(c -> c.accept(onSuccess.apply(numbers)));
        } else onError.accept(null);

    }

    private int sum(String numbers) {
        return Arrays.asList(numbers.split(SEPARATORS))
            .stream()
            .map(Integer::parseInt)
            .mapToInt(Integer::intValue)
            .sum();
    }
}