Example Operator interface for "Rethinking Interfaces" in Medium

Operator.java

import java.util.List;

interface Operation {

    boolean     isOperation (String input);
    int         operate     (List<Integer> numbers);
}

Sum.java

class Sum implements Operation {

    public boolean isOperation(String operator) {
        return operator.equals("+");
    }

    public int operate (List<Integer> numbers) {
        return numbers
                .stream()
                .mapToInt(Integer::intValue)
                .reduce(
                        (a,b) -> a+b
                ).getAsInt();
    }
}