Second example for "Rethinking interfaces"

OperationInterfaceAsClass.java

class OperationInterfaceAsClass {

    Predicate<String>                   isOperation;
    Function<List<Integer>, Integer>    operate;

    private OperationInterfaceAsClass(){};

    public static OperationInterfaceAsClass of(
            Predicate<String>                   isOperation,
            Function<List<Integer>, Integer>    operate
    ){
        OperationInterfaceAsClass toBuild = new OperationInterfaceAsClass();

        toBuild.isOperation    = isOperation;
        toBuild.operate        = operate;

        return toBuild;
    }
}

Program.java

class Program {
    public static void main(String[] args) {
        //Note, because now we can create them on the fly, we win in freedom as to where and how
        //to define them.
        var Sum = OperationInterfaceAsClass.of(
                        operator -> operator.equals("+"),
                        numbers -> numbers
                                    .stream()
                                    .mapToInt(Integer::intValue)
                                    .reduce(
                                            (a,b) -> a+b
                                    ).getAsInt()
                );

        var Times = OperationInterfaceAsClass.of(
                operator -> operator.equals("x"),
                numbers -> numbers
                        .stream()
                        .mapToInt(Integer::intValue)
                        .reduce(
                                (a,b) -> a*b
                        ).getAsInt()
        );
    }
}