Third example for "Rethinking interfaces". Now making totally equivalent our "interface class" to implement our interface.

OperationInterfaceAsClass.java

class OperationInterfaceAsClass implements Operation {

    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;
    }


    @Override
    public boolean isOperation(final String input) {
        return this.isOperation.test(input);
    }

    @Override
    public int operate(final List<Integer> numbers) {
        return this.operate.apply(numbers);
    }
}