Transforming it to an abstract approach instead of interface approach (having some part of the code already implemented).

OperationInterfaceAsClass.java

class OperationAbstractAsClass implements Operation {

    Predicate<String>                   isOperation;
    IntBinaryOperator                   reductionOperation;

    private OperationAbstractAsClass(){};

    public static OperationAbstractAsClass of(
            Predicate<String>                   isOperation,
            IntBinaryOperator                   reductionOperation
    ){
        OperationAbstractAsClass toBuild = new OperationAbstractAsClass();

        toBuild.isOperation         = isOperation;
        toBuild.reductionOperation  = reductionOperation;

        return toBuild;
    }


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

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

    private int operateWithGivenReductionOperation(final List<Integer> numbers) {
        return numbers
                .stream()
                .mapToInt(Integer::intValue)
                .reduce(
                        reductionOperation
                ).getAsInt();
    }
}