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javalambdafunctional-programming
Liepkalne
Liepkalne

Reputation: 21

Java8 FP if/else if/else replace by lambdas

I have a problem with Java 8 if statements. Can someone show me a way to write this code without if's using only Java 8 lambdas? The solution shouldn't have if's, while's or for's. Is it even possible?

if (first_number == second_number) {
  return "PERFECT";
} else if (first_number > second_number) {
  return "ABUNDANT";
} else {
  return "DEFICIENT";
}

Upvotes: 1

Views: 7084

Answers (2)

Pflaumenkern
Pflaumenkern

Reputation: 56

How about this solution with lambdas? I'm assuming integer values, but shouldn't be too hard to switch to float or double.

    int first_number = 10;
    int second_number = 20;
    IntBinaryOperator subtract = (n1, n2) -> n1 - n2; // Subtract second from first value
    IntUnaryOperator determineSign = n -> Integer.signum(n); // Determine the sign of the subtraction
    IntFunction<String> message = key -> { // Sign of 0 means value has been 0 (first_number == second_number). Sign of 1 = positive value, hence equals first_number > second_number, otherwise return the default.
        Map<Integer, String> messages = new HashMap<>();
        messages.put(0, "PERFECT");
        messages.put(1, "ABUNDANT");
        return messages.getOrDefault(key, "DEFICIENT");
    };

     return message.apply(determineSign.applyAsInt(subtract.applyAsInt(first_number, second_number)));

Edit: Andreas mentioned valid concerns, I agree you wouldn't do it like that. But I think its more about proving that it is possible using lambdas. :) Another method (ab)using Optional:

    int first_number = 20;
    int second_number = 20;

    Optional<Integer> dummy = Optional.of(0); // Dummy allowing to call filter on the Optional
    Predicate<Integer>isAbundant = i -> first_number > second_number; // Check if input is abundant
    Predicate<Integer> isPerfect = i -> first_number == second_number; // Check if input is perfect
    Supplier<String> other = () -> dummy.filter(isAbundant).map(i -> "ABUNDANT").orElse("DEFICIENT"); // Fallback, if input is not perfect. Check for abundant or return default
    Supplier<String> validate = () -> dummy.filter(isPerfect).map(i -> "PERFECT").orElse(other.get()); // Check if input is perfect or use fallback

    return validate.get();

Upvotes: 0

Andreas
Andreas

Reputation: 159145

No "if's, while's or for's", but no lambdas either:

return (first_number == second_number ? "PERFECT" :
        first_number > second_number ? "ABUNDANT" : "DEFICIENT");

The ? : is called the conditional operator in the Java Language Specification (see 15.25. Conditional Operator ? :), but is commonly known as the ternary operator, since it is the only operator in Java with 3 parts.

Upvotes: 7

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