Return null in a method that returns doubles java

Question

I am trying to test for a null input at the start of the method. And if found true I want to return null even though the method would usually return doubles.

I need to keep the type of method as double

public double computeMean (double[] grades) {
        if (grades == null) {
            return null;

Cannot convert from null to double

Answer 1

You should use

Double wrapper class(mind the uppercase "d")

not double primitive type as your return type, if you want to be able to return null .

Answer 2

Explanation

null is a value that can only be used for objects. A double however is a primitive, it does not use the object-system. Which is why you can not return null if you specified double as return type.

So, what are your options?

---

Double wrapper

You can instead use Double , the wrapper class for double which uses the object-system.

Since Java provides automatic conversion between double and Double whenever needed (autoboxing), this can be quite handy to use.

Note that using Double brings quite some overhead to just a small double and that people regularly tend to forget to check for null when converting a Double to a double . Ie

// foo() returns Double
double value = foo(); // Bad code, it could be null!

Instead, users must remember to check the resulting value:

Double result = foo();
if (result == null) {
    ...
} else {
    double value = result;
    ...
}

---

OptionalDouble

The modern, and probably better alternative, is to use Optional (you need at least Java 8 for this).

It was designed to be used whenever a method naturally might sometimes not return a result. For example if the array passed in empty . That case is completely okay and not to be considered as error.

This also solves the problem of users forgetting to check the result. Optional forces them to check it, else they can not get hands on the underlying value.

In order to avoid the performance overhead of Optional<Double> (wrapper class again), there is also OptionalDouble which internally uses double (primitive). Here is the code:

public OptionalDouble computeMean(double[] grades) {
    if (grades == null) {
        return OptionalDouble.empty();
    }
    ...
    return OptionalDouble.of(result);
}

And the usage:

OptionalDouble result = computeMean(...);

From there the user has a couple of options (see the documentation ), for example

double value = result.orElse(10.4);
// or
double value = result.orElseThrow();
// or
if (!result.isPresent()) {
    ...
} else {
    double value = result.getAsDouble();
}

---

Exception

The last option is to actually just throw an exception. You should consider this whenever an user is doing something that is not intended and against what you consider correct usage (indicate this in your methods documentation).

I would actually say that in your specific situation, this is the case. It is impossible to compute a mean on null . It is different to passing in an empty array, where the I would go for an empty Optional . For a null array I would throw an exception, to indicate a bad usage.

A good exception for this situation is IllegalArgumentException , here is the code:

public double computeMean(double[] grades) {
    if (grades == null) {
        throw IllegalArgumentException("Grades must not be null!");
    }
    ...
}

Another idiomatic exception for exactly this use case is to throw NullPointerException . There is even a compact helper method to do all of this in one line:

public double computeMean(double[] grades) {
    Objects.requireNonNull(grades); // Yes, thats it

    ...
}

---

Result

Putting all of that together, I would do the following two changes:

• Throw NullPointerException if grades is null , using Objects#requireNonNull
• Return an empty OptionalDouble if grades is empty

public OptionalDouble computeMean(double[] grades) {
    Objects.requireNonNull(grades);

    if (grades.length == 0) {
        return OptionalDouble.empty();
    }

    ...
    return OptionalDouble.of(result);
}