Null can not be compared to double - why?

Question

To my understanding following code should run fine.

However, When I try to compile, it is failing at compareTo

public class Test {

    Comparable<?> getMinimum() {
        return null;
    }

    public int getPreviousValue(double nv) {
        return getMinimum().compareTo(nv);
    }
}

So my question is why compilation if failing - are we not allowed to compare null with double?

because the double type is a primitive (non-reference) type, and primitive types cannot be null — Elkhan Ibrahimov, Sep 14 '18 at 11:02
Probably because `double` cannot be null, but `Double` (the class) can be. That would be my guess. — Nathan Mills, Sep 14 '18 at 11:02
Are we??? Checkout this post https://stackoverflow.com/questions/9090077/how-to-check-if-a-double-is-null — Wale, Sep 14 '18 at 11:02
Your question already answered [here](https://stackoverflow.com/questions/5855188/can-we-call-any-method-on-null-object) and [here](https://stackoverflow.com/questions/2707322/what-is-null-in-java) ;) — Rostyslav Barmakov, Sep 14 '18 at 11:04
@AniruddhParihar that will not work anyway! using Double results in somthing.getDouble(); which will never assign a null to returned value! — Wale, Sep 14 '18 at 11:04
@W.Seun : i think it will work! by just changing the signature of Comparable like this : Comparable getMinimum() — Aniruddh Parihar, Sep 14 '18 at 11:08

GhostCat · Answer 1 · 2018-09-14T13:08:17.550

The problem here is not the null result of that method.

The problem is that the compiler doesn't know how you could call compareTo(double) on something that your getMinimum() returns.

You have to change the the signature to Comparable<Double> getMinimum() to make it work! You can only compare against a specific type of number if your Comparable supports that!

Of course, at runtime, you will then face a NPE. And also of course: some code inspection tools could identify this specific NPE situation already at compile time.

score 2 · Answer 2 · answered Sep 14 '18 at 11:06

Compiler is getting confused at Comparable<?>. Make the Comparable of specific type (Double in you case) to make the compiler happy. :)

public class Test {

    Comparable<Double> getMinimum() {
        return null;
    }

    public int getPreviousValue(double nv) {
        return getMinimum().compareTo(nv);
    }
}

score 1 · Answer 3 · answered Sep 14 '18 at 11:03

1

You don't have a nullpointer, you have a problem invoking compareTo with argument double NOT Double!

answered Sep 14 '18 at 11:03

fl0w

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Null can not be compared to double - why?

3 Answers3