CODEWITHSUNDEEP
javanullboxing
Vishrant
Vishrant

Reputation: 16638

Boxing of null value is allowed then why unboxing is not allowed?

I was reading jls §5.1.7 and it says that there are 9 types of boxing, 9th one being Boxing From the null type to the null type

Then I read that Unboxing Conversion of null throws a NullPointerException. Ok that is obvious. Then why boxing of null does not throws a NullPointerExceptionand what is the use of boxing null value?

Upvotes: 1

Views: 882

Answers (2)

locoyou
locoyou

Reputation: 1697

I think the doc you provide give the answer. "This rule is necessary because the conditional operator applies boxing conversion to the types of its operands, and uses the result in further calculations."

If one of second and third operands of ?: is not boolean or numeric expressions, then boxing may be used. For example, the type of true?1:2 is int, while the type of true?null:1 is Integer. In the second example, auto-boxing is employed. And in the run time, the second example's type will be null type because when boxing null type, a null type will be got.

Upvotes: 0

Bartek Maraszek
Bartek Maraszek

Reputation: 1414

Converting null to Integer does not throw NullPointerException because null is a valid value for any reference, e.g: Integer intObject = null; // fine

However, you cannot do this:

int intPrimitive = intObject; // not fine

Because when you try to convert an Integer to int the Integer.intValue() is called behind the scenes, but calling any method on a null reference throws NPE:

Object whatever = null;
whatever.anyMethod(); // always throws NPE

Because the reference points to null, not an actual object of the chosen type.

Upvotes: 1

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