CODEWITHSUNDEEP
javaternary-operatorautoboxing
Praveen Kumar
Praveen Kumar

Reputation: 1624

returning null through conditional operator when return value is int

Can anyone explain the output of this code ?

public class Main
{
   int temp()
   {
      return(true ? null : 0);  
   }


public static void main(String[] args)
{
    Main m=new Main();
    System.out.println(m.temp());
 }
}

Upvotes: 3

Views: 4730

Answers (5)

ajay.patel
ajay.patel

Reputation: 1967

Lets take this one by one:

first compilation: why does it compile successfully? Have a look at below code:

int getIntValue(){

return new Integer(0));//note that i am returning a reference here and hence even I can possibly pass a null. 

}

Here unboxing happens and you see this code compiling properly. Even this code runs fine.

Now coming to your code:

int temp()
   {
      return(true ? null : 0);  
   }

Couple of things here, first this is making use of ternary operator. Java specification says that if any operand is of type T and other operand is primitive, primitive is first autoboxed and type T is returned as a result of the operation. And hence here, 0 is first wrapper (autoboxed) to Integer and than return type is basically converted to Integer type(remember, we can pass null here). Now when you pass null as return type, at runtime this is casted to int premitive type.

So what we are basically doing is as below:

int i =(int)null;

And above code basically gives you nullpointerexception.

Upvotes: 7

AllTooSir
AllTooSir

Reputation: 49402

Can anyone explain the output of this code ?

This will always through a NullPointerException . Trying to unbox null to int is a NullPointerException.

return(true ? null : 0);

The condition is always true and hence the return expression evaluates to null. The second and third operands are null and 0 respectively. Since null can be a value of a reference , the entire expression will be typed to Integer as it is the closest match to 0 and null . Since the return type is primitive int and hence the Integer null reference should be unboxed to int , while doing that it should throw NPE as int cannot hold null , but Integer can.

Refer the JLS.

The type of a conditional expression is determined as follows:

  1. If one of the second and third operands is of primitive type T, and the type of the other is the result of applying boxing conversion (§5.1.7) to T, then the type of the conditional expression is T.

  2. If one of the second and third operands is of the null type and the type of the other is a reference type, then the type of the conditional expression is that reference type.

Upvotes: 2

anshulkatta
anshulkatta

Reputation: 2064

It will be NullPointerException , because int cannot be assigned null

however , this code will always return null , because here you stated the condition always as true , so true part of ternary statement , that is null will get returned.

but this works

public class Test 
 {

        Integer temp()
        {
           return(true ? null : 0);  
        }


     public static void main(String[] args)
     {
         Test m=new Test();
         System.out.println(m.temp());
      }

 }

Because Integer can hold null value , primitive int cannot.

Upvotes: -2

Bathsheba
Bathsheba

Reputation: 234795

A NullPointerException will be thrown. This is because the ternary will evaluate to a boxed type containing null, and when you unbox a boxed type containing null (which Java must do to return an int) you get that exception.

For more details see Conversion from null to int possible?

Upvotes: 0

stinepike
stinepike

Reputation: 54722

it should throw a NullPointerException

because return(true ? null : 0); in this statement it will always return null;

Upvotes: 0

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