Short form for Java If the operator returns a NullPointerException when one of the returned objects is zero

Question

Why does this code return an error: java.lang.NullPointerException

Object obj = null; Long lNull = null; Long res = obj == null ? lNull : 10L; 

But the following method works without errors:

 Object obj = null; Long res = obj == null ? null : 10L; 

Answer 1

In the second case, the compiler can conclude that null must be of type Long . In the first case, this is not so - and assumes that the expression returns a Long . You can see that this is so (e.g. fix), for example,

 Long res = obj == null ? lNull : (Long) 10L; 

which does not throw a NullPointerException.

Answer 2

The error occurs because in your case for the standard one, an unboxing value of the boxed type is required :

If one of the second and third operands has a primitive type T , and the type of the other is the result of applying the box transform (section 5.1.1) to T , then the conditional expression type is T

In your case, T is long , because 10L is long and lNull is long , that is, the result of applying the box to long conversion.

The standard further states that

If necessary, the decompression conversion is performed as a result.

This is the exception. Note that if you invert the condition, the exception will no longer be thrown:

 Long res = obj != null ? lNull : 10L; 

You can fix the problem by explicitly asking for long instead of using long , i.e.

 Long res = obj == null ? lNull : Long.valueOf(10L); 

Answer 3

JLS, section 15.25 talks about the type of expression of a conditional statement for various combinations of types of the second and third operands. There are many tables comparing two types in all relevant combinations with the type of result.

 3rd → long 2nd ↓ ... Long long ... null lub(null,Long) 

In your first example, there is Long and a Long , which gives Long . This requires lNull be unpacked, which explains NullPointerException .

In your second example, there is a literal null (not a null variable) and Long . This results in "lub (null, Long)" or Long , and no decompression is performed, so no NPE is observed.

You can avoid NPE by using your first example, or cast 10L like Long , because a null and a Long give a Long .

 3rd → Long 2nd ↓ ... Long Long