Short form for Java If statement returns NullPointerException when one of the returned objects is null [duplicate]

Answer 1

In the second case, the compiler can infer that the null must be a Long type. In the first case, it doesn't - and assumes that the expression returns a long. You can see that this is the case (i.e. fix it) like,

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

which does not yield a NullPointerException.

Answer 2

The JLS, Section 15.25 talks about the type of the conditional operator expression for various combinations of the types of the second and third operands. There are lots of tables mapping the two types in all relevant combinations to the result type.

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

Your first example has a Long and a long, which yields long. This requires lNull to be unboxed, which explains the NullPointerException.

Your second example has a null literal (not a null variable) and a long. This results in "lub(null,Long)" or Long, and no unboxing is performed, so no NPE is observed.

You can avoid the NPE by using your first example or by casting 10L as a Long, because a null and a Long yield a Long.

3rd → Long
2nd ↓  
...
Long  Long

Answer 3

The error happens because in your case the standard requires unboxing the value of a boxed type:

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.

In your case T is long, because 10L is long and lNull is Long, i.e. the result of applying boxing conversion to long.

The standard further says that

If necessary, unboxing conversion is performed on the result.

This is what causes the exception. Note that if you invert the condition, the exception would 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);