带有三元运算符的 NullPointerException

Question

Why does this work:

String a = null;
String b = a != null && a.equals("Nan") ? "Nan" : a;
System.out.println(b);

but this produces NPE:

Double value = null;
Double v = value != null && value.isNaN() ? 0.0 : value;
System.out.println(v);

Rewriting it as:

Double value = null;
Double v;
if (value != null && value.isNaN()) {
    v = 0.0;
} else {
    v = value;
}

of course works as expected. But why do I get NPE using the ternary/conditional operator when using Double and no NPE when using String ? What am I missing?

Answer 1

The type of the conditional expression is inferred as double , so you get a NullPointerException when the program attempts to unbox value (which is null ) as a primitive double .

To make the expression have a Double result (which can store null ), cast 0.0 to Double .

Double v = value != null && value.isNaN() ? (Double) 0.0 : value

See §15.25.2. Numeric Conditional Expressions for more details.

Answer 2

Just to extend the above accepted answer which is to the point but you may as well avoid casting here.

   Double value = null;
   Double v = value != null && value.isNaN() ? Double.valueOf(0.0) : value;
   System.out.println(v);

The Double.valueOf(0.0) will provide a Double in first type of condition which will make the conditional operator to evaluate into the type of Double since value is as well of type Double . So if no participant of condition is of primitive type double , it will not try any unboxing and a NPE will not be thrown.