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NullPointerException with ternary operator

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?

Solution

  • 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.