Casting an instance of Object to primitive type or Object Type

Question

when I need to cast an instance of type Object to a double , which of the following is better and why ?

Double d = (Double) object;

Or

double d = (double) object;

Mascarenas

If you try to compile these two options you'll notice something about the second... – Hatten

Answer 1

The difference is that the first form will succeed if object is null - the second will throw a NullPointerException. So if it's valid for object to be null, use the first - if that indicates an error condition, use the second.

This:

double d = (double) object;

is equivalent to:

Double tmp = (Double) object;
double t = tmp.doubleValue();

(Or just ((Double)object).doubleValue() but I like separating the two operations for clarity.)

Note that the cast to double is only valid under Java 7 - although it's not clear from the Java 7 language enhancements page why that's true.

Carnallite

You can do double d = (double) (Double) object; in versions of java pre 1.7, this will throw the NPE from the (double) cast if object is null. – Telephonist

@bmorris591: Indeed - or just call doubleValue() directly. – Carnallite

@bmorris591: I am getting NPE in both jdk 1.6 and 1.7 ! – Mascarenas

@mataug: Then presumably object is null. – Carnallite

From @bmorris591 comment I assumed that NPE will be thrown only in pre 1.7, and in 1.7 I would probably get 0 . – Mascarenas

@mataug: No, prior to Java 7 it wouldn't compile. – Carnallite