Java: many ways of casting a (long) Object to double

Question

I have an Object obj that I know is actually a long. In some Math code I need it as double.

Is it safe to directly cast it to double?

double x = (double)obj;

Or should I rather cast it first to long and then to double.

double x = (double)(long)obj;

I also found another (less readable) alternative:

double x = new Long((long)obj).doubleValue();

What are the dangers/implications of doing either?

Solution Summary:

• obj is a Number and not a long.
• Java 6 requires explicit casting, e.g.: double x = ((Number)obj).doubleValue()
• Java 7 has working cast magic: double x = (long)obj

For more details on the Java6/7 issue also read discussion of TJ's answer.

Edit: I did some quick tests. Both ways of casting (explicit/magic) have the same performance.

Solution 1

As every primitive number in Java gets cast to its boxing type when an object is needed (in our case Long) and every boxed number is an instance of Number the safest way for doing so is:

final Object object = 0xdeadbeefL;
final double d = ((Number)object).doubleValue();

The danger here is, as always, that the Object we want to cast is not of type Number in which case you will get a ClassCastException. You may check the type of the object like

if(object instanceof Number) ...

if you like to prevent class cast exceptions and instead supply a default value like 0.0. Also silently failing methods are not always a good idea.

Solution 2

I have an Object obj that I know is actually a long.

No, you don't. long is a primitive data type, and primitive types in Java are not objects. Note that there's a difference between the primitive type long and java.lang.Long, which is a wrapper class.

You cannot cast a Long (object) to a long (primitive). To get the long value out of a Long, call longValue() on it:

Long obj = ...;

long value = obj.longValue();

Is it safe to directly cast it to double?

If it's actually a primitive long, then yes, you can cast that to a double. If it's a Long object, you don't need to cast, you can just call doubleValue() on it:

double x = obj.doubleValue();

Comments

[T.J. Crowder]

"Long => double is not" Works for me, presumably the Long gets auto-unboxed.

[assylias]

Object o = 5L; double d = (double) o; does not work. But double d = (Long) o; does. ;-)

[T.J. Crowder]

@ assylias: I never said it did. :-) I said Long => double works. And it does, see my answer.

[assylias]

You only need one cast: double d = (Long) obj; or double d = (long) obj;

[T.J. Crowder]

@assylias: I prefer the clarity, as the cast is there whether I write it or not. (And of course, in the above, I do need two casts. I could only rely on the implicit cast if I were assigning to a variable.) Also, double d = (long)obj; fails, you can't cast Object to long.

[assylias]

Object o = 15L; double d = (long) o; compiles and runs without exception.

[T.J. Crowder]

@assylias: On what? It doesn't for me on Oracle's JDK 1.6: pastie.org/5519551

[T.J. Crowder]

@Juve: Must be a Java 7 enhancement.

[Neet]

Ehm, he said he has an 'Object' so there is no primitive long which can be casted to 'double'.

[jarek]

I like this answer best. It shortly explains the boxing and I tried the object instanceof Number test via the debugger to see what is going on in my JVM. Thx!