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

问题:

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.

回答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.

回答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();

回答3:

Simple casting should work perfectly fine. This will be faster than going via the wrapper classes, getting string representation and then parsing to double, create new instance again using the long value - and more importantly, it's more readable.

double d = (double)15234451L;

回答4:

You can cast it to a Long (since the Object is not a long but a Long), and then cast the Long to a double:

double d = (double)(Long)obj;

For instance, this has the expected output of 2.6666666666666665:

public class CastDouble {
    public static final void main(String[] args) {
        Object o = 15L;

        System.out.println(40 / (double)(Long)o);
    }
}

回答5:

You only need one cast, from Object to Long or long (which implicitly casts to Long then applies unboxing):

Object o = 5L;
double d = (long) o; //Apparently only works on Java 7+
//or
double d = (Long) o;