Why can't I set a Double object equal to an int? - Java

Question

Why do I get an error when attempting to initialize a Double to an int, even though it does not throw an exception when using the primitive type, double?

Double a = 1;   // error - incompatible types
Double b = 1.0; // OK
double c = 1;   // OK

Why is the behavior different between the class Double and the primitive type, double?

Answer 1

When you initialize your Double as:

Double a = 1;

There needs to be done 2 things:

• Boxing int to Integer
• Widening from Integer to Double

Athough, boxing is fine, but widening from Integer to Double isn't valid. So, it fails to compile.

Note that, Java doesn't support Widening followed by Boxing conversion, as specified in JLS §5.2:

Assignment contexts allow the use of one of the following:

• an identity conversion (§5.1.1)
• a widening primitive conversion (§5.1.2)
• a widening reference conversion (§5.1.5)
• a boxing conversion (§5.1.7) optionally followed by a widening reference conversion
• an unboxing conversion (§5.1.8) optionally followed by a widening primitive conversion.

Your 2^nd assignment goes through boxing conversion.
While 3^rd assignment goes through widening conversion.