Assigning integer value to a Float wrapper in Java

Question

The following works

float a=3;

but the following doesn't:

Float a=3;

Shouldn't 3 be automatically promoted to float (as widening conversions don't require an explicit cast) and then Boxed to Float type ?

Is it because of a rule I read in Khalid Mogul's Java book ?

Widening conversions can't be followed by any boxing conversions

Answer 1

The reason why Float a=3; won't work is because the compiler wraps the 3 into it's Integer object (in essence, the compiler does this: Float a = new Integer(3); and that's already a compiler error). Float object isn't and Integer object (even though they come from the same Number object).

The following works:

Number a = 3;

which in essence is translated by the compiler as:

Number a = new Integer(3);

or as Joachim Sauer mentioned,

Number a = Integer.valueOf(3);

Hope this helps.

Answer 2

Float               Integer
  ^                    ^
  |                    |
  |                    |
  v                    v
float <----------->   int

There is a boxing/unboxing conversion betwen the primitive and the wrapper, and there is a promotion from one numeric primitive to another. But Java is not able to make this conversion twice (convert from int to Float, in your case).

Answer 3

Float a= 3.0f; will work.