Converting from Java primitive to wrapper classes

Question

I am mystified by the behavior of the Java compiler when assigning primitives to wrapper class references. Please see the code below. The lines with comments don\'t compile.

Answer 1

Let's look at the types of conversions allowed in an assignment context.

Principally:

Assignment contexts allow the use of one of the following:

• an identity conversion

• a widening primitive conversion

• a widening reference conversion

• a boxing conversion optionally followed by a widening reference conversion

• an unboxing conversion optionally followed by a widening primitive conversion.

(Note my emphasis on one.)

Most of your examples that do not compile, for example

Integer s11 = (short)7;

require a widening primitive conversion followed by a boxing conversion. This is not a permitted conversion.

But then you might wonder why the following example does compile:

Byte s9 = (short)7;

This is a narrowing primitive conversion followed by a boxing conversion.

This is a special case:

In addition, if the expression is a constant expression of type byte, short, char, or int [...] a narrowing primitive conversion followed by a boxing conversion may be used if the type of the variable is:

• Byte and the value of the constant expression is representable in the type byte.

• Short and the value of the constant expression is representable in the type short.

• Character and the value of the constant expression is representable in the type char.

This special case is necessary because there is no way to express an integer literal of a type narrower than int.