java - Why final byte variable cannot be converted to Integer during auto-boxing? -

there no error when try autobox i2 byte,but when vise-versa(b1 integer),then error occurs.

        final byte b1 = 1;         integer i1 = b1; //error          final int i2 = 1;         byte b2 = i2;// no error          byte b3 = 1;         int i3 = b3; // no error 

can suggest read jls sec 5.2, linked in answer previous similar question.

assignment contexts allow use of 1 of 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 widening reference conversion

• an unboxing conversion (§5.1.8) optionally followed widening primitive conversion.

...

in addition, if expression constant expression (§15.28) of type byte, short, char, or int:

• a narrowing primitive conversion may used if type of variable byte, short, or char, , value of constant expression representable in type of variable.

• a narrowing primitive conversion followed boxing conversion may used if type of variable is:

  • byte , value of constant expression representable in type byte.
  • ...

taking cases in reverse order:

    byte b3 = 1;     int i3 = b3; // no error 

assigning byte int widening conversion.

    final int i2 = 1;     byte b2 = i2;// no error 

this same previous question: can assign constant-valued int byte, provided value of int fits byte.

    final byte b1 = 1;     integer i1 = b1; //error 

you're trying widening primitive conversion, followed boxing conversion. that's not 1 of cases listed here, it's error.

you can fix explicit widening cast:

    integer i1 = (int) b1; //ok