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javaautoboxing

Why during autoboxing final long to Byte compilation error happens, but final int to Byte is ok?

There is no error during auto-boxing of constants with int and short types to Byte, but constant with long type do has error. Why?

final int i = 3;
Byte b = i; // no error

final short s = 3;
Byte b = s; // no error


final long l = 3;
Byte b = l; // error
Solution

  • From JLS Sec 5.2, "Assignment contexts" (emphasis mine):

    In addition, if the expression is a constant expression (§15.28) of type byte, short, char, or int:

    • A narrowing primitive conversion may be used if the type of the variable is byte, short, or char, and the value of the constant expression is representable in the type of the variable.

    • 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.
      • ...

    It's simply not allowed for longs by the spec.

    Note that the second bullet point here says that this happens irrespective of the boxing: assigning a constant long expression to a byte variable would similarly fail:

    // Both compiler errors.
byte primitive = 0L;
Byte wrapped = 0L;