Java Primitive conversions in assignment context Long and int

Question

Long ll = 102; // Error
Byte bb = 101; // No error

Why Long assignment is resulting in compile time error while Byte assignme

Answer 1

See 5.1.7 Boxing Conversion of the JLS

• If p is a value of type int, then boxing conversion converts p into a reference r of class and type Integer, such that r.intValue() == p

Because 102 is an integer literal, it's type is int and auto boxing will convert it to Integer (as the spec says), but an Integer can not be casted to Long.

Thus when you use a long literal or cast the int literal to long the JLS will use the boxing conversion and the result will be a Long object.

This will be fine

Long long1 = (long) 102;
Long long2 = 102L;
Long long3 = 102l;

The second one

Byte bb = 101;

works, because of the 5.2. Assignment Conversion

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.

So 101 is a integer literal, but there is an assignment that needs a narrowing conversion (int -> byte) and the value of the int is within the byte value range. Thus it is representable as the variable type (see spec) and it is converted.

This will NOT WORK of course

 Byte bb = 128; // can not be represented as the variable type. Thus no narrowing conversion.