Compiler error when doing type casting in Java

Question

See the following expressions in Java.

int temp = -254; Integer temp2 = (Integer) temp; // compiles because of autoboxing Integer temp3 = (Integer) -254; // doesn't compile - illegal start of type. Integer temp4 = (Integer) 10-254; // compiles Integer temp5 = (Integer) (int) -254; // compiles Integer temp6 = -254; // compiles Integer temp7 = (int) -254; // compiles 

In the above expressions, why are these expressions (Integer) 10-254 and (int) -254 valid, while the expression (Integer) -254 not compiled, although the constant -254 can be perfectly evaluated to Integer ?

Answer 1

This is an interesting edge case, the compiler is trying to perform integer subtraction in the class Integer and literal int (254).

Note that the following compilations are more explicit:

  Integer temp3 = (Integer)(-254) 

Answer 2

In particular, this corresponds to section 15.16 of the third edition of the JLS :

The expression expressed converts at run time the value of one numeric type to the same value of another numeric type; or confirms, when compiling, the time when the type of the expression is Boolean; or checking, at startup, that the reference value refers to an object whose class is compatible with the specified reference type.

CastExpression:
(PrimitiveType Dimsopt) UnaryExpression
(ReferenceType) UnaryExpressionNotPlusMinus