java - Any idea why I need to cast an integer literal to (int) here?

Question

In the following example

int i = -128;
Integer i2 = (Integer) i; // compiles

Integer i3 = (Integer) -128; /*** Doesn't compile ***/

Integer i4 = (Integer) (int) -128; // compiles
Integer i4 = -128; // compiles
Integer i5 = (int) -128; // compiles
Integer i6 = (Integer) (-128); // compiles
Integer i7 = (Integer) 0-128; // compiles

I can't cast -128 with (Integer) but I can cast (int) -128.

I always thought -128 was of int type and casting it with (int) should be redundant.

The error on the line with i3 is

cannot find symbol variable Integer

I tried this with Java 6 update 29 and Java 7 update 1.

EDIT: You get the same behavior with +128 instead of -128. It does appear to be confusion between unary and binary operators.

Answer 1

The compiler tries to subtract 128 from (Integer) instead of casting -128 to Integer. Add () to fix it

Integer i3 = (Integer) -128; // doesn't compile
Integer i3 = (Integer) (-128); // compiles

According to BoltClock in the comments the cast to int works as intended, because it is a reserved word and therefore can't be interpreted as an identifier, which makes sense to me.

And Bringer128 found the JLS Reference 15.16.

 CastExpression:
    ( PrimitiveType Dims

_opt ) UnaryExpression ( ReferenceType ) UnaryExpressionNotPlusMinus

As you can see, casting to a primitive type requires any UnaryExpression, whereas casting to a reference type requires a UnaryExpressionNotPlusMinus. These are defined just before the CastExpression at JLS 15.15.