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

Question

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 Dimsopt ) 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.

I found the JLS reference. 15.16.

 CastExpression:
    ( PrimitiveType Dimsopt ) 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.

You need to either change the cast to a primitive type:

... (int) -128;

Or you can change the expression to the right of the cast to a non-plus-minus unary expression:

... (Integer) (-128);  // Either
... (Integer) 0 - 128; // Or

The compiler interprets the - as the two-arg minus operator, i.e. it's trying to subtract 128 from some other number named Integer, but there's no such variable in scope.

This compiles:

Integer i3 = (Integer) (-128)

This may have to do with syntax parsing. Notice that

Integer i4 = (Integer) (-128); 

works just fine.

In general, you should not cast to Integer class. This involves something called auto-boxing, and can cause some subtle errors in your code. The prefered method of doing what you want is:

Integer i6 = Integer.valueOf(-128)