javax.lang.model.util.Elements.getConstantExpression() and negative argument value like 0xfd cause invalid string result "0xfd". While java.lang.Byte.decode("0xfd") and byte assignment like byte b = 0xfd cause to either exception or compiling error.

You may reproduce it using following code:
===
package tmp;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.lang.model.element.TypeElement;
import javax.lang.model.SourceVersion;
import java.util.Set;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.annotation.processing.SupportedSourceVersion;

@SupportedSourceVersion(SourceVersion.RELEASE_6)
@SupportedAnnotationTypes("*")
public class Proc extends AbstractProcessor {
   public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
       if(roundEnv.processingOver()) {
           byte b = -3;
           System.out.println("byte: " + processingEnv.getElementUtils().getConstantExpression(b));
       }
       return true;
   }
}
===

The result is like this:
# javac -processor tmp.Proc java.lang.Object
byte: 0xfd