Bad concatenation syntax not detected by javac

Question

The following test case produces no compile errors but bad results. In my app the string '45' was inserted as though it were between the concatenation operators. This condition occurred in jdk 1.3.1_07.
* Demo of uncaught invalid syntax ?
public class JavacTestCase {
     String plTrain = "210";
     String pieceMk = "131D1N";
     public JavacTestCase() {
          // the whitespace-separated operators '+ +' appear to be invalid syntax
          String commod2 = plTrain +     + '-' + pieceMk;
Pardon me if this is a feature, not a bug,
Bill Winspur

Answer 1

This is not invalid syntax.
The dash/hyphen is ASCII decimal 45. Since '-' is a char primitive type and is preceded by the unary operator, it will be converted to an int. The second plus is treated as a unary operator, so + '-' becomes +45. Normal String concatenation then takes place so you end up with "45" within the resulting String.