Equivalent of comma operator in Java?

Question

I am trying to do a source transformation on some Java code that results in code where for every expression, a method is called if the expression is evaluated.

(The use case is a simplistic line coverage measure. I've done this sort of thing before in JavaScript: <my-expression> becomes (covered("path/to/file.js", 12), <my-expression>) or something, where 12 is the line number of the expression).

Java doesn't have a comma operator. I thought about wrapping expressions in a method call, e.g. my covered function would be declared public static <T> T covered(String file, int line, T expr) and return its third argument, so I could write covered("path/to/file.java", 12, myExpression()) but it doesn't work for expressions that have type void.

Is there an easy way to accomplish this? Evil code is okay; this is generated code.

Answer 1

(I see the problem, now.)

The only context in Java where a void expression can legally occur is when it is a statement expression, or the 1st or 3rd part in a classic for statement. So:

• If the expression is used as an expression statement:

  covered(...); <my-expression>;
  

• If the expression is used as the 1st or 3rd part of a for,

  covered(...), <my-expression>
  

• Otherwise

  covered(..., <my-expression>)
  

  or some such. (This requires an overload of covered for each primitive type, and also an overload with signature <T> overload(..., <T>).)

I think this can be determined purely based on the syntax. No type analysis or method overload resolution is required to figure out whether a void method is actually being called.