I've seen a "_" as variable prefix. What's that mean?

Question

Thanks for replying!

Answer 1

More precisely, it often means this is a class-level variable (that is, one defined for the class - could be a static class variable or a per-object instance variable). By "often" I mean that this is a common coding convention, to help distinguish class-level variables from local variables. Without such a convention, it is easy to unwittingly mask your class-level variable with a local one using the same name:
class Zorch
  private boolean enabled;
  ... (many source lines later) ...
  public void zorchBlast()
    boolean enabled;  // this declaration masks the class-level variable
    enabled = true;   // only sets local boolean, not class-level one
    }(What's really bad is if the local declaration of enabled isn't at the beginning of zorchBlast, but instead is buried 10-20 lines into the method. I saw this in a code review just yesterday, it really happens.)
With the coding convention "Prefix all class-level variables with '_'" this code now looks like:
class Zorch
  private boolean _enabled;
  public void zorchBlast()
    boolean enabled;  // this declaration is clearly local, no '_'
    boolean _enabled; // this declaration masks the class-level one,
                      // but is easier to spot - hey, what's this local variable
                      // doing with a leading '_'?
    (etc...)Other common conventions to highlight class-level variables:
- leading 'm' or 'm_', meaning "member" (popularized by Microsoft)
- trailing '_'
- always reference class-level variables using explicit 'this.'
But there is nothing inherent in the '_' prefix that this must be a class-level variable, and there is nothing in the Java compiler to enforce this one way or another, it is just a common style.
(My personal preference is trailing '_'. I think my aversion to prefix '_' goes back to my C++ days, when some compilers had problems with variables with leading '_'s. Also, I find it easier to say these variable names - "enabled_" is just spoken "enabled", but I find the need to say "_enabled" as "underscore enabled" - when either discussing the code with someone else, or even just verbalizing it to myself while coding.)
-- Paul