> The problem is, especially in a hot loop ... The proper way to do something like this in java is either log.trace(..., ...) or if (log.traceEnabled()) log.trace(...)

The former still creates strings, for the garbage collector to mop up even when log.traceEnabled() is false, no?

Also, even if the former or latter is implemented as:

  fn trace(log, str, args...) {
     if (!log.tracing) return;
     // ...
  }

Most optimising JIT compilers will code hoist the if-condition when log.tracing is false, anyway.