# The JPF Logging API #
There is one simple rule: do not use `System.out` or `System.err` for any permanent logging


Of course we all do this temporarily during debugging, but it really shouldn't stay in the code. The logging infrastructure is quite easy to use. Just declare a static `Logger` instance with an appropriate id (either package or logging topic) at the top of your class, and then use the `Logger` API to create output:

~~~~~~~~ {.java}
...
import java.util.logging.Level;
import java.util.logging.Logger;

package x.y.z;

class MyClass .. {
  static Logger log = JPF.getLogger("x.y.z");
  ...
    log.severe("there was an error");
  ...
    log.warning("there was a problem");
  ...
    log.info("something FYI");
  ...
    if (log.isLoggable(Level.FINE)){    // (1) don't create garbage
        log.fine("this is some detailed info about: " + something);
    }
  ...
}
~~~~~~~~

Note that there is only one instance for each `Logger` ID, i.e. you can have a corresponding static field in all your relevant classes, and don't have to share the fields. Another aspect that is mostly important for the lower log levels (e.g. `FINE`) is that you should't concatenate log messages in operations that occur frequently, since the corresponding `StringBuilder` instances can cause performance degradations even if the log level is not set (the arguments still get evaluated). In this case, encapsulate the logging in `log.isLoggable(level){..}` blocks.