====== Пример простенького логгера ======

<code java>

import static java.lang.System.out;
import static java.lang.System.err;
import java.time.LocalDateTime;

enum LogLevel { Debug, Info, Warning, Error, Fatal }

final class Logger
{

    private static Logger instance = null;

    public void debug(String message, Object ...format) {
        logCommit(LogLevel.Info, message, format);
    }

    public void info(String message, Object ...format) {
        logCommit(LogLevel.Info, message, format);
    }

    public void log(String message, Object ...format) {
        logCommit(LogLevel.Info, message, format);
    }
    
    public void warning(String message, Object ...format) {
        logCommit(LogLevel.Warning, message, format);
    }
    
    public void error(String message, Object ...format) {
        logCommit(LogLevel.Error, message, format);
    }
    
    public void fatal(String message, Object ...format) {
        logCommit(LogLevel.Fatal, message, format);
        System.exit(1);
    }

    private void logCommit(LogLevel level, String message, Object ...format) {
        var now = LocalDateTime.now();
        var level_s = levelString(level);
        message = message.format(message, format);
        if (level == LogLevel.Warning || level == LogLevel.Error || level == LogLevel.Fatal) {
            err.println(now.toString() + " [" + level_s + "] " + message);
        } else {
            out.println(now.toString() + " [" + level_s + "] " + message);
        }
    }

    private static String levelString(LogLevel level) {
        switch (level) {
            case Debug: return "DEBG";
            case Info: return "INFO";
            case Warning: return "WARN";
            case Error: return "EROR";
            case Fatal: return "FATL";
            default  : return "????";
        }
    }

    public static Logger getInstance() {
        if (Logger.instance == null) {
            Logger.instance = new Logger();
        }
        return Logger.instance;
    }

}

class HelloWorld
{

    static private final Logger logger = Logger.getInstance();

    public static void main(String[] args) {
        logger.log("Hello, %s!", "World");
        logger.fatal("Stop with code 1");
        logger.log("Hello, %s!", "World");
    }

}
</code>