Replacing ConsoleAppender to FileAppender in log4j

I have created a Logger over the log4j logger with ConsoleAppender. The code is as follows,

public class AppLogger{ 

   static {
        ConsoleAppender ca = new ConsoleAppender(new PatternLayout(PatternLayout.TTCC_CONVERSION_PATTERN));
        org.apache.log4j.Logger.getRootLogger().addAppender(ca);
   }
   public AppLogger(Class classname) {
        logger = LoggerFactory.getLogger(classname);
   }

   public void debug(String message){
        logger.debug(message);
   }
...
...
}

I have not used the log4j.properties file because I am not sure where to put it. The above code is working fine and logging properly on the console. Now I want to change it to use it with a FileAppender.

I simply updated the static constructors as follows,

    FileAppender fa = new FileAppender();
    fa.setFile("d:/log.txt");
    fa.setLayout(new PatternLayout(PatternLayout.DEFAULT_CONVERSION_PATTERN));
    org.apache.log4j.Logger.getRootLogger().addAppender(fa);

And added fa insted of ca. But now I am getting this error,

log4j:ERROR No output stream or file set for the appender named [null].

I am not sure which property I am missing to define fro the File Appender.

You forgot to activate the options:

FileAppender fa = new FileAppender();
fa.setFile("d:/log.txt");
fa.setLayout(new PatternLayout(PatternLayout.DEFAULT_CONVERSION_PATTERN));
fa.activateOptions(); \\ ← activate the options
org.apache.log4j.Logger.getRootLogger().addAppender(fa);

The method OptionHandler#activateOptions() activate the options that were previously set with calls to option setters.