Here is an example of how to use the FixedWindowRollingPolicy class from the logback library to rollover log files based on a fixed number of files:

import ch.qos.logback.core.rolling.RollingFileAppender;
import ch.qos.logback.core.rolling.FixedWindowRollingPolicy;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class MyApp {
    public static void main(String[] args) {
        // create a new logger
        Logger logger = LoggerFactory.getLogger("MyApp");

        // create a new RollingFileAppender
        RollingFileAppender<ILoggingEvent> fileAppender = new RollingFileAppender<>();
        fileAppender.setFile("mylog.log");

        // create a new FixedWindowRollingPolicy
        FixedWindowRollingPolicy rollingPolicy = new FixedWindowRollingPolicy();
        rollingPolicy.setFileNamePattern("mylog.%i.log");
        rollingPolicy.setMinIndex(1);
        rollingPolicy.setMaxIndex(10);

        // set the rolling policy for the RollingFileAppender
        fileAppender.setRollingPolicy(rollingPolicy);

        // start the RollingFileAppender
        fileAppender.start();

        // add the RollingFileAppender to the logger
        logger.addAppender(fileAppender);

        // log some messages
        logger.debug("Debug message");
        logger.info("Info message");
        logger.warn("Warning message");
        logger.error("Error message");
    }
}

In this example, the FixedWindowRollingPolicy class is used to rollover log files based on a fixed number of files.

The setFileNamePattern method is used to specify the file name pattern for the log files, in this case, "mylog.%i.log" where %i is the index of the file.

The setMinIndex and setMaxIndex methods are used to set the range of index numbers for the log files. In this example, the min index is 1 and the max index is 10, so the log files will be named "mylog.1.log", "mylog.2.log", "mylog.3.log" and so on. Once the max index is reached, the oldest file is deleted and the index is reset to the min index.

The start() method should be called after setting the policies and appender, to start the appender.

Once the configuration is done, logback is ready to use and you can use the logger to log message.

Please note that, the example provided here is an indicative one, the actual implementation and usage of FixedWindowRollingPolicy class might vary based on the specific use case.