Here is an example of a class extending RollingPolicyBase in Logback:

import ch.qos.logback.core.rolling.RollingPolicyBase;
import ch.qos.logback.core.rolling.TimeBasedRollingPolicy;

public class MyRollingPolicy extends RollingPolicyBase {
    private TimeBasedRollingPolicy timeBasedRollingPolicy;

    public void setTimeBasedRollingPolicy(TimeBasedRollingPolicy timeBasedRollingPolicy) {
        this.timeBasedRollingPolicy = timeBasedRollingPolicy;
    }

    @Override
    public void start() {
        if (timeBasedRollingPolicy == null) {
            throw new IllegalStateException("timeBasedRollingPolicy must be set");
        }
        // additional initialization code
        super.start();
    }

    @Override
    public void rollover() {
        // rollover logic
    }
}

This class is an example of a custom rolling policy in Logback, which is used to determine when log files should be rolled over. In this example, the custom rolling policy is based on time and is set using the setTimeBasedRollingPolicy method. The start method is overridden to perform additional initialization and to check if the timeBasedRollingPolicy has been set. The rollover method is overridden to define the specific rollover logic.