Need some help with this java assignment

Create a Java class named Circle that implements java.io.Serializable interface (for more information see: https://docs.oracle.com/javase/7/docs/api/java/io/Serializable.html) and models a circle based on radius. Radius cannot be less than zero. Implement the getter and setter method for radius. Also include an overriding of toString in the circle class. Create the class' comparator which compares two objects of type Circle.