It is indeed possible to put variables into interfaces. More accurately, it is possible to put public static final constants into interfaces. MAX_RINGS is arbitrarily assigned the value 5; real-world phones can ring longer.

Note that even without the public static final keywords, the constant still has all of those properties.

The book says to leave the keywords out because they are already implied.

public interface Ringable {
	int MAX_RINGS = 5;
	public void ring();
}