You probably imported the
java.util.Arrays class and called its
toString(double[]) method, which can not be overridden.
toString(double[]) is a static method of the
Arrays class. Static methods can't be overridden (only hidden).
Arrays also doesn't have an accessible constructor, so you can't extend it.
You should name your class something different than
Arrays, and you should make the methods static. If it's a utility class, give it a private constructor without parameters and an empty implementation.