Such a class is known as a utility class. An example of a utility class is the Java standard class library’s Math class. Advanced techniques: Fields and methods in Java Learn seven advanced ...

It’s a good idea to use an abstract class when you need to implement mutable state. As an example, the Java Collections Framework includes the AbstractList class, which uses the state of variables.

public final class NegExpr implements Expr { } For example, if we want to write the following class and try to compile it, we get the following error: javac DivideExpr.java DivideExpr.java:1: error: ...