Figure 1 shows a UML class diagram for the simple class and type hierarchy used as this article’s example. The diagram models two classes, one interface, and three type declarations.

Every Java class inherits from Object, so familiarity with this superclass is fundamental to understanding the Java class hierarchy. Also see my Java tip introducing composition vs. inheritance .

Sealed classes, proposed in JEP 409 and available since Java 17, let developers limit and control how deeply a component's type hierarchy can extend. With sealed classes, a developer can essentially ...

UML is used as a representation of the Java programming language. Using sample class and sequence diagrams, you can see how UML conveys unambiguous code-mapping information to developers.