In this Java 101 tutorial, I’ve explained how interfaces differ from classes, and showed you how to declare, implement, and extend interfaces in your Java programs.

The Java 2 SDK organizes its vast collection of classes and interfaces into a tree-like hierarchy of packages within packages, which is equivalent to directories within directories.

This post explains how to use classes in Java to build objects and organize your code. Also learn: constructors, static methods, and more!

If you want to extend the Expr class, you must explicitly add the permits clause. Only a library developer can add that clause. Sealed class rules Sealed classes offer precise control over inheritance ...

Don't overcomplicate your classes. That's what SOLID's single responsibility principle in Java means. Here's an example of how to do SOLID right in Java.