Book Image

Learn Java with Projects

By : Dr. Seán Kennedy, Maaike van Putten
5 (3)
Book Image

Learn Java with Projects

5 (3)
By: Dr. Seán Kennedy, Maaike van Putten

Overview of this book

Learn Java with Projects stands out in the world of Java guides; while some books skim the surface and others get lost in too much detail, this one finds a nice middle ground. You’ll begin by exploring the fundamentals of Java, from its primitive data types through to loops and arrays. Next, you’ll move on to object-oriented programming (OOP), where you’ll get to grips with key topics such as classes, objects, encapsulation, inheritance, polymorphism, interfaces, and more. The chapters are designed in a way that focuses on topics that really matter in real-life work situations. No extra fluff here, so that you get more time to spend on the basics and form a solid foundation. As you make progress, you’ll learn advanced topics including generics, collections, lambda expressions, streams and concurrency. This book doesn't just talk about theory—it shows you how things work with little projects, which eventually add up to one big project that brings it all together. By the end of this Java book, you’ll have sound practical knowledge of Java and a helpful guide to walk you through the important parts of Java.
Table of Contents (22 chapters)
1
Part 1: Java Fundamentals
9
Part 2: Object-Oriented Programming
15
Part 3: Advanced Topics

Applying sealed classes

Sealed classes were introduced in Java 17. What we are going to cover here relates to classes but the same logic applies to interfaces (Chapter 10). With inheritance, you can extend from any class (or interface) using the extends keyword, unless the class is final of course.

Note

Interfaces cannot be final because their whole rationale is to be implemented.

Consider the following scenario: what if you wanted your class to be available for inheritance, but only for certain classes? In other words, you want to scope the subclasses allowed. So far, inheritance, using extends, enables every class to become a subclass, whereas final prevents a class from having subclasses.

This is where sealed classes are useful – they enable you to specify what subclasses are allowed. Just to reiterate, this also applies to interfaces, where we can specify what classes are allowed to implement the interface.

Before we look at an example, there are some new...