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

Mastering upcasting and downcasting

Earlier, we touched upon why we get ClassCastException errors. The rule is that a reference can refer to objects of its own type or objects of subclasses. In effect, a reference can point across and down the inheritance hierarchy, but never up. If a reference does point up the hierarchy, you will get a ClassCastException error. Recall that the reason this occurs is that the subclass reference could have extra methods that any superclass object would have no code for. Whether that is the case or not is immaterial, could have is enough.

Keep in mind that assignment works from right to left; so, when reading code involving upcasting/downcasting, the direction in the hierarchy is from right to left as well. In addition, remember that the compiler is always looking at the reference type.

Now, let’s discuss, with the aid of code examples, both upcasting and downcasting. Let’s start with upcasting.

Upcasting

With upcasting, you...