This chapter gives you a quick introduction to the concepts of object-oriented programming (OOP) and covers some enhancements that have been introduced since Java 8. We will also try to cover a few good OOD practices wherever applicable and demonstrate them using specific code examples.

One can spend many hours reading articles and practical advice on OOD in books and on the internet. Doing this can be beneficial for some people. But, in our experience, the fastest way to get hold of OOD is to try its principles early in your own code. That is exactly the goal of this chapter—to give you a chance to see and use the OOD principles so that the formal definition makes sense immediately.

One of the main criteria of well-written code is the clarity of the intent. A well-motivated and clear design helps achieve this. The code is run by a computer, but it is maintained...

In this recipe, you will learn the first two OOP concepts—object/class and encapsulation. These concepts are at the foundation of OOD.

The term object usually refers to an entity that couples data and procedures that can be applied to this data. Neither data nor procedures are required, but one of them is—and, typically, both are—always present. The data is called object fields (or properties), while procedures are called methods. Field values describe the object's state. Methods describe the object's behavior. Every object has a type, which is defined by its class—the template used for the object's creation. An object is also...

In this recipe, you will learn about three types of inner classes:

• Inner class: This is a class defined inside another (enclosing) class. Its accessibility from outside the enclosing class is regulated by the public, protected, and private access modifiers. An inner class can access the private members of the enclosing class, and the enclosing class can access the private members of its inner class, but a private inner class or private members of a non-private inner class cannot be accessed from outside the enclosing class.
• Method-local inner class: This is a class defined inside a method. Its accessibility is restricted to within the method.
• Anonymous inner class: This is a class without a declared name that's defined during object instantiation based on the interface only or the extended class.

In this recipe, you will learn more about two important OOP concepts, inheritance and polymorphism, which have been mentioned already and used in the examples of the previous recipes. Together with aggregation, these concepts make the design more extensible.

The extended class is called the base class, superclass, or parent class. The new extension of the class is called a subclass or child class.

To demonstrate the power of inheritance and polymorphism, let&apos...

In this recipe, you will learn the last of the OOP concepts, called interface, and further practice the usage of aggregation and polymorphism as well as inner classes and inheritance.

An interface defines the signatures of the methods one can expect to see in the class that implements the interface. It is the public face of the functionality that's accessible to a client and is thus often called an Application Program Interface (API). It supports polymorphism and aggregation, and facilitates a more flexible and extensible design.

An interface is implicitly abstract, which means it cannot be instantiated. No object can be created based on an interface only, without implementing it...

In this recipe, you will learn about two new features that were first introduced in Java 8—the default and static methods in an interface.

A default method in an interface allows us to add a new method signature without changing the classes that have implemented this interface before a new method signature was added. The method is called default because it provides functionality in case this method is not implemented by the class. If, however, the class implements it, the interface's default implementation is ignored and overridden by the class implementation.

A static method in an interface can provide functionality in the same way a static method...

In this recipe, you will learn about a new feature that was introduced in Java 9, the private interface method, which is of two types—static and non-static.

A private interface method must have an implementation (a body with a code). A private interface method not used by other methods of the same interface does not make sense. The purpose of a private method is to contain functionality that is common between two or more methods with a body in the same interface or to isolate a section of code in a separate method for better structure and readability. A private interface method cannot be overridden—not by a method of any other interface, nor by a method in...

In this recipe, you will learn how to use the java.util.Optional class for representing optional values instead of using null references. It was introduced in Java 8 and further enhanced in Java 9—where three more methods were added—or(), ifPresentOrElse(), and stream(). We will demonstrate all of them.

The Optional class is a wrapper around a value, which can be null or a value of any type. It was intended to help to avoid the dreaded NullPointerException. But, so far, the introduction of Optional helped to accomplish it only to a degree and mostly in the area of streams and functional programming.

The vision that motivated the creation of the Optional...