Understanding classes and instances in Java 9

In the previous chapter, we learned some of the basics of the object-oriented paradigm, including classes and objects. We started working on the backend for a Web Service related to 2D shapes. We ended up creating a UML diagram with the structure of many classes, including their hierarchy, fields, and methods. It is time to take advantage of JShell to start coding a basic class and work with its instances in JShell.

In Java 9, a class is always the type and blueprint. The object is the working instance of the class, and therefore, objects are also known as instances.

Note

Classes are first-class citizens in Java 9 and they will be the main building blocks of our object-oriented solutions.

One or more variables can hold a reference to an instance. For example, consider that we have the following three variables of the Rectangle type:

• rectangle1
• rectangle2
• rectangle10
• rectangle20

Let's consider that the rectangle1 variable holds a reference to an instance of the Rectangle class with its width set to 36 and its height set to 20. The rectangle10 variable holds a reference to the same instance referenced by rectangle1. Thus, we have two variables that hold a reference to the same Rectangle object.

The rectangle2 variable holds a reference to an instance of the Rectangle class with its width set to 22 and its height set to 41. The rectangle20 variable holds a reference to the same instance referenced by rectangle2. We have another two variables that hold a reference to the same Rectangle object.

The following picture illustrates the situation where many variables of the Rectangle type that hold a reference to a single instance. The variable names are at the left-hand side and the rectangles with their width and height values represent a specific instance of the Rectangle class.

We will work with many variables that hold a reference to a single instance in JShell later in this chapter.