The following lines declare a new minimal Rectangle class in Java. The code file for the sample is included in the java_9_oop_chapter_03_01 folder, in the example03_01.java file.

class Rectangle {
}

The class keyword, followed by the class name (Rectangle), composes the header of the class definition. In this case, we don't specify a parent class or superclass for the Rectangle class. A pair of curly braces ({}) encloses the class body after the class header. In the forthcoming chapters, we will declare classes that inherit from another class, and therefore, they will have a superclass. In this case, the class body is empty. The Rectangle class is the simplest possible class we can declare in Java 9.

The following lines represent an equivalent way of creating the Rectangle class. However, we don't need...