As with any programming language, Java has a grammar that is applied to the language elements. The elements are building blocks used to compose language structures that allow the programmer to express intent. Elements themselves have different levels of complexity. Lower-level (simpler) elements enable building higher-level (more complex) ones. For a more detailed and systematic treatment of Java grammar and language elements, please see the Java Specification (https://docs.oracle.com/javase/specs).
In this book, we start with input elements that belong to one of the lowest levels. They are called input elements because they serve as input to the Java compiler.