Java in a Nutshell, 5th Edition [Electronic resources] نسخه متنی

2.1. Java Programs from the Top Down

Before we begin our bottom-up exploration of Java syntax,
let's take a moment for a top-down overview of a
Java
program. Java programs consist of one or more files, or

compilation units , of Java source code. Near the end of the
chapter, we describe the structure of a Java file and explain how to
compile and run a Java program. Each compilation unit begins with an
optional package declaration followed by zero or
more import declarations. These declarations
specify the namespace within which the compilation unit will define
names, and the namespaces from which the compilation unit imports
names. We'll see package and
import again in Section 2.10 later in this chapter.

The optional package and import
declarations are followed by zero or more
reference type definitions.
These are typically class or
interface definitions, but in Java 5.0 and later,
they can also be enum definitions or annotation
definitions. The general features of reference types are covered
later in this chapter, and detailed coverage of the various kinds of
reference types is in Chapters Chapter 3 and
Chapter 4.

Type definitions include members such as fields, methods, and
constructors. Methods are
the most important type member. Methods are blocks of Java code
comprised of

statements .
Most statements include

expressions ,
which are built using

operators
and values known as

primitive data
types . Finally, the
keywords used to
write statements, the punctuation characters that
represent operators, and the literals values that appear in a program
are all

tokens ,
which are described next. As the name of this section implies, this
chapter moves from describing the smallest units, tokens, to
progressively larger units. Since the concepts build upon one
another, we recommend reading this chapter sequentially.