Anatomy of a Java Program
The application program in Listing
1.1 has the following components:
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Comments
-
Reserved words
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Modifiers
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Statements
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Blocks
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Classes
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Methods
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The main method
To build a program, you need to
understand these basic elements. They are explained in the sections that
follow.
1.1. Comments
The first line in Welcome.java in Listing 1.1 is a
comment that documents what
the program is and how the program is constructed. Comments help programmers to
communicate and understand the program. Comments are not programming statements
and thus are ignored by the compiler. In Java, comments are preceded by two
slashes (//) on a line, called a line
comment, or enclosed between /*
and */
on one or several lines, called a paragraph
comment. When the compiler sees //,
it ignores all text after //
on the same line. When it sees /*,
it scans for the next */
and ignores any text between /*
and */.
// This application program prints Welcome to Java! /* This application program prints Welcome to Java! */ /* This application program prints Welcome to Java! */
Note
In addition to the two comment styles, //
and /*, Java supports comments of a special type, referred to as
javadoc comments. javadoc comments begin with
/**
and end with */. They are used for documenting classes, data, and
methods. They can be extracted into an HTML file using JDK's javadoc command. For more information, see java.sun.com/j2se/javadoc.
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1.2. Reserved Words
Reserved words, or keywords, are words
that have a specific meaning to the compiler and cannot be used for other
purposes in the program. For example, when the compiler sees the word class, it understands that the word after class is the name for the class. Other reserved words in Listing 1.1 are
public,
static,
and void. Their use will be introduced later in the book.
Tip
Because Java is case-sensitive, public
is a reserved word, but Public is not. Nonetheless, for clarity and readability, it would
be best to avoid using reserved words in other forms. (See Appendix A, "Java
Keywords.")
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1.3. Modifiers
Java uses certain reserved words called modifiers that specify the
properties of the data, methods, and classes and how they can be used. Examples
of modifiers are public
and static.
Other modifiers are private,
final,
abstract,
and protected.
A public datum, method, or class can be accessed by other classes. A
private datum or method cannot be accessed by other classes.
1.4. Statements
A statement represents an action
or a sequence of actions. The statement System.out.println("Welcome
to Java!"); in the program in Listing 1.1 is a statement to display the greeting "Welcome to
Java!" Every statement in Java ends with a semicolon (;).
1.5. Blocks
The braces in the program form a block that groups the
components of the program. In Java, each block begins with an opening brace
({)
and ends with a closing brace (}).
Every class has a class block that groups the data and methods of the class. Every method
has a method block
that groups the statements in the method. Blocks can be nested, meaning that one
block can be placed within another, as shown in the following code.
1.6. Classes
The class
is the essential Java construct. To program in Java, you must understand classes
and be able to write and use them. The mystery of classes will be unveiled
throughout the book. For now, though, it is enough to know that a program is
defined by using one or more classes.
1.7. Methods
What is System.out.println?
System.out
is known as the standard output object. println
is a method in the
object, which consists of a collection of statements that perform a sequence of
operations to display a message to the standard output device. If you run the
program from the command window, the output from the System.out.println is displayed in the command window. The method can be
used even without fully understanding the details of how it works. It is used by
invoking a statement with a string argument. The string argument is enclosed in
parentheses. In this case, the argument is "Welcome
to Java!" You can call the same println
method with a different argument to print a different message.
1.8. The main Method
Every Java application must have a user-declared main method that defines where the program execution begins.
The JVM executes the application by invoking the main method. The main
method looks like this:
public static void main(String[] args) { // Statements; }
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