When you start learning Java, it can sometimes seem as if learning one aspect necessitates knowledge of several others. This article addresses that common frustration by providing a short overview of several key features of Java. It is excerpted from chapter two of Java2: The Complete Reference, 5th edition, written by Herbert Schildt (McGraw-Hill, 2004; ISBN: 0072224207).
An Overview of Java - Lexical Issues (Page 9 of 9 )
Now that you have seen several short Java programs, it is time to more formally describe the atomic elements of Java. Java programs are a collection of whitespace, identifiers, comments, literals, operators, separators, and keywords. The operators are described in the next chapter. The others are described next.
Java is a free-form language. This means that you do not need to follow any special indentation rules. For example, the Example program could have been written all on one line or in any other strange way you felt like typing it, as long as there was at least one whitespace character between each token that was not already delineated by an operator or separator. In Java, whitespace is a space, tab, or newline.
Identifiers are used for class names, method names, and variable names. An identifier may be any descriptive sequence of uppercase and lowercase letters, numbers, or the underscore and dollar-sign characters. They must not begin with a number, lest they be confused with a numeric literal. Again, Java is case-sensitive, so VALUEis a different identifier than Value. Some examples of valid identifiers are:
AvgTemp count a4 $test this_is_ok
Invalid variable names include:
2count high-temp Not/ok
A constant value in Java is created by using a literal representation of it. For example, here are some literals:
100 98.6 ‘X’ “This is a test”
Left to right, the first literal specifies an integer, the next is a floating-point value, the third is a character constant, and the last is a string. A literal can be used anywhere a value of its type is allowed.
As mentioned, there are three types of comments defined by Java. You have already seen two: single-line and multiline. The third type is called a documentation comment. This type of comment is used to produce an HTML file that documents your program. The documentation comment begins with a /**and ends with a */. Documentation comments are explained in Appendix A.
In Java, there are a few characters that are used as separators. The most commonly used separator in Java is the semicolon. As you have seen, it is used to terminate statements. The separators are shown in the following table:
Used to contain lists of parameters in method
definition and invocation. Also used for defining
precedence in expressions, containing expressions
in control statements, and surrounding cast types.
Used to contain the values of automatically
initialized arrays. Also used to define a block
of code, for classes, methods, and local scopes.
Used to declare array types. Also used when
dereferencing array values.
Separates consecutive identifiers in a variable
declaration. Also used to chain statements together
Used to separate package names from subpackages
and classes. Also used to separate a variable or
method from a reference variable.
The Java Keywords
There are 49 reserved keywords currently defined in the Java language (see Table 2-1). These keywords, combined with the syntax of the operators and separators, form the definition of the Java language. These keywords cannot be used as names for a variable, class, or method.
abstract continue goto package synchronized assert default if private this boolean do implements protected throw break double import public throws byte else instanceof return transient case extends int short try catch final interface static void char finally long strictfp volatile class float native super while const for new switch
The keywords const and goto are reserved but not used. In the early days of Java, several other keywords were reserved for possible future use. However, the current specification for Java only defines the keywords shown in Table 2-1. The assertkeyword was added by Java 2, version 1.4
In addition to the keywords, Java reserves the following: true, false, and null. These are values defined by Java. You may not use these words for the names of variables, classes, and so on.
The Java Class Libraries
The sample programs shown in this chapter make use of two of Java’s built-in methods: println( )and print( ). As mentioned, these methods are members of the System class, which is a class predefined by Java that is automatically included in your programs. In the larger view, the Java environment relies on several built-in class libraries that contain many built-in methods that provide support for such things as I/O, string handling, networking, and graphics. The standard classes also provide support for windowed output. Thus, Java as a totality is a combination of the Java language itself, plus its standard classes. As you will see, the class libraries provide much of the functionality that comes with Java. Indeed, part of becoming a Java programmer is learning to use the standard Java classes. Throughout Part I of this book, various elements of the standard library classes and methods are described as needed. In Part II, the class libraries are described in detail.
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