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Java in a Nutshell

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How Java Differs from C

2.4 No Preprocessor

Java does not include any kind of preprocessor like the C cpp preprocessor. It may seem hard to imagine programming without #define, #include, and #ifdef, but in fact, Java really does not require these constructs.

Defining Constants

Any variable declared final in Java is a constant--its value must be specified with an initializer when it is declared, and that value may never be changed. The Java equivalent of a C #define'ed constant is a static final variable declared within a class definition. If the compiler can compute the value of such a static final variable at compile-time, it uses the computed value to pre-compute other compile-time constants that refer to the value. The variable java.lang.Math.PI is an example of such a constant. It is declared like this:

public final class Math {
    public static final double PI = 3.14159.....;

Note two things about this example. First, the C convention of using CAPITAL letters for constants is also a Java convention. Second, note the advantage Java constants have over C preprocessor constants: Java constants have globally unique hierarchic names, while constants defined with the C preprocessor always run the risk of a name collision. Also, Java constants are strongly typed and allow better type-checking by the compiler than C preprocessor constants.

Defining Macros

The C preprocessor allows you to define macros--a construct that looks like a function invocation but that is actually replaced directly with C code, saving the overhead of a function call. Java has no equivalent to this sort of macro, but compiler technology has advanced to a point where macros are rarely necessary any more. A good Java compiler should automatically be able to "inline" short Java methods where appropriate.

Including Files

Java does not have a #include directive, but it does not need one. Java defines a mapping of fully qualified class names (like java.lang.Math) to a directory and file structure (like java/lang/Math.class). This means that when the Java compiler needs to read in a specified class file, it knows exactly where to find it and does not need a special directive to tell it where to look.

Furthermore, Java does not make the distinction between declaring a variable or procedure and defining it that C does. This means that there is no need for C-style header files or function prototypes--a single Java object file serves as the interface definition and implementation for a class.

Java does have an import statement, which is superficially similar to the C preprocessor #include directive. What this statement does, however, is tell the compiler that the current file is using the specified classes, or classes from the specified package, and allows us to refer to those classes with abbreviated names. For example, since the compiler implicitly imports all the classes of the java.lang package, we can refer to the constant java.lang.Math.PI by the shorter name Math.PI.

Conditional Compilation

Java does not have any form of the C #ifdef or #if directives to perform conditional compilation. In theory, conditional compilation is not necessary in Java since it is a platform-independent language, and thus there are no platform dependencies that require the technique. In practice, however, conditional compilation is still often useful in Java--to provide slightly different user interfaces on different platforms, for example, or to support optional inclusion of debugging code in programs.

While Java does not define explicit constructs for conditional compilation, a good Java compiler (such as Sun's javac) performs conditional compilation implicitly--that is, it does not compile code if it can prove that the code will never be executed. Generally, this means that code within an if statement testing an expression that is always false is not included. Thus, placing code within an if (false) block is equivalent to surrounding it with #if 0 and #endif in C.

Conditional compilation also works with constants, which, as we saw above, are static final variables. A class might define the constant like this:

private static final boolean DEBUG = false;

With such a constant defined, any code within an if (DEBUG) block is not actually compiled into the class file. To activate debugging for the class, it is only necessary to change the value of the constant to true and recompile the class.

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