Hi friend,


Java does not have a preprocessor. It provides similar functionality (#define, typedef, and so forth) to that provided by the C++ preprocessor, but with far more control. Constant data members are used in place of the #define directive, and class definitions are used in lieu of typedef. The end result is that Java source code is much more consistent and easier to read than C++ source code. Additionally, as you learned earlier, Java programs don't use header files; the Java compiler builds class declarations directly from the source code files, which contain both class declarations and method implementations.

The C++ header file for a ball class.

The C++ ball class.

#define COLOR_RED 1
#define COLOR_YELLOW 2
#define COLOR_BLUE 3
#define MATERIAL_RUBBER 1
#define MATERIAL_LEATHER 2

class ball {
float diameter;
int color;
int material;
};

To move this code to Java, the only change is to get rid of the preprocessor #define directives and the semicolon at the end of the class declaration. You get rid of the #define directives by declaring Java class members that are static and final. For data members in Java, the static keyword means there is only one copy for the entire class, and the final keyword means that they are constant, which is usually the motive for using #define in C/C++ code. The resulting Java version of this class. Keep in mind that the Java version is not stored in a header file, because Java doesn't support header files; in Java, definitions and declarations are combined in one place, the .java source file.

The Java ball class.

class ball {
// Constants
static final int COLOR_RED = 1;
static final int COLOR_YELLOW = 2;
static final int COLOR_BLUE = 3;
static final int MATERIAL_RUBBER = 1;
static final int MATERIAL_LEATHER = 2;

// Variables
float diameter;
int color;
int material;
}

The Java version of ball pulls all the constants inside the class definition as static final integers. Within this class, you would then refer to them just as you would the previous C++ versions. However, outside this class they are inaccessible, because they have been left at their default access type. To make them visible by other classes, you simply declare their access type as public. The statement about default access isn't entirely true;

The Java ball class with public constants.

class ball {
// Constants
public static final int COLOR_RED = 1;
public static final int COLOR_YELLOW = 2;
public static final int COLOR_BLUE = 3;
public static final int MATERIAL_RUBBER = 1;
public static final int MATERIAL_LEATHER = 2;

// Variables
float diameter;
int color;
int material;
}


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Thanks.

The Java Native Interface, JNI, is just that; an interface. It's an API that allows Java code to interact with code written in another language, typically C or C++. Because the JNI is an interface, JVM vendors are free to implement the virtual machine as they see fit. As long as the JVM follows the specification of the JNI, all native code written to the specification should work with that JVM.