It is no secret that the Java language is highly derived from the C and C++ languages. Because C++ is currently one of the more popular game programming languages, it is important to understand the aspects of C++ that Java inherits. Of possibly even more importance are the aspects of C++ that Java doesn't support. Because Java is an entirely new language, it was possible for the language architects at Sun to pick and choose which features from C++ to implement in Java and how to implement them.

The focus of this appendix is to point out the differences between Java and C++. If you are a C++ programmer, you can appreciate the differences between Java and C++. Even if you don't have any C++ experience, you can gain some insight into the Java language by understanding the C++ discrepancies that it clears up in its implementation. If you have a C/C++ game you are thinking of porting to Java, this appendix will help you sort out the major areas to target in your porting efforts.

The Preprocessor

All C/C++ compilers implement a stage of compilation known as the preprocessor. Part of the responsibility of the C++ preprocessor is to perform an intelligent search and replace on identifiers that have been declared using the #define or #typedef directives. Although most advocators of C++ discourage this usage of the preprocessor, which was inherited from C, it is still widely used by most C++ programmers. Most of the processor definitions in C++ are stored in header files, which complement the actual source code files.

The problem with the preprocessor approach is that it provides an easy way for programmers to inadvertently add unnecessary complexity to a program. Many programmers using the #define and #typedef directives end up inventing their own sublanguage within the confines of a particular project. This results in other programmers having to go through the header files and sort out all of the #define and #typedef information to understand a program, which makes code maintenance and reuse almost impossible. An additional problem with the preprocessor approach is that it is very weak when it comes to type checking and validation.

Java does not have a preprocessor. It provides similar functionality (#define, #typedef, and so on) 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 the #typedef directive. The end result is that Java source code is much more consistent and easier to read than C++ source code. Additionally, Java programs don't use header files; the Java compiler builds class definitions directly from the source code files, which contain both class definitions and method implementations.

Pointers

Most developers agree that the misuse of pointers causes the majority of bugs in C/C++ programming. Put simply, when you have pointers, you have the ability to trash memory. C++ programmers regularly use complex pointer arithmetic to create and maintain dynamic data structures. In return, C++ programmers spend a lot of time hunting down complex bugs caused by their complex pointer arithmetic.

The Java language does not support pointers. Java provides similar functionality by making heavy use of references. Java passes all arrays and objects by reference. This approach prevents common errors due to pointer mismanagement. It also makes programming easier in a lot of ways because the correct usage of pointers is easily misunderstood by all but the most seasoned programmers.

You might be thinking that the lack of pointers in Java will keep you from being able to implement many data structures such as dynamic arrays. The reality is that any pointer task can be carried out just as easily, and more reliably, with objects and arrays of objects. You then benefit from the security provided by the Java runtime system; it performs boundary checking on all array indexing operations.

Structures and Unions

C++ has three types of complex data types: classes, structures, and unions. Java only implements one of these data types: classes. Java forces programmers to use classes when the functionality of structures and unions is desired. Although this sounds like more work for the programmer, it actually ends up being more consistent, because classes can imitate structures and unions with ease. The Java designers really wanted to keep the language simple, so it only made sense to eliminate aspects of the language that overlapped.

Functions

In C, code is organized into functions, which are defaulted as global subroutines accessible to a program. C++ added classes and, in doing so, provided class methods, which are functions that are connected to classes. C++ class methods are very similar to Java class methods. However, because C++ still supports C, nothing is discouraging C++ programmers from using functions. This results in a mixture of function and method use that makes for confusing programs.

Java has no functions. Being a more pure object-oriented language than C++, Java forces programmers to bundle all routines into class methods. No limitation is imposed by forcing programmers to use methods instead of functions. As a matter of fact, implementing routines as methods encourages programmers to better organize code. Keep in mind that, strictly speaking, nothing is wrong with the procedural approach of using functions; it just doesn't mix well with the object-oriented paradigm that defines the core of Java.

Multiple Inheritance

Multiple inheritance is a feature of C++ that enables you to derive a class from multiple parent classes. Although multiple inheritance is indeed powerful, it is complicated to use correctly and causes lots of problems otherwise. It is also very complicated to implement from the compiler perspective.

Java takes the high road and provides no direct support for multiple inheritance. You can implement functionality similar to multiple inheritance by using interfaces in Java. Java interfaces provide object method descriptions, but contain no implementations.

Strings

C and C++ have no built-in support for text strings. The standard technique adopted among C and C++ programmers is that of using null-terminated arrays of characters to represent strings.

In Java, strings are implemented as first class objects (String and StringBuffer), meaning that they are at the core of the Java language. Java's implementation of strings as objects provides several advantages:

• The manner in which you create strings and access the elements of strings is consistent across all strings on all systems.
• Because the Java string classes are defined as part of the Java language, and not part of some extraneous extension, Java strings function predictably every time.
• The Java string classes perform extensive runtime checking, which helps eliminate troublesome runtime errors.