That First Gulp of Java
When we told a newbie friend that we had just co-authored a book on Java, he said, “You may be surprised to learn that I've heard of Java.” We weren't. In little more than a year Java has gone from an obscure back-room project at Sun to an all but universal topic of interest and speculation in the computing community—and we would be astonished if it hadn't.
If Java were just another powerful, expressive, object-oriented programming language, a few developers would have glanced at it, said, “that's nice.” and gone back to plowing their way through C++ code. If it merely offered a far more flexible, secure, and transparent way to enrich web page content with small programs, a few web surfers would have gotten about as excited as they did over “plug-ins”. If it only provided a more convenient way to distribute large applications across heterogeneous platforms, a few large companies would have begun investigating its potential.
Java combines all these features and many more, however, and it is this extraordinary combination that uniquely qualifies Java to capture the interest of an entire industry, and to demand significant investment by most of the big names in that industry: Borland, Intel, Microsoft, Novell, Oracle, SGI, and Symantec among many others. Even a fairly brief technical overview should suffice to explain the unprecedented enthusiasm that has greeted Java.
Like C++, Java capitalizes on the popularity of the C language, and preserves much of its compact, expressive character. Unlike C++, it makes no attempt to maintain backward compatibility and, for that reason and others, it has many advantages over its currently popular cousin.
While C++ provides syntax to achieve encapsulation, inheritance, polymorphism, and other features of object-oriented programming, it also supports traditional structured (and unstructured) programming. Java does not. Every Java program we create, even the first “hello world,” is object-oriented, preventing us from backsliding into old, familiar—and less productive—ways.
The need to support two utterly different types of programming makes C++ needlessly complicated. For example, a parameter can be of either a built-in type or a class type, and may be passed by value, by reference, or via a pointer: six possible combinations. In Java, a built-in parameter is always passed by value, a class-type parameter is always passed by reference: just two combinations, and no decision required. Much simpler. Furthermore, stronger typing eliminates many of the implicit type conversions that introduce unexpected ambiguity.
“Wait! No pointers?” Yup. No pointers. C needed them to support array processing and a primitive form of call by reference. Java supports true call by reference (as C++ does) and implements arrays as a built-in type. So, no pointers. No uninitialized pointers. No forgot-to-check-for-NULL pointers. No tears-at-midnight-trying-to-find-the-mis-aimed pointers.
Global variables also meet a cruel but well deserved fate. In Java, every datum is neatly encapsulated, accessible only by operations of the class of which it's a member. Thus another major source of programming mistakes disappears entirely. Automatic garbage collection gets rid of yet another: the “memory leaks” that result when we fail to release memory allocated dynamically.
Java also discards a mechanism that “seemed like a good idea at the time”: the preprocessor.
Early C compilers avoided multiple parsing passes by supporting forward declarations of functions and data. Programmers could ensure consistency among declarations, definitions, and uses by placing the declarations in separate header files, then directing the preprocessor to “#include” them in source files. C and C++ implementations have continued this tradition into an age in which systems are so large that we must pre-compile the header files themselves to get acceptable rebuilding times—and header-file maintenance becomes a major task in itself.
In Java, class definitions are not split between header and source files. Simple “import” statements tell the compiler which classes are needed by the current source file, and the compiler does the rest. Declaration order becomes unimportant. This scheme requires a bit more sophistication from the compiler, but one result of eliminating the preprocessor is that Java programs generally take less time to build than C++ programs. Another is that the developer's life becomes much easier.
The C/C++ preprocessor solves problems other than centralization of declarations, of course, but the Java compiler handles most of these other problems easily. And one other major problem disappears altogether:
The need to support multiple platforms obliges C and C++ developers to devote considerable effort to maintaining alternate blocks of code, and using conditional-compilation directives to ensure that only the correct blocks are compiled when they rebuild a particular system version.
Java makes all that tedium go away, because the language is independent of any particular machine architecture. Java programs are fully portable, not only the source code but the executable code as well, courtesy of the Java Virtual Machine (JVM).