2.13. Differences Between C and Java

If you are a C or C++ programmer, you should have found much of the syntax of Javaparticularly at the level of operators and statementsto be familiar. Because Java and C are so similar in some ways, it is important for C and C++ programmers to understand where the similarities end. C and Java differ in important ways, as summarized in the following list:

No preprocessor

Java does not include a preprocessor and does not define any analogs of the #define, #include, and #ifdef directives. Constant definitions are replaced with static final fields in Java. (See the java.lang.Math.PI field for an example.) Macro definitions are not available in Java, but advanced compiler technology and inlining has made them less useful. Java does not require an #include directive because Java has no header files. Java class files contain both the class API and the class implementation, and the compiler reads API information from class files as necessary. Java lacks any form of conditional compilation, but its cross-platform portability means that this feature is rarely needed.

No global variables

Java defines a very clean namespace. Packages contain classes, classes contain fields and methods, and methods contain local variables. But Java has no global variables, and thus there is no possibility of namespace collisions among those variables.

Well-defined primitive type sizes

All the primitive types in Java have well-defined sizes. In C, the size of short, int, and long types is platform-dependent, which hampers portability.

No pointers

Java classes and arrays are reference types, and references to objects and arrays are akin to pointers in C. Unlike C pointers, however, references in Java are entirely opaque. There is no way to convert a reference to a primitive type, and a reference cannot be incremented or decremented. There is no address-of operator like &, dereference operator like * or ->, or sizeof operator. Pointers are a notorious source of bugs. Eliminating them simplifies the language and makes Java programs more robust and secure.

Garbage collection

The Java Virtual Machine performs garbage collection so that Java programmers do not have to explicitly manage the memory used by all objects and arrays. This feature eliminates another entire category of common bugs and all but eliminates memory leaks from Java programs.

No goto statement

Java doesn't support a goto statement. Use of goto except in certain well-defined circumstances is regarded as poor programming practice. Java adds exception handling and labeled break and continue statements to the flow-control statements offered by C. These are a good substitute for goto.

Variable declarations anywhere

C requires local variable declarations to be made at the beginning of a method or block, while Java allows them anywhere in a method or block. Many programmers prefer to keep all their variable declarations grouped together at the top of a method, however.

Forward references

The Java compiler is smarter than the C compiler in that it allows methods to be invoked before they are defined. This eliminates the need to declare functions in a header file before defining them in a program file, as is done in C.

Method overloading

Java programs can define multiple methods with the same name, as long as the methods have different parameter lists.

No struct and union types

Java doesn't support C struct and union types. A Java class can be thought of as an enhanced struct, however.

No bitfields

Java doesn't support the (infrequently used) ability of C to specify the number of individual bits occupied by fields of a struct.

No typedef

Java doesn't support the typedef keyword used in C to define aliases for type names. Java's lack of pointers makes its type-naming scheme simpler and more consistent than C's, however, so many of the common uses of typedef are not really necessary in Java.

No method pointers

C allows you to store the address of a function in a variable and pass this function pointer to other functions. You cannot do this with Java methods, but you can often achieve similar results by passing an object that implements a particular interface. Also, a Java method can be represented and invoked through a java.lang.reflect.Method object.