By: Tamil Selvan in Java Tutorials on 2007-09-15
There are a number of C++ features that Java does not support. In some cases, a specific C++ feature simply didn't relate to the Java environment. In other cases, the designers of Java eliminated some of the duplication of features that exists in C++. In still other instances, a feature of C++ is not supported by Java because it was deemed too dangerous for Internet applets.
Perhaps the single biggest difference between Java and C++ is that Java does not support pointers. As a C++ programmer you know that the pointer is one of C++'s most powerful and important language features. It is also one of its most dangerous when used improperly. Pointers don't exist in Java for two reasons:
- Pointers are inherently insecure. For example, using a C++-style pointer, it is possible
to gain access to memory addresses outside a program's code and data. A malicious
program could make use of this fact to damage the system, perform unauthorized
accesses (such as obtaining passwords), or otherwise violate security restrictions.
- Even if pointers could be restricted to the confines of the Java run-time system (which
is theoretically possible, since Java programs are interpreted), the designers of Java
believed that they were inherently troublesome.
Note Since pointers don't exist in Java, neither does the -> operator.
Here are a few more of the most important "omissions":
- Java does not include structures or unions. These were felt to be redundant since the
class encompasses them.
- Java does not support operator overloading. Operator overloading is sometimes a
source of ambiguity in a C++ program, and the Java design team felt that it causes
more trouble than benefit.
- Java does not include a preprocessor nor does it support the preprocessor directives.
The preprocessor plays a less important role in C++ than it does in C. The designers
of Java felt that it was time to eliminate it entirely.
- Java does not perform any automatic type conversions that result in a loss of
precision. For example, a conversion from long integer to integer must be explicitly
- All the code in a Java program is encapsulated within one or more classes. Therefore,
Java does not have what you normally think of as global variables or global functions.
- Java does not allow default arguments. In C++, you may specify a value that a
parameter will have when there is no argument corresponding to that parameter when
the function is invoked. This is not allowed in Java.
- Java does not support the inheritance of multiple superclasses by a subclass.
- Although Java supports constructors, it does not have destructors. It does, however,
add the finalize() function.
- Java does not support typedef.
- It is not possible to declare unsigned integers in Java.
- Java does not allow the goto.
- Java does not have the delete operator.
- The << and >> in Java are not overloaded for I/O operations.
- In Java, objects are passed by reference only. In C++, objects may be passed by
value or by reference.
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