2.1 Jb0105: java oop: similarities and differences between java

In Java, you can initialize primitive data members in the class definition. You can also initialize them in the constructor. If you fail to initialize them, they will be initialized to zero (or equivalent) automatically.

Like C++, Java supports constructors that may be overloaded. As in C++, if you fail to provide a constructor, a default constructor will be provided for you. If you provide a constructor, the default constructor is not provided automatically.

All objects in Java are passed by reference, eliminating the need for the copy constructor used in C++.

(In reality, all parameters are passed by value in Java. However, passing a copy of a reference variable makes it possible for code in the receiving method to access the object referred to by the variable, and possibly to modify the contents of that object. However, code in the receiving method cannot cause the original reference variable to refer to a different object.)

There are no destructors in Java. Unused memory is returned to the operating system by way of a garbage collector , which runs in a different thread from the main program. This leads to a whole host of subtle and extremely important differences between Java and C++.

Like C++, Java allows you to overload functions (methods) . However, default arguments are not supported by Java.

Unlike C++, Java does not support templates. Thus, there are no generic functions or classes. (Note, generics similar to C++ templates were introduced into Java in version 5 subsequent to the original publication of this document.)

Unlike C++, several "data structure" classes are contained in the "standard" version of Java. (Note, the Standard Template Library was introduced into the C++ world subsequent to the original publication of this document.)

More specifically, several "data structure" classes are contained in the standard class library that is distributed with the Java Development Kit (JDK). For example, the standard version of Java provides the containers Vector and Hashtable that can be used to contain any object through recognition that any object is an object of type Object . However, to use these containers, you must perform the appropriate upcasting and downcasting, which may lead to efficiency problems. (Note, the upcasting and downcasting requirements were eliminated in conjunction with the introduction of "generics" into Java mentioned earlier.)

Multithreading is a standard feature of the Java language.

Although Java uses the same keywords as C++ for access control: private , public , and protected , the interpretation of these keywords is significantly different between Java and C++.

There is no virtual keyword in Java. All non-static methods use dynamic binding, so the virtual keyword isn't needed for the same purpose that it is used in C++.

Java provides the final keyword that can be used to specify that a method cannot be overridden and that it can be statically bound. (The compiler may elect to make it inline in this case.)

The detailed implementation of the exception handling system in Java is significantly different from that in C++.

Unlike C++, Java does not support operator overloading. However, the (+) and (+=) operators are automatically overloaded to concatenate strings, and to convert other types to string in the process.

As in C++, Java applications can call functions written in another language. This is commonly referred to as native methods . However, applets cannot call native methods.

Unlike C++, Java has built-in support for program documentation. Specially written comments can be automatically stripped out using a separate program named javadoc to produce program documentation.

Generally Java is more robust than C++ due to the following:

• Object handles (references) are automatically initialized to null.
• Handles are checked before accessing, and exceptions are thrown in the event of problems.
• You cannot access an array out of bounds.
• The potential for memory leaks is prevented (or at least greatly reduced) by automatic garbage collection.

Miscellaneous

This section contains a variety of miscellaneous information.

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