Tuesday, July 17, 2007

Java vs. C++ - Part Two

These are some more differences between Java and C++.

Class attributes and behaviors Attributes are "instance variables".
Behaviors are "methods"
Attributes are "data members".
Behaviors are "member functions"
Extending Class Object Every class is a subclass of Object and so inherits the 11 methods defined by class object, you never create a class from scratch. Even if a class doesn't explicitly use extends in its definition, it implicitly extends Object You can create classes from scratch. A class will inherit attributes and behaviors of a base class ONLY when its declaration explicitly implies that it should.
Packages Every class belongs to a package even if not explicitly specified it will be the default package ( the current directory ) Classes do not have packages. Their containing header files are simply included in source files in which they are to be used
Constructor name Methods other than the constructor of a class ARE ALLOWED to have the same name as the class AND to specify return types but these are not constructors and won't be called when an object is created. It is NOT ALLOWED for any member function other than the constructor to have the same name as the class name AND it is INVALID to specify a return type for any constructor.
Initializing attributes Instance variables CAN be initialized where they are declared IN THE CLASS BODY, by the class's constructor, or they can be assigned values by "set" methods. Instance variables that are not explicitly initialized by the programmer are initialized by the compiler (primitive numeric variables are set to 0, booleans are set to false and references are set to null). Data members can be initialized ONLY by the class's constructor, or they can be assigned values by "set" methods. Instance variables that are not explicitly initialized by the programmer are NOT automatically initialized (they will have whatever values that happened to be stored in their allocated memory space).
Accessing hidden attributes An instance variable hidden by a local variable (having the same name) can be accessed in a method by:
(this is a reference to the current object)
Such a hidden data member can be accessed in the member function by:
(this here is a pointer to the current object)
Access modifiers Each instance variable or method declaration in a Class definition must be preceded by an access modifier. Access modifiers are followed by a colon and apply to all following member declarations until overridden by another access modifier. If they were omitted they are implicitly applied by the compiler:
for classes: members are private by default.
for structs: members are public by default.
Package access Members that have no access modifier preceding their declaration are accessible to all the classes included in the same package. Members are either public or private and the only way that another class could access a non-public member of a different class is by inheritance of protected members or by being declared as a friend class of that class.
Memory leaks Are less likely to occur because Java performs automatic garbage collection to help return memory back to the system. When an object is no longer used in a program ( there are no references to the object e.g. if null was assigned to the objects reference ) it is marked for garbage collection. Before the garbage collector returns memory resources to the system it calls the finalize method of the object to be destroyed. Are common because memory is not automatically reclaimed to the system it is the programmer's responsibility to do that himself by freeing the memory in the Class destructor when its task is over.
Multiple inheritance Is not supported but interfaces are supported that allow a class to acquire multiple functionalities from any number of interfaces Is supported.
Over-ridden superclass methods Can be accessed from the subclass by:
Such an over-ridden base-class member function can be accessed by the derived class by:
Calling superclass constructor To explicitly call the superclass Parent constructor from the subclass Child constructor:
public Child( int a, int b )
super( a );
x = b;
The calling statement must be the first statement in the subclass constructor
To do the same thing here it goes like this:
Child( int a, int b )
: Parent( a )
x = b;
here we use the member initializer ( : ) to call the Parent constructor before the body of the constructor is executed
Polymorphism and dynamic binding Applies automatically.
When a reference to a subclass object is assigned to a superclass reference which is then used to invoke a superclass method that is overridden in the subclass definition the java compiler only checks that the class of the reference really has that method and the java interpreter calls the method of the actual data type of the object at execution time.
Does NOT apply automatically.
When a pointer to a derived-class object is assigned to a base-class pointer which is then used to invoke a base-class member function that is overridden in the derived class definition the compiler treats the object exactly as an object of the base-class and the base-class member function is called for the object. This is overcome by declaring the function to be a vertual one and in this case the compiler generates code that would at run time access the appropriate version of the function for the actual object's data type.
final methods and final classes

A method that is declared final cannot be overridden in a subclass.

A class that is declared final cannot be a superclass( cannot have subclasses).

Does not have an equivalent in C++
const functions Java has no equivalent for C++'s const functions A member function can be declared as const, indicating that calling that member function does not change the object.
Abstract classes Are declared by using abstract and have one or more abstract mehtods
public abstract ClassName {
private int instanceVariable;
public int someMethod1()....
public void someMethod2()...
public abstract int method();
//abstract method
Must have one or more pure virtual functions
class ClassName {
virtual int someFunction1()...
virtual int someFunction2()...
virtual int function() = 0;
// pure virtual funtion
int dataMember;
GUI support Normally has packages that support frames, windows, Graphical User Interface components. Does not normally support them. Appropriate third party libraries must be obtained to offer their support.

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