Classes in Java provide a way to further modularize and organize your
code. Like C++ classes, Java classes are designed to encapsulate
real world objects. They combine attributes of an object (data members
or variables) with functions and methods of an objects. In this fashion,
unlike old C structures, C++ and Java classes combine what an object is,
has and does into a single entity. Classes in both C++ and Java practice
"inheritance". In C++, you have base classes and derived classes and you
declare a class to inherit from another with "class CHILD : public
PARENT". In Java, you use the keyword "extends" to do the same thing.
Example:
"public class CHILD extends PARENT". One aspect in which Java is
different than C++ is that unlike C++, Java will allow you to nest
classes inside of each other.
Example 1 - Declaring a Class:
import
javax.swing.*;
public
class
CreatingAClass
{
public
void main(String[] args)
{
System.exit(0);
}//close main() function
}//close
CreatingAClass class |
Example 2 - Class Inheritance:
import
javax.swing.*;
public
class
ParentSuperClass
{
public
void main(String[] args)
{
System.exit(0);
}//close main() function
public
void DoParentStuff() { }
}//close
ParentSuperClass class
public
class
ChildSubClass extends
ParentSuperClass
{
public
void main(String[] args)
{
System.exit(0);
}//close main() function
public void
DoChildStuff() { }
}//close
ChildSubClass class |
Note: To instantiate an object of the
classes defined above you would use the keyword "new". They don't call it a
"pointer" in Java, but you can tell that's exactly what it is under the hood
when you see the code:
Java (no pointers):
ChildSubClass CHILD = new ChildSubClass();
C++ (pointers) ChildSubClass
* CHILD = new ChildSubClass();
In the C++ example, you are creating a new object and allocating memory for
it on the heap (free store) and using a pointer to access the memory address
that references the object. In the Java example, they simply define this
method as the way to instantiate an object.
As objects created on the stack in C++, Java objects access their methods
and functions via the dot operator. Example:
ParentSuperClass PARENT = new
ParentSuperClass();
PARENT.DoParentStuff();
ChildSubClass CHILD = new ChildSubClass();
CHILD.DoChildStuff();
As in nature, in C++ and Java a child object inherits the data members,
functions and methods of its parent class. So you may:
CHILD.DoParentStuff();
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