Over Loading and overriding

OverLoading :
Method overloading means having two or more methods with the same name but different signatures in the same scope.
These two methods may exist in the same class or anoter one in base class and another in derived class.
   Ex:
  

class Person

{

    private String fName;

    private String lName;

    Person()

    {

        this.fName = "";

        this.lName = "";

    }

    Person(String fName)

    {

        this.fName = fName;

        this.lName = "";

    }

    Person(String fName, String lName)

    {

        this.fName = fName;

        this.lName = lName;

    }

}

In the above example calling the over loaded method as follows
    Person(); /* as a constructor and call method without parameter*/
    Person(userFirstName); /* as a constructor and call method with one parameter(like User's first Name)*/
    Person(userFirstName,userLastName); /*as a constructor and call method with one parameter(like User's first Name)*/
When you have required same reason that take different signatures, but conceptually do the same thing.
   
Overriding:
Method overriding means having a different implementation of the same method in the inherited class.
These two methods would have the same signature, but different implementation.
One of these would exist in the base class and another in the derived class. These cannot exist in the same class.


In a derived class, if you include a method definition that has the same name and exactly the same number and types of
parameters as a method already defined in the base class, this new definition replaces the old definition of the method.

Explanation

A subclass inherits methods from a superclass.
Sometimes, it is necessary for the subclass to modify the methods defined in the superclass. This is referred to as method overriding.
The following example demonstrates method overriding.

class Circle
{

    //declaring the instance variable

    protected double radius;

    public Circle(double radius)
    {
          this.radius = radius;
    }

    // other method definitions here

    public double getArea()
    {
          return Math.PI*radius*radius;

    }//this method returns the area of the circle

}// end of class circle

When the getArea method is invoked from an instance of the Circle class, the method returns the area of the circle.
The next step is to define a subclass to override the getArea() method in the Circle class. The derived class will be the Cylinder class. The getArea() method in the Circle class computes the area of a circle, while the getArea method in the Cylinder class computes the surface area of a cylinder.

The Cylinder class is defined below.

class Cylinder extends Circle
{

    //declaring the instance variable

    protected double length;

    public Cylinder(double radius, double length)
    {
         super(radius);
         this.length = length;
    }

    // other method definitions here

    public double getArea()
    {
         // method overriden here
         return 2*super.getArea()+2*Math.PI*radius*length;
    }//this method returns the cylinder surface area

}// end of class Cylinder

When the overriden method (getArea) is invoked for an object of the Cylinder class,
 the new definition of the method is called and not the old definition from the superclass(Circle).

instantiate the above two classes as shown below:
Circle myCircle;
myCircle = new Circle(1.20);
Cylinder myCylinder;
myCylinder = new Cylinder(1.20,2.50);