class TwoDPoint { 
   double x; 
   double y; 
} 

However, this is not more than a data structure. For example, we want this object to response the message: How far is it from the origin ? Thus, we can extend this class by adding: 

class TwoDPoint { 
   double x; 
   double y; 

   double length() { 
      return (Math.sqrt(this.x*this.x - this.y*this.y)); 
   } 

} 

Whenever we call the following; 

   TwoDPoint p = new TwoDPoint(); 
   p.x = 4.0; 
   p.y = 5.0; 
   System.out.println("Distance from the origin: "+p.length()); 

Please note that we used a special word: "this" In order to access the fields of an object, we use "this" keyword. It can be assumed as a copy of the calling object, which is "p" in our sample code segment. 

We created an object by using an empty constructor; which is not explicitly defined in the class definition. Instead, we can define a new constructor for TwoDPoint class: 

  TwoDPoint(double xv, double yv) { 
     this.x = xvalue; 
     this.y = yvalue; 
  } 

  So, we can create a new object (which is called instance of a class, officially) as: 

  TwoDPoint p2 = new TwoDPoint(3, 4); 
  System.out.println("Distance from the origin: "+p2.length()); 

  This code segment produces the same result as the first one.