A Simple Example with a Class and 1 Object

As I am a retired COBOL programmer, I don’t know anything about Object Oriented programming so please forgive me if this example seems a bit elementary. Classes are templates for real-life objects. In the example below, a Rectangle class is created. Each object in this class (i.e. each rectangle) has a width and a height. These are called instance variables. An object called my_rectangle is created in the Rectangle class and its width and height are supplied. Finally, a variable called area is created, calculated and displayed:

 

Java > cat Rectangle.java

class Rectangle

  {

  double width;

  double height;

  }

class RectangleExample

  {

  public static void main(String args[])

    {

    Rectangle my_rectangle = new Rectangle();

    double area;

    my_rectangle.width = 3;

    my_rectangle.height = 4;

    area = my_rectangle.width * my_rectangle.height;

    System.out.println("Area = " + area);

    }

  }

Java > javac Rectangle.java

Java > java RectangleExample

Area = 12.0

Java >