19. Using static methods and constant data

• Not all things in OOP ought to be object-based.

• Here is a good example of a statement that calls a static field and a static method to get the job done:

  
      // pi times r squared:
      // calculate the area of a circle with a given radius.
      double area = Math.PI * Math.pow(radius, 2);  
  
  

• Any constant data is a perfect candidate to be declared both static and final. For example,

  
      public static final int DAYS_IN_JANUARY = 31;