Overloading, Overriding, Runtime Types and Object Orientation - is A vs Has A

• is a defines a direct relationship between a superclass and a subclass
• has a identifies a relationship in which one object contains another object (defined by field variables)

Examples

• A circle is a shape that has a center point and a radius. (JJ pg 138)

    public class Circle extends Shape { // a circle is a shape
        Point  center;                  // a circle has a point
        double radius;                  // a circle has a radius
    }

• Define a class hierarchy for the following classes (BB pg14):
  1. An Employee class that maintains an employee number.
  2. A Full-time employee class that maintains an employee number, hours worked per week and calculates it's own pay using a salary() method.
  3. A Retired employee class that maintains an employee number, the number of years worked, and calculates it's own pay using a salary() method.

    public class Employee {     // an employee
        long   id;              // has an id, and
        String status;          // a status
    }

    abstract class EmployeeStatus extends Employee {
        abstract double salary();
    }
    
    // fulltime is a status
    class FullTime extends EmployeeStatus { 
        double hrs;        
        double salary(){
             return hrs * 60.0;
        }
    }
    
    // retired is a status
    class Retired extends EmployeeStatus {  
        int years;
        double salary() {
            return 0;
        }
    }    

• Create classes for 2DShape, Circle, Square and Point. Points have an (x,y) location. Circles have an (x,y) location and a radius. Squares have an (x,y) location and a length. (BB pg17)

    class Point {       // a point
        int x;          // has an x-location, and
        int y;          // a y-location
    }

    class 2DShape {     // all 2DShapes
        Point p;        // have a point
    }
    
    class Circle extends 2DShape {  // a circle is a 2DShape
        double radius;              // and has a radius
    }
    
    class Square extends 2DShape {  // a circle is a 2DShape
        double length;              // and has length
    }