Re: How do you make a group level in Java?

"Travis" <NoSpam@Spamstopper.org> wrote in message
news:Xns9C73D9A286C98NoSoyswbellnet@216.196.97.140...


See Effective Java by Joshua Bloch. In the first edition, see chapter 5. I
think there is a newer edition which I do not have yet. Basically in Java
you have classes/objects and they can be composed of other classes/objects.

Here is some sample code for Chapter 5 Item 20: Replace unions with class
hierarchies:

C code:

/* Discriminated union */
#include "math.h"
typedef enum {RECTANGLE, CIRCLE} shapeType_t;

typedef struct {
    double length;
    double width;
} rectangleDimensions_t;

typedef struct {
    double radius;
} circleDimensions_t;

typedef struct {
    shapeType_t tag;
    union {
        rectangleDimensions_t rectangle;
        circleDimensions_t circle;
    } dimensions;
} shape_t;

double area(shape_t *shape) {
    switch(shape->tag) {
      case RECTANGLE: {
        double length = shape->dimensions.rectangle.length;
        double width = shape->dimensions.rectangle.width;
        return length * width;
      }
      case CIRCLE: {
        double r = shape->dimensions.circle.radius;
        return M_PI * (r*r);
      }
      default: return -1.0; /* Invalid tag */
    }
}

int main() {
    shape_t rectangle, circle;
    rectangle.tag = RECTANGLE;
    rectangle.dimensions.rectangle.length = 2.0;
    rectangle.dimensions.rectangle.width = 3.0;
    printf("Rectangle area: %f\n", area(&rectangle));

    circle.tag = CIRCLE;
    circle.dimensions.circle.radius = 1.0;
    printf("Circle area: %f\n", area(&circle));

    return 0;
}

//***************************************************

Java code:

// Page 101
abstract class Shape {
    abstract double area();
}

class Circle extends Shape {
    final double radius;

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

    double area() { return Math.PI * radius*radius; }
}

class Rectangle extends Shape {
    final double length;
    final double width;

    Rectangle(double length, double width) {
        this.length = length;
        this.width = width;
    }

    double area() { return length * width; }
}

// Page 102
class Square extends Rectangle {
    Square(double side) {
        super(side, side);
    }

    double side() {
        return length; // or equivalently, width
    }
}

//***************************************************

Java test code:

// Test program for class hierarchy on page 101
public class ShapeTest {
    public static void main(String args[]) {
        Shape rectangle = new Rectangle(2.0, 3.0);
        Shape circle = new Circle(2.0);
        Shape square = new Square(15.0);

        System.out.println("Rectangle area: " + rectangle.area());
        System.out.println("Circle area: " + circle.area());
        System.out.println("Square area: " + square.area());
    }
}