the first code on interface

Hi, please could you evaluate the following training code (the first one on interface)?
To keep the classes clear, I put all the printing stuff to a separate class Print - this was my intention.
The code is a bit larger but easy to read, I hope.

I don't like two things there, please give me advice on them.
1. these "returns 0" - I have no knowledge how to treat the methods which are not applicable for particular shape
2. when I want to create another shape I will have to also add appropriate code block to the print method

If the concept is totally stupid, just tell me.

Thanks a lot.

package interface; public class Shape1 { public static void main(String[] args){ Shape[] shapes = new Shape[5]; shapes[0] = new Circle(2.0); shapes[1] = new Globe(4.0); shapes[2] = new Rectangle(5.0, 3.0); shapes[3] = new Cube(5.0); shapes[4] = new Block(5.0, 6.0, 7.0); Print p = new Print(); for(int i = 0; i < shapes.length; i++){ p.print(shapes[i]); } } } interface Shape { double PI = 3.14; public abstract double getRadius(); public abstract double getWidth(); public abstract double getHeight(); public abstract double getSide(); public abstract double area(); public abstract double circumference(); } class Circle implements Shape { protected double r; public Circle(double r) { this.r = r; } public double getRadius() { return r; } public double getWidth() { return 0; } public double getHeight() { return 0; } public double getSide() { return 0; } public double area() { return PI*r*r; } public double circumference() { return 2*PI*r; } } class Globe implements Shape { protected double r; public Globe(double r) { this.r = r; } public double getRadius() { return r; } public double getWidth() { return 0; } public double getHeight() { return 0; } public double getSide() { return 0; } public double area() { return 4/3*PI*r*r*r; } public double circumference() { return 4*PI*r*r;} } class Rectangle implements Shape { protected double w, h; public Rectangle(double w, double h) { this.w = w; this.h = h; } public double getRadius() { return 0; } public double getWidth() { return w; } public double getHeight() { return h; } public double getSide() { return 0; } public double area() { return w*h; } public double circumference() { return (w + h) * 2; } } class Cube implements Shape { protected double s; public Cube(double s) { this.s = s; } public double getRadius() { return 0; } public double getWidth() { return 0; } public double getHeight() { return 0; } public double getSide() { return s; } public double area() { return s*s*s; } public double circumference() { return s*s*6; } } class Block implements Shape { protected double w, h, d; public Block(double w, double h, double d) { this.w = w; this.h = h; this.d = d; } public double getRadius() { return 0; } public double getWidth() { return 0; } public double getHeight() { return 0; } public double getSide() { return 0; } public double area() { return w*h*d; } public double circumference() { return w*h*2 + w*d*2 + h*d*2; } } class Print { void print (Shape a){ Boolean shapeExists = false; if (a instanceof Circle){ shapeExists = true; System.out.println(a.getClass()); System.out.println("Circle r=" + a.getRadius()); System.out.println("Area=" + a.area()); System.out.println("Circumference=" + a.circumference()+ "\n"); } if (a instanceof Globe){ shapeExists = true; System.out.println(a.getClass()); System.out.println("Globe r=" + a.getRadius()); System.out.println("Area=" + a.area()); System.out.println("Circumference=" + a.circumference()+ "\n"); } if (a instanceof Rectangle){ shapeExists = true; System.out.println(a.getClass()); System.out.println("Rectangle w=" + a.getWidth()); System.out.println("Rectangle h=" +a.getHeight()); System.out.println("Area=" + a.area()); System.out.println("Circumference=" + a.circumference()+ "\n"); } if (a instanceof Cube){ shapeExists = true; System.out.println(a.getClass()); System.out.println("Cube s=" + a.getSide()); System.out.println("Area=" + a.area()); System.out.println("Circumference=" + a.circumference()+ "\n"); } if (shapeExists == false) System.out.println( "Print doesn't support shape of " + a.getClass()+"\n"); } }

[ October 03, 2008: Message edited by: frantisek nejedly ]
[ October 03, 2008: Message edited by: frantisek nejedly ]

I don't like two things there, please give me advice on them.
1. these "returns 0" - I have no knowledge how to treat the methods which are not applicable for particular shape
2. when I want to create another shape I will have to also add appropriate code block to the print method

#1 - Instead of returning 0 you could return a negative value, clearly indicating that it is not a valid value. You could also throw an exception.

#2 - The print method should be part of the Shape interface, and thus be implemented by each class implementing the interface.

Yep thats a good idea about putting the print on the Shape interface, in that way each shape knows how to print itself (similar to toString())and you dont need instanceof (or the Print class really).

Possibly you could introduce another interface(s) that extends Shape and move some of the shape specific into that. You would leave print(), and any other truly generic method in the top level Shape interface and have something like RoundShape, RectangularShape etc.

HTH

#1 - Instead of returning 0 you could return a negative value, clearly indicating that it is not a valid value. You could also throw an exception.

I see.
a - Can be the option of returnig negative value from NA methods considered as a normal and valid solution in this situation?
b - What if the negative return value is a valid one (of course not for shapes here)?

#2 - The print method should be part of the Shape interface, and thus be implemented by each class implementing the interface.

I see. I will play with it using Tom's suggestions.

Thank you very much.

You could throw an UnsupportedOperationException. I think though you should try to code it so they cannot be called on an object where it doesnt make sense....

I understand, I haven't learnt exceptions yet, regarding this, could be the code, as it is, considered as an acceptable one, including standalone Print class?

Well I wouldn't claim to be an expert on what code "should" look like....but its not bad at all. You're using interfaces in the calling code which is good, the Print class does what it says on the tin, just printing.

The main problem as we said is that if you add a new shape you need to remember to update Print class to handle it, whereas if the print method was on the Shape interface you would be forced to implement it in the new Shape class by the compiler, thereby preventing "Oh I didnt know the Print class did that" errors.

Thanks a lot Tom.

No bother

Did you mean something like this?

code deleted
[ October 04, 2008: Message edited by: frantisek nejedly ]

Note that if you have to implement interface methods by using 'return 0' or something like that, you should take it as a suggestion that either the class isn't really an instance of what the interface defines, or the interface itself is badly designed.

In this case, there's some of both: a "Shape" is generally a two-dimensional figure, so Spheres and Blocks shouldn't be shapes. A Shape, being two-dimensional, should have a height; and since "Radius" is a property of only one kind of Shape, it shouldn't be a property of the Shape interface.

Consider a design where "Shape" had only getArea() and print() methods, and where "Triangle" and "Trapezoid" replaced "Cube", "Block", and "Globe". M ethods like getRadius() and getWidth(), etc, belong only to the classes they apply to.

final version, no 'return 0' needed
package interface57; public class Shape57 { public static void main(String[] args){ Shape[] shapes = new Shape[5]; shapes[0] = new Circle(2.0); shapes[1] = new Globe(4.0); shapes[2] = new Rectangle(5.0, 3.0); shapes[3] = new Cube(5.0); shapes[4] = new Block(5.0, 6.0, 7.0); for(int i = 0; i < shapes.length; i++){ shapes[i].print(); } } } interface Shape { double PI = 3.14; public abstract void print(); } class Circle implements Shape { protected double r; public Circle(double r) { this.r = r; } public double getRadius() { return r; } public double area() { return PI*r*r; } public double circumference() { return 2*PI*r; } public void print (){ System.out.println(getClass()); System.out.println("Circle r=" + getRadius()); System.out.println("Area=" + area()); System.out.println("Circumference=" + circumference()+ "\n"); } } class Globe implements Shape { protected double r; public Globe(double r) { this.r = r; } public double getRadius() { return r; } public double area() { return 4/3*PI*r*r*r; } public double circumference() { return 4*PI*r*r;} public void print (){ System.out.println(getClass()); System.out.println("Globe r=" + getRadius()); System.out.println("Area=" + area()); System.out.println("Circumference=" + circumference()+ "\n"); } } class Rectangle implements Shape { protected double w, h; public Rectangle(double w, double h) { this.w = w; this.h = h; } public double getWidth() { return w; } public double getHeight() { return h; } public double area() { return w*h; } public double circumference() { return (w + h) * 2; } public void print(){ System.out.println(getClass()); System.out.println("Rectangle w=" + getWidth()); System.out.println("Rectangle h=" +getHeight()); System.out.println("Area=" + area()); System.out.println("Circumference=" + circumference()+ "\n"); } } class Cube implements Shape { protected double s; public Cube(double s) { this.s = s; } public double getSide() { return s; } public double area() { return s*s*s; } public double circumference() { return s*s*6; } public void print() { System.out.println(getClass()); System.out.println("Cube s=" + getSide()); System.out.println("Area=" + area()); System.out.println("Circumference=" + circumference()+ "\n"); } } class Block implements Shape { protected double w, h, d; public Block(double w, double h, double d) { this.w = w; this.h = h; this.d = d; } public double area() { return w*h*d; } public double circumference() { return w*h*2 + w*d*2 + h*d*2; } public void print() { System.out.println(getClass()); System.out.println("Block "); System.out.println("Area=" + area()); System.out.println("Circumference=" + circumference()+ "\n"); } }

[ October 04, 2008: Message edited by: frantisek nejedly ]
[ October 04, 2008: Message edited by: frantisek nejedly ]