Instantiating shapes using the Factory Design Pattern in Java

Question

Trying to learn the factory design pattern, came up with some code based on a Shape Factory (found this example here). Is this the right way to implement the factory pattern?

interface Shape{
 void draw();
}
class Rectangle implements Shape{
 @Override
 public void draw() {
 System.out.println(" Drawing A Rectangle!");
 }
}
class Circle implements Shape{
 @Override
 public void draw() {
 System.out.println(" Drawing A cIRCLE!");
 }
}
class Triangle implements Shape{
 @Override
 public void draw() {
 System.out.println(" Drawing A Triangle!");
 }
}
class ShapeFactory{
 public static Shape getShape(String shapeType)
 {
 Shape shape = null;
 switch(shapeType.toUpperCase()){
 case "CIRCLE":
 shape = new Circle();
 break;
 case "RECTANGLE":
 shape = new Rectangle();
 break;
 case "TRIANGLE":
 shape = new Triangle();
 break;
 }
 return shape;
 }
}
public class FactoryExample {
 public static void main(String[] args) {
 Shape s = ShapeFactory.getShape("rectangle");
 s.draw();
 s = ShapeFactory.getShape("triangle");
 s.draw();
 }
}

• Thijs Riezebeek: Try using constants to be passed to the factory for creation. This prevents typos from being introduced, e.g. ShapeFactory.createShape(Shape.TRIANGLE);

Answer 1

You could rewrite getShape as follows:

public static Shape getShape(String shapeType) {
 switch (shapeType.trim().toUpperCase()) {
 case "CIRCLE":
 return new Circle();
 case "RECTANGLE":
 return new Rectangle();
 case "TRIANGLE":
 return new Triangle();
 default:
 return null;
 }
}

Answer 2

Naming the method getShape(...) suggests that it's going to retrieve the same cached instance with every call. A more appropriate name to convey what the method does would be makeShape(...), or maybe newShape(...). (Unfortunately, you followed a bad example.)