Question

I have to produce a class diagram for a condition, condition was something like:

Fruits have colors and fruits can grow. Apples are fruits.
They have seeds. Seeds can make new apples.

I would like to have some experts advice before I implement any design.

I was thinking some thing like this but I am no expert so please help me out here

abstract class Fruit
{
    protected $_color;

    abstract function grow();

    protected function seed(Fruit $fruit)
    {
        return $fruit;
    }
}

class Apple extends Fruit
{

}

How can this be achieved using composition or inheritance ?

Answer 1

There are fruits that have seeds. Apple is one of those fruits. So I would use a simple plain inheritance design:

abstract class Fruit
{
     private $color;

     public function getColor(){/* */};
     public function setColor($color){/* */}

     abstract public function grow();
}

abstract class SeedableFruit extends Fruit
{
    public function seed()
    {
        //Do something
    }
}

class Apple extends SeedableFruit
{
    public function grow()
    {
        //Do something
    }
}

In this way you cannot reuse the implementation of the grow method of a non-SeedableFruit in a Fruit with seeds, but if you think how natural evolution works, it makes sense… because in that case the two fruits have a common ancestor that grows in the same way, so the problem could be resolved inserting that common ancestor in the class hierarchy.

The alternative design was to use a DecoratorPattern to describe a fruit that can have seeds, and then create an Apple with seeds with a call like new SeedableFruitDecorator(new Apple());. But I think it’s not the correct way, since an Apple has always seeds.